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23
24 title: 'JAVAC(1) JDK @@VERSION_SHORT@@ | JDK Commands'
25 date: @@COPYRIGHT_YEAR@@
26 lang: en
27 ---
28
29 ## Name
30
31 javac - read Java declarations and compile them into class files
32
33 ## Synopsis
34
35 `javac` \[*options*\] \[*sourcefiles-or-classnames*\]
36
37 *options*
38 : Command-line options.
39
40 *sourcefiles-or-classnames*
41 : Source files to be compiled (for example, `Shape.java`) or
42 the names of previously compiled classes to be processed for annotations
43 (for example, `geometry.MyShape`).
44
45 ## Description
46
47 The `javac` command reads _source files_ that contain module, package and type
48 declarations written in the Java programming language, and compiles them into _class files_
49 that run on the Java Virtual Machine.
50
51 The `javac` command can also [process annotations](#annotation-processing)
52 in Java source files and classes.
53
54 Source files must have a file name extension of `.java`.
55 Class files have a file name extension of `.class`.
56 Both source and class files normally have file names that identify the contents.
57 For example, a class called `Shape` would be declared in a source file
58 called `Shape.java`, and compiled into a class file called `Shape.class`.
59
60 There are two ways to specify source files to `javac`:
61
62 - For a small number of source files, you can list their file names on
63 the command line.
64
65 - For a large number of source files, you can use the [`@`*filename*](#option-at)
66 option on the command line to specify an _argument file_ that lists
67 their file names. See [Standard Options] for a description of the
68 option and [Command-Line Argument Files] for a description of
69 `javac` argument files.
70
71 The order of source files specified on the command line or in an
72 argument file is not important. `javac` will compile the files together,
73 as a group, and will automatically resolve any dependencies between
74 the declarations in the various source files.
75
76 `javac` expects that source files are arranged in one or more directory
77 hierarchies on the file system, described in [Arrangement of Source
78 Code].
79
80 To compile a source file, `javac` needs to find the declaration of
81 every class or interface that is used, extended, or implemented by the
82 code in the source file. This lets `javac` check that the code has the
83 right to access those classes and interfaces. Rather than specifying
84 the source files of those classes and interfaces explicitly, you can
85 use command-line options to tell `javac` where to search for their
86 source files. If you have compiled those source files previously, you
87 can use options to tell `javac` where to search for the corresponding
88 class files. The options, which all have names ending in "path", are
89 described in [Standard Options], and further described in
90 [Configuring a Compilation] and [Searching for Module, Package and Type Declarations].
91
92 By default, `javac` compiles each source file to a class file in the
93 same directory as the source file. However, it is recommended to
94 specify a separate destination directory with the [`-d`](#option-d) option.
95
96 Command-line [options] and [environment variables] also control how
97 `javac` performs various tasks:
98
99 - Compiling code to run on earlier releases of the JDK.
100 - Compiling code to run under a debugger.
101 - Checking for stylistic issues in Java source code.
102 - Checking for problems in `javadoc` comments (`/** ... */`).
103 - Processing annotations in source files and class files.
104 - Upgrading and patching modules in the compile-time environment.
105
106 `javac` supports [Compiling for Earlier Releases Of The Platform]
107 and can also be invoked from Java code using one of a number of [APIs]
108
109 ## Options
110
111 `javac` provides [standard options], and [extra options] that are either
112 non-standard or are for advanced use.
113
114 Some options take one or more arguments.
115 If an argument contains spaces or other whitespace characters,
116 the value should be quoted according to the conventions of the
117 environment being used to invoke javac.
118 If the option begins with a single dash (`-`) the argument should
119 either directly follow the option name, or should be separated with a
120 colon (`:`) or whitespace, depending on the option. If the option begins with
121 a double dash (`--`), the argument may be separated either by whitespace
122 or by an equals (`=`) character with no additional whitespace.
123 For example,
124
125 -Aname="J. Duke"
126 -proc:only
127 -d myDirectory
128 --module-version 3
129 --module-version=3
130
131 In the following lists of options, an argument of *path* represents
132 a search path, composed of a list of file system locations separated
133 by the platform path separator character, (semicolon `;` on Windows,
134 or colon `:` on other systems.) Depending on the option, the
135 file system locations may be directories, JAR files or JMOD files.
136
137 ### Standard Options
138
139 <a id="option-at">`@`*filename*</a>
140 : Reads options and file names from a file. To shorten or simplify the
141 `javac` command, you can specify one or more files that contain arguments
142 to the `javac` command (except [`-J`](#option-J) options). This lets you to create
143 `javac` commands of any length on any operating system.
144 See [Command-Line Argument Files].
145
146 <a id="option-A">`-A`*key*\[`=`*value*\]</a>
147 : Specifies options to pass to annotation processors. These options are not
148 interpreted by `javac` directly, but are made available for use by
149 individual processors. The *key* value should be one or more identifiers
150 separated by a dot (`.`).
151
152 <a id="option-add-modules">`--add-modules` *module*`,`*module*</a>
153 : Specifies root modules to resolve in addition to the initial modules, or
154 all modules on the module path if *module* is `ALL-MODULE-PATH`.
155
156 <a id="option-boot-class-path">`--boot-class-path` *path* or `-bootclasspath` *path*</a>
157 : Overrides the location of the bootstrap class files.
158
159 **Note:** This can only be used when compiling for releases prior to JDK 9.
160 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
161 [`-target`](#option-target) for details. For JDK 9 or later, see [`--system`](#option-system).
162
163 <a id="option-class-path">`--class-path` *path*, `-classpath` *path*, or `-cp` *path*</a>
164 : Specifies where to find user class files and annotation processors. This
165 class path overrides the user class path in the `CLASSPATH` environment
166 variable.
167
168 - If `--class-path`, `-classpath`, or `-cp` are not specified, then the
169 user class path is the value of the `CLASSPATH` environment variable,
170 if that is set, or else the current directory.
171
172 - If not compiling code for modules, if the [`--source-path`](#option-source-path) or -sourcepath`
173 option is not specified, then the user class path is also searched for source files.
174
175 - If the [`-processorpath`](#option-processor-path) option is not specified, then the class path is
176 also searched for annotation processors.
177
178 <a id="option-d">`-d` *directory*</a>
179 : Sets the destination directory (or _class output directory_) for class files.
180 If a class is part of a package, then `javac` puts the class file in a
181 subdirectory that reflects the module name (if appropriate) and package name.
182 The directory, and any necessary subdirectories, will be created if they
183 do not already exist.
184
185 If the `-d` option is not specified, then `javac` puts each class file in
186 the same directory as the source file from which it was generated.
187
188 Except when compiling code for multiple modules, the contents of the
189 class output directory will be organized in a package hierarchy.
190 When compiling code for multiple modules, the contents of the output
191 directory will be organized in a module hierarchy, with the contents of each
192 module in a separate subdirectory, each organized as a package
193 hierarchy.
194
195 **Note:**
196 When compiling code for one or more modules, the class output directory will
197 automatically be checked when searching for previously compiled classes.
198 When not compiling for modules, for backwards compatibility,
199 the directory is _not_ automatically checked for previously compiled classes,
200 and so it is recommended to specify the class output directory as one
201 of the locations on the user class path, using the `--class-path` option or one of
202 its alternate forms.
203
204 <a id="option-deprecation">`-deprecation`</a>
205 : Shows a description of each use or override of a deprecated member or
206 class. Without the `-deprecation` option, `javac` shows a summary of the
207 source files that use or override deprecated members or classes. The
208 `-deprecation` option is shorthand for `-Xlint:deprecation`.
209
210 <a id="option-enable-preview">`--enable-preview`</a>
211 : Enables preview language features. Used in conjunction with either
212 [`-source`](#option-source) or [`--release`](#option-release).
213
214 <a id="option-encoding">`-encoding` *encoding*</a>
215 : Specifies character encoding used by source files, such as EUC-JP and
216 UTF-8. If the `-encoding` option is not specified, then the platform default
217 converter is used.
218
219 <a id="option-endorseddirs">`-endorseddirs` *directories*</a>
220 : Overrides the location of the endorsed standards path.
221
222 **Note:** This can only be used when compiling for releases prior to JDK 9.
223 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
224 [`-target`](#option-target) for details.
225
226 <a id="option-extdirs">`-extdirs` *directories*</a>
227 : Overrides the location of the installed extensions.
228 `directories` is a list of directories, separated by the platform path separator
229 (`;` on Windows, and `:` otherwise).
230 Each JAR file in the specified directories is searched for class files.
231 All JAR files found become part of the class path.
232
233 If you are compiling for a release of the platform that supports the
234 Extension Mechanism, then this option specifies the directories that
235 contain the extension classes.
236 See [Compiling for Other Releases of the Platform].
237
238 **Note:** This can only be used when compiling for releases prior to JDK 9.
239 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
240 [`-target`](#option-target) for details.
241
242 <a id="option-g">`-g`</a>
243 : Generates all debugging information, including local variables. By default,
244 only line number and source file information is generated.
245
246 <a id="option-g-custom">`-g:`\[`lines`, `vars`, `source`\]</a>
247 : Generates only the kinds of debugging information specified by the
248 comma-separated list of keywords. Valid keywords are:
249
250 `lines`
251 : Line number debugging information.
252
253 `vars`
254 : Local variable debugging information.
255
256 `source`
257 : Source file debugging information.
258
259 <a id="option-g-none">`-g:none`</a>
260 : Does not generate debugging information.
261
262 <a id="option-h">`-h` *directory*</a>
263 : Specifies where to place generated native header files.
264
265 When you specify this option, a native header file is generated for each
266 class that contains native methods or that has one or more constants
267 annotated with the [`java.lang.annotation.Native`](
268 ../../api/java.base/java/lang/annotation/Native.html)
269 annotation. If the class is part of a package, then the compiler puts the
270 native header file in a subdirectory that reflects the module name
271 (if appropriate) and package name.
272 The directory, and any necessary subdirectories, will be created if they
273 do not already exist.
274
275 <a id="option-help">`--help`, `-help` or `-?`</a>
276 : Prints a synopsis of the standard options.
277
278 <a id="option-help-extra">`--help-extra` or `-X`</a>
279 : Prints a synopsis of the set of extra options.
280
281 <a id="option-help-lint">`--help-lint`</a>
282 : Prints the supported keys for the `-Xlint` option.
283
284 <a id="option-implicit">`-implicit:`\[`none`, `class`\]</a>
285 : Specifies whether or not to generate class files for implicitly referenced
286 files:
287
288 - `-implicit:class` --- Automatically generates class files.
289
290 - `-implicit:none` --- Suppresses class file generation.
291
292 If this option is not specified, then the default automatically generates
293 class files. In this case, the compiler issues a warning if any class files
294 are generated when also doing annotation processing. The warning is not
295 issued when the `-implicit` option is explicitly set.
296 See [Searching for Module, Package and Type Declarations].
297
298 <a id="option-J">`-J`*option*</a>
299 : Passes *option* to the runtime system, where *option* is one of the Java
300 options described on [java](java.html) command. For example, `-J-Xms48m`
301 sets the startup memory to 48 MB.
302
303 **Note:** The `CLASSPATH` environment variable, `-classpath` option, `-bootclasspath`
304 option, and `-extdirs` option do not specify the classes used to run
305 `javac`. Trying to customize the compiler implementation with these options
306 and variables is risky and often does not accomplish what you want. If you
307 must customize the compiler implementation, then use the `-J` option to
308 pass options through to the underlying Java launcher.
309
310 <a id="option-limit-modules">`--limit-modules` *module*`,`*module*\*</a>
311 : Limits the universe of observable modules.
312
313 <a id="option-module">`--module` *module-name* (`,`*module-name*)* or `-m` *module-name* (`,`*module-name*)*</a>
314 : Compiles those source files in the named modules that are newer
315 than the corresponding files in the output directory.
316
317 <a id="option-module-path">`--module-path` *path* or `-p` *path*</a>
318 : Specifies where to find application modules.
319
320 <a id="option-module-source-path">`--module-source-path` *module-source-path*</a>
321 : Specifies where to find source files when compiling code
322 in multiple modules. See [The Module Source Path Option].
323
324 <a id="option-module-version">`--module-version` *version*</a>
325 : Specifies the version of modules that are being compiled.
326
327 <a id="option-nowarn">`-nowarn`</a>
328 : Generate only mandatory warnings.
329
330 <a id="option-parameters">`-parameters`</a>
331 : Generates metadata for reflection on method parameters. Stores formal
332 parameter names of constructors and methods in the generated class file so
333 that the method `java.lang.reflect.Executable.getParameters` from the
334 Reflection API can retrieve them.
335
336 <a id="option-proc">`-proc:`\[`none`, `only`, `full`\]</a>
337 : Controls whether annotation processing and compilation are done.
338
339 - `-proc:none` means that compilation takes place without annotation
340 processing
341
342 - `-proc:only` means that only annotation processing is done,
343 without any subsequent compilation.
344
345 - `-proc:full` means annotation processing and compilation are done.
346
347 If this option is not used, annotation processing and compilation
348 are done if at least one other option is used to explicitly
349 configure annotation processing.
350
351 <a id="option-processor">`-processor` *class1*\[`,`*class2*`,`*class3*...\]</a>
352 : Names of the annotation processors to run. This bypasses the default
353 discovery process.
354
355 <a id="option-processor-module-path">`--processor-module-path` *path*</a>
356 : Specifies the module path used for finding annotation processors.
357
358 <a id="option-processor-path">`--processor-path` *path* or `-processorpath` *path*</a>
359 : Specifies where to find annotation processors. If this option is not used,
360 then the class path is searched for processors.
361
362 <a id="option-profile">`-profile` *profile*</a>
363 : Checks that the API used is available in the specified profile.
364 This option is deprecated and may be removed in a future release.
365
366 **Note:** This can only be used when compiling for releases prior to JDK 9.
367 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
368 or [`-target`](#option-target) for details.
369
370 <a id="option-release">`--release` *release*</a>
371 : Compiles source code according to the rules of the Java programming language
372 for the specified Java SE release, generating class files which target
373 that release.
374 Source code is compiled against the combined Java SE and JDK API for the
375 specified release.
376
377 The supported values of *release* are the current Java SE release and a
378 limited number of previous releases, detailed in the command-line help.
379
380 For the current release, the Java SE API consists of the `java.*`,
381 `javax.*`, and `org.*` packages that are exported by the Java SE modules in
382 the release; the JDK API consists of the `com.*` and `jdk.*` packages that
383 are exported by the JDK modules in the release, plus the `javax.*` packages
384 that are exported by standard, but non-Java SE, modules in the release.
385
386 For previous releases, the Java SE API and the JDK API are as defined in
387 that release.
388
389 **Note:** When using `--release`, you cannot also use the [`--source`](#option-source)/`-source` or
390 [`--target`](#option-target)/`-target` options.
391
392 **Note:** When using `--release` to specify a release that supports the Java
393 Platform Module System, the `--add-exports` option cannot be used to
394 enlarge the set of packages exported by the Java SE, JDK, and standard
395 modules in the specified release.
396
397 <a id="option-s">`-s` *directory*</a>
398 : Specifies the directory used to place the generated source files. If a
399 class is part of a package, then the compiler puts the source file in a
400 subdirectory that reflects the module name (if appropriate) and package name.
401 The directory, and any necessary subdirectories, will be created if they
402 do not already exist.
403
404 Except when compiling code for multiple modules, the contents of the
405 source output directory will be organized in a package hierarchy.
406 When compiling code for multiple modules, the contents of the source output directory will be
407 organized in a module hierarchy, with the contents of each
408 module in a separate subdirectory, each organized as a package
409 hierarchy.
410
411 <a id="option-source">`--source` *release* or `-source` *release*</a>
412 : Compiles source code according to the rules of the Java programming language
413 for the specified Java SE release.
414 The supported values of *release* are the current Java SE release and a
415 limited number of previous releases, detailed in the command-line help.
416
417 If the option is not specified, the default is to compile source code
418 according to the rules of the Java programming language for the current
419 Java SE release.
420
421 <a id="option-source-path">`--source-path` *path* or `-sourcepath` *path*</a>
422 : Specifies where to find source files.
423 Except when compiling multiple modules together, this is the source code path
424 used to search for class or interface definitions.
425
426 **Note:** Classes found through the class path might be recompiled when their source
427 files are also found. See [Searching for Module, Package and Type Declarations].
428
429 <a id="option-system">`--system` *jdk* \| `none`</a>
430 : Overrides the location of system modules.
431
432 <a id="option-target">`--target` *release* or `-target` *release*</a>
433 : Generates `class` files suitable for the specified Java SE release.
434 The supported values of *release* are the current Java SE release and a
435 limited number of previous releases, detailed in the command-line help.
436
437 **Note:** The target release must be equal to or higher than the source release.
438 (See [`--source`](#option-source).)
439
440 <a id="option-upgrade-module-path">`--upgrade-module-path` *path*</a>
441 : Overrides the location of upgradeable modules.
442
443 <a id="option-verbose">`-verbose`</a>
444 : Outputs messages about what the compiler is doing. Messages include
445 information about each class loaded and each source file compiled.
446
447 <a id="option-version">`--version` or `-version`</a>
448 : Prints version information.
449
450 <a id="option-Werror">`-Werror`</a>
451 : Terminates compilation when warnings occur.
452
453 ### Extra Options
454
455 <a id="option-add-exports">`--add-exports` *module*`/`*package*`=`*other-module*(`,`*other-module*)\*</a>
456 : Specifies a package to be considered as exported from its defining module
457 to additional modules or to all unnamed modules when the value of
458 *other-module* is `ALL-UNNAMED`.
459
460 <a id="option-add-reads">`--add-reads` *module*`=`*other-module*(`,`*other-module*)\*</a>
461 : Specifies additional modules to be considered as required by a given
462 module.
463
464 <a id="option-default-module-for-created-files">`--default-module-for-created-files` *module-name*</a>
465 : Specifies the fallback target module for files created by annotation
466 processors, if none is specified or inferred.
467
468 <a id="option-disable-line-doc-comments">`--disable-line-doc-comments`</a>
469 : Disables support for documentation comments with lines beginning ///.
470
471 <a id="option-Djava.endorsed.dirs">`-Djava.endorsed.dirs=`*dirs*</a>
472 : Overrides the location of the endorsed standards path.
473
474 **Note:** This can only be used when compiling for releases prior to JDK 9.
475 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
476 or [`-target`](#option-target) for details.
477
478 <a id="option-Djava.ext.dirs">`-Djava.ext.dirs=`*dirs*</a>
479 : Overrides the location of installed extensions.
480
481 **Note:** This can only be used when compiling for releases prior to JDK 9.
482 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
483 or [`-target`](#option-target) for details.
484
485 <a id="option-patch-module">`--patch-module` *module*`=`*path*</a>
486 : Overrides or augments a module with classes and resources in JAR files or
487 directories.
488
489 <a id="option-Xbootclasspath">`-Xbootclasspath:`*path*</a>
490 : Overrides the location of the bootstrap class files.
491
492 **Note:** This can only be used when compiling for releases prior to JDK 9.
493 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
494 or [`-target`](#option-target) for details.
495
496 <a id="option-Xbootclasspath-a">`-Xbootclasspath/a:`*path*</a>
497 : Adds a suffix to the bootstrap class path.
498
499 **Note:** This can only be used when compiling for releases prior to JDK 9.
500 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
501 or [`-target`](#option-target) for details.
502
503 <a id="option-Xbootclasspath-p">`-Xbootclasspath/p:`*path*</a>
504 : Adds a prefix to the bootstrap class path.
505
506 **Note:** This can only be used when compiling for releases prior to JDK 9.
507 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
508 or [`-target`](#option-target) for details.
509
510 <a id="option-Xdiags">`-Xdiags:`\[`compact`, `verbose`\]</a>
511 : Selects a diagnostic mode.
512
513 <a id="option-Xdoclint">`-Xdoclint`</a>
514 : Enables recommended checks for problems in documentation comments.
515
516 <a id="option-Xdoclint-custom">`-Xdoclint:`(`all`|`none`|\[`-`\]*group*)\[`/`*access*\]</a>
517 : Enables or disables specific groups of checks in documentation comments.
518
519 *group* can have one of the following values:
520 `accessibility`, `html`, `missing`, `reference`, `syntax`.
521
522 The variable *access* specifies the minimum visibility level of classes and
523 members that the `-Xdoclint` option checks. It can have one of the
524 following values (in order of most to least visible):
525 `public`, `protected`, `package`, `private`.
526
527 The default *access* level is `private`.
528
529 When prefixed by `doclint:`, the *group* names and `all` can be used with `@SuppressWarnings`
530 to suppress warnings about documentation comments in parts of the code being compiled.
531
532 For more information about these groups of checks, see the [DocLint](javadoc.html#doclint)
533 section of the `javadoc` command documentation.
534 The `-Xdoclint` option is disabled by default in the `javac` command.
535
536 For example, the following option checks classes and members (with all
537 groups of checks) that have the access level of protected and higher (which
538 includes protected and public):
539
540 > `-Xdoclint:all/protected`
541
542 The following option enables all groups of checks for all access levels,
543 except it will not check for HTML errors for classes and members that have the
544 access level of package and higher (which includes package, protected and
545 public):
546
547 > `-Xdoclint:all,-html/package`
548
549 <a id="option-Xdoclint-package">`-Xdoclint/package:`\[`-`\]*packages*(`,`\[`-`\]*package*)\*</a>
550 : Enables or disables checks in specific packages. Each *package* is either
551 the qualified name of a package or a package name prefix followed by `.*`,
552 which expands to all sub-packages of the given package. Each *package* can
553 be prefixed with a hyphen (`-`) to disable checks for a specified package
554 or packages.
555
556 For more information, see the [DocLint](javadoc.html#doclint)
557 section of the `javadoc` command documentation.
558
559 <a id="option-Xlint">`-Xlint`</a>
560 : Enables all recommended warnings. In this release, enabling all available
561 warnings is recommended.
562
563 <a id="option-Xlint-custom">`-Xlint:`\[`-`\]*key*(`,`\[`-`\]*key*)\*</a>
564 : Enables and/or disables warning categories using the one or more of the keys described
565 below separated by commas. The keys `all` and `none` enable or disable all categories
566 (respectively); other keys enable the corresponding category, or disable it if preceded
567 by a hyphen (`-`).
568
569 Supported values for *key* are:
570
571 - `all`: Enables all warning categories.
572
573 - `auxiliaryclass`: Warns about an auxiliary class that is hidden in a
574 source file, and is used from other files.
575
576 - `cast`: Warns about the use of unnecessary casts.
577
578 - `classfile`: Warns about the issues related to classfile contents.
579
580 - `dangling-doc-comments`: Warns about extra or misplaced documentation
581 comments near the beginning of a declaration.
582
583 - `deprecation`: Warns about the use of deprecated items.
584
585 - `dep-ann`: Warns about the items marked as deprecated in `javadoc` but
586 without the `@Deprecated` annotation.
587
588 - `divzero`: Warns about the division by the constant integer 0.
589
590 - `empty`: Warns about an empty statement after `if`.
591
592 - `exports`: Warns about the issues regarding module exports.
593
594 - `fallthrough`: Warns about the falling through from one case of a
595 switch statement to the next.
596
597 - `finally`: Warns about `finally` clauses that do not terminate normally.
598
599 - `identity`: Warns about use of a value-based class where an identity
600 class is expected
601
602 - `incubating`: Warns about the use of incubating modules.
603
604 - `lossy-conversions`: Warns about possible lossy conversions
605 in compound assignment.
606
607 - `missing-explicit-ctor`: Warns about missing explicit constructors in
608 public and protected classes in exported packages.
609
610 - `module`: Warns about the module system-related issues.
611
612 - `opens`: Warns about the issues related to module opens.
613
614 - `options`: Warns about the issues relating to use of command line
615 options.
616
617 - `output-file-clash`: Warns if any output file is overwritten during compilation.
618 This can occur, for example, on case-insensitive filesystems.
619
620 - `overloads`: Warns about the issues related to method overloads.
621
622 - `overrides`: Warns about the issues related to method overrides.
623
624 - `path`: Warns about the invalid path elements on the command line.
625
626 - `preview`: Warns about the use of preview language features.
627
628 - `processing`: Warns about the issues related to annotation processing.
629
630 - `rawtypes`: Warns about the use of raw types.
631
632 - `removal`: Warns about the use of an API that has been marked for
633 removal.
634
635 - `restricted`: Warns about the use of restricted methods.
636
637 - `requires-automatic`: Warns developers about the use of automatic
638 modules in requires clauses.
639
640 - `requires-transitive-automatic`: Warns about automatic modules in
641 requires transitive.
642
643 - `serial`: Warns about the serializable classes that do not provide a
644 serial version ID. Also warns about access to non-public members from a
645 serializable element.
646
647 - `static`: Warns about the accessing a static member using an instance.
648
649 - `strictfp`: Warns about unnecessary use of the `strictfp` modifier.
650
651 - `synchronization`: Warns about synchronization attempts on instances
652 of value-based classes. This key is a deprecated alias for `identity`,
653 which has the same uses and effects. Users are encouraged to use the
654 `identity` category for all future and existing uses of `synchronization`.
655
656 - `text-blocks`: Warns about inconsistent white space characters in text
657 block indentation.
658
659 - `this-escape`: Warns about constructors leaking `this` prior to subclass initialization.
660
661 - `try`: Warns about the issues relating to the use of try blocks (that
662 is, try-with-resources).
663
664 - `unchecked`: Warns about the unchecked operations.
665
666 - `varargs`: Warns about the potentially unsafe `vararg` methods.
667
668 - `none`: Disables all warning categories.
669
670 With the exception of `all` and `none`, the keys can be used with
671 the `@SuppressWarnings` annotation to suppress warnings in a part
672 of the source code being compiled.
673
674 See [Examples of Using -Xlint keys].
675
676 <a id="option-Xmaxerrs">`-Xmaxerrs` *number*</a>
677 : Sets the maximum number of errors to print.
678
679 <a id="option-Xmaxwarns">`-Xmaxwarns` *number*</a>
680 : Sets the maximum number of warnings to print.
681
682 <a id="option-Xpkginfo">`-Xpkginfo:`\[`always`, `legacy`, `nonempty`\]</a>
683 : Specifies when and how the `javac` command generates `package-info.class`
684 files from `package-info.java` files using one of the following options:
685
686 `always`
687 : Generates a `package-info.class` file for every `package-info.java`
688 file. This option may be useful if you use a build system such as Ant,
689 which checks that each `.java` file has a corresponding `.class` file.
690
691 `legacy`
692 : Generates a `package-info.class` file only if `package-info.java`
693 contains annotations. This option does not generate a
694 `package-info.class` file if `package-info.java` contains only
695 comments.
696
697 **Note:** A `package-info.class` file might be generated but be empty if all the
698 annotations in the `package-info.java` file have
699 `RetentionPolicy.SOURCE`.
700
701 `nonempty`
702 : Generates a `package-info.class` file only if `package-info.java`
703 contains annotations with `RetentionPolicy.CLASS` or
704 `RetentionPolicy.RUNTIME`.
705
706 <a id="option-Xplugin">`-Xplugin:`*name* *args*</a>
707 : Specifies the name and optional arguments for a plug-in to be run.
708 If *args* are provided, *name* and *args* should be quoted or otherwise
709 escape the whitespace characters between the name and all the arguments.
710 For details on the API for a plugin, see the API documentation for
711 [jdk.compiler/com.sun.source.util.Plugin](../../api/jdk.compiler/com/sun/source/util/Plugin.html).
712
713 <a id="option-Xprefer">`-Xprefer:`\[`source`, `newer`\]</a>
714 : Specifies which file to read when both a source file and class file are
715 found for an implicitly compiled class using one of the following options.
716 See [Searching for Module, Package and Type Declarations].
717
718 - `-Xprefer:newer`: Reads the newer of the source or class files for a
719 type (default).
720
721 - `-Xprefer:source` : Reads the source file. Use `-Xprefer:source` when
722 you want to be sure that any annotation processors can access
723 annotations declared with a retention policy of `SOURCE`.
724
725 <a id="option-Xprint">`-Xprint`</a>
726 : Prints a textual representation of specified types for debugging purposes.
727 This does not perform annotation processing or compilation. The format of
728 the output could change.
729
730 <a id="option-XprintProcessorInfo">`-XprintProcessorInfo`</a>
731 : Prints information about which annotations a processor is asked to process.
732
733 <a id="option-XprintRounds">`-XprintRounds`</a>
734 : Prints information about initial and subsequent annotation processing
735 rounds.
736
737 <a id="option-Xstdout">`-Xstdout` *filename*</a>
738 : Sends compiler messages to the named file. By default, compiler messages go
739 to `System.err`.
740
741 ## Environment Variables
742
743 ### CLASSPATH
744
745 If the [`--class-path`](#option-class-path) option or any of its alternate forms are not specified,
746 the class path will default to the value of the `CLASSPATH` environment
747 variable if it is set.
748 However, it is recommended that this environment variable should _not_ be set,
749 and that the `--class-path` option should be used to provide an explicit
750 value for the class path when one is required.
751
752 ### JDK\_JAVAC\_OPTIONS
753
754 The content of the `JDK_JAVAC_OPTIONS` environment variable, separated by
755 white-spaces ( ) or white-space characters (`\n`, `\t`, `\r`, or `\f`) is
756 prepended to the command line arguments passed to `javac` as a list of
757 arguments.
758
759 The encoding requirement for the environment variable is the same as the
760 `javac` command line on the system. `JDK_JAVAC_OPTIONS` environment variable
761 content is treated in the same manner as that specified in the command line.
762
763 Single quotes (`'`) or double quotes (`"`) can be used to enclose arguments
764 that contain whitespace characters. All content between the open quote and the
765 first matching close quote are preserved by simply removing the pair of quotes.
766 In case a matching quote is not found, the launcher will abort with an error
767 message. `@`*files* are supported as they are specified in the command line.
768 However, as in `@`*files*, use of a wildcard is not supported.
769
770 **Examples of quoting arguments containing white spaces:**
771
772 > `export JDK_JAVAC_OPTIONS='@"C:\white spaces\argfile"'`
773
774 > `export JDK_JAVAC_OPTIONS='"@C:\white spaces\argfile"'`
775
776 > `export JDK_JAVAC_OPTIONS='@C:\"white spaces"\argfile'`
777
778 ## Command-Line Argument Files
779
780 An argument file can include command-line options and source file names in any
781 combination. The arguments within a file can be separated by spaces or new line
782 characters. If a file name contains embedded spaces, then put the whole file
783 name in double quotation marks.
784
785 File names within an argument file are relative to the current directory, not
786 to the location of the argument file. Wildcards (`*`) are not allowed in these
787 lists (such as for specifying `*.java`). Use of the at sign (`@`) to
788 recursively interpret files is not supported. The `-J` options are not supported
789 because they're passed to the launcher, which does not support argument files.
790
791 When executing the `javac` command, pass in the path and name of each argument
792 file with the at sign (`@`) leading character. When the `javac` command
793 encounters an argument beginning with the at sign (`@`), it expands the
794 contents of that file into the argument list.
795
796
797 ### Examples of Using javac @filename
798
799 Single Argument File
800 : You could use a single argument file named `argfile` to hold all `javac`
801 arguments:
802
803 > `javac @argfile`
804
805 This argument file could contain the contents of both files shown in the
806 following **Two Argument Files** example.
807
808 Two Argument Files
809 : You can create two argument files: one for the `javac` options and the
810 other for the source file names. Note that the following lists have no
811 line-continuation characters.
812
813 Create a file named `options` that contains the following:
814
815 **Linux and macOS:**
816
817 ```
818 -d classes
819 -g
820 -sourcepath /java/pubs/ws/1.3/src/share/classes
821 ```
822
823 **Windows:**
824
825 ```
826 -d classes
827 -g
828 -sourcepath C:\java\pubs\ws\1.3\src\share\classes
829 ```
830
831 Create a file named `sources` that contains the following:
832
833 ```
834 MyClass1.java
835 MyClass2.java
836 MyClass3.java
837 ```
838
839 Then, run the `javac` command as follows:
840
841 > `javac @options @sources`
842
843 Argument Files with Paths
844 : The argument files can have paths, but any file names inside the files are
845 relative to the current working directory (not `path1` or `path2`):
846
847 > `javac @path1/options @path2/sources`
848
849 ## Arrangement of Source Code
850
851 In the Java language, classes and interfaces can be organized into
852 packages, and packages can be organized into modules. `javac` expects
853 that the physical arrangement of source files in directories of the
854 file system will mirror the organization of classes into packages, and
855 packages into modules.
856
857 It is a widely adopted convention that module names and package names
858 begin with a lower-case letter, and that class names begin with an
859 upper-case letter.
860
861 ### Arrangement of Source Code for a Package
862
863 When classes and interfaces are organized into a package, the package
864 is represented as a directory, and any subpackages are represented as
865 subdirectories.
866
867 For example:
868
869 - The package `p` is represented as a directory called `p`.
870
871 - The package `p.q` -- that is, the subpackage `q` of package `p` --
872 is represented as the subdirectory `q` of directory `p`. The
873 directory tree representing package `p.q` is therefore `p\q`
874 on Windows, and `p/q` on other systems.
875
876 - The package `p.q.r` is represented as the directory tree `p\q\r`
877 (on Windows) or `p/q/r` (on other systems).
878
879 Within a directory or subdirectory, `.java` files represent classes
880 and interfaces in the corresponding package or subpackage.
881
882 For example:
883
884 - The class `X` declared in package `p` is represented by the file
885 `X.java` in the `p` directory.
886
887 - The class `Y` declared in package `p.q` is represented by the file
888 `Y.java` in the `q` subdirectory of directory `p`.
889
890 - The class `Z` declared in package `p.q.r` is represented by the file
891 `Z.java` in the `r` subdirectory of `p\q` (on Windows) or `p/q`
892 (on other systems).
893
894 In some situations, it is convenient to split the code into
895 separate directories, each structured as described above, and
896 the aggregate list of directories specified to `javac`.
897
898 ### Arrangement of Source Code for a Module
899
900 In the Java language, a module is a set of packages designed for
901 reuse. In addition to `.java` files for classes and interfaces, each
902 module has a source file called `module-info.java` which:
903
904 1. declares the module's name;
905
906 2. lists the packages exported by the module (to allow reuse by other
907 modules);
908
909 3. lists other modules required by the module (to reuse their exported
910 packages).
911
912 When packages are organized into a module, the module is represented
913 by one or more directories representing the packages in the module,
914 one of which contains the `module-info.java` file. It may be convenient,
915 but it is not required, to use a single directory, named after the module,
916 to contain the `module-info.java` file alongside the directory tree which
917 represents the packages in the module (i.e., the _package hierarchy_
918 described above). The exact arrangement of source code for a module
919 is typically dictated by the conventions adopted by a development
920 environment (IDE) or build system.
921
922 For example:
923
924 - The module `a.b.c` may be represented by the directory `a.b.c`, on all
925 systems.
926
927 - The module's declaration is represented by the file
928 `module-info.java` in the `a.b.c` directory.
929
930 - If the module contains package `p.q.r`, then the `a.b.c` directory
931 contains the directory tree `p\q\r` (on Windows) or `p/q/r`
932 (on other systems).
933
934 The development environment may prescribe some directory hierarchy
935 between the directory named for the module and the source files to
936 be read by `javac`.
937
938 For example:
939
940 - The module `a.b.c` may be represented by the directory `a.b.c`
941
942 - The module's declaration and the module's packages may be in
943 some subdirectory of `a.b.c`, such as `src\main\java` (on Windows)
944 or `src/main/java` (on other systems).
945
946
947 ## Configuring a Compilation
948
949 This section describes how to configure `javac` to perform a basic compilation.
950
951 See [Configuring the Module System] for additional details for use when compiling
952 for a release of the platform that supports modules.
953
954 ### Source Files
955
956 * Specify the source files to be compiled on the command line.
957
958 If there are no compilation errors, the corresponding class files will
959 be placed in the [output directory].
960
961 Some systems may limit the amount you can put on a command line;
962 to work around those limits, you can use [argument files](#command-line-argument-files).
963
964 When compiling code for modules, you can also specify source files indirectly,
965 by using the [`--module`](#option-module) or `-m` option.
966
967 ### Output Directory
968
969 * Use the [`-d`](#option-d) option to specify an output directory in which to put the compiled class files.
970
971 This will normally be organized in a [package hierarchy](#package-hierarchy),
972 unless you are compiling source code from multiple modules, in which case it will be
973 organized as a [module hierarchy](#module-hierarchy).
974
975 When the compilation has been completed, if you are compiling one or more modules,
976 you can place the output directory on the module path for the Java [launcher](java.html);
977 otherwise, you can place the place the output directory on the class path
978 for the Java launcher.
979
980 ### Precompiled Code
981
982 The code to be compiled may refer to libraries beyond what is provided by the platform.
983 If so, you must place these libraries on the class path or module path.
984 If the library code is not in a module, place it on the class path;
985 if it is in a module, place it on the module path.
986
987 * Use the [`--class-path`](#option-class-path) option to specify libraries to be placed on the class path.
988 Locations on the class path should be organized in a [package hierarchy](#package-hierarchy).
989 You can also use alternate forms of the option: `-classpath` or `-cp`.
990
991 * Use the [`--module-path`](#option-module-path) option to specify libraries to be placed on the module path.
992 Locations on the module path should either be modules or directories of modules.
993 You can also use an alternate form of the option: `-p`.
994
995 See [Configuring the Module System] for details on how to modify the default
996 configuration of library modules.
997
998 **Note**: the options for the class path and module path are not mutually
999 exclusive, although it is not common to specify the class path when compiling
1000 code for one or more modules.
1001
1002 ### Additional Source Files
1003
1004 The code to be compiled may refer to types in additional source files that are not
1005 specified on the command line.
1006 If so, you must put those source files on either the source path or module path.
1007 You can only specify one of these options: if you are not compiling code for a module,
1008 or if you are only compiling code for a single module, use the source path;
1009 if you are compiling code for multiple modules, use the module source path.
1010
1011 * Use the [`--source-path`](#option-source-path) option to specify the locations of additional source
1012 files that may be read by javac.
1013 Locations on the source path should be organized in a [package hierarchy](#package-hierarchy).
1014 You can also use an alternate form of the option: `-sourcepath`.
1015
1016 * Use the [`--module-source-path`](#option-module-source-path) option one or more times to specify the location
1017 of additional source files in different modules that may be read by javac,
1018 or when compiling source files in multiple modules.
1019 You can either specify the locations for each module [individually](#module-specific-form),
1020 or you can organize the source files so that you can specify the locations all
1021 [together](#module-pattern-form). For more details, see [The Module Source Path Option].
1022
1023 If you want to be able to refer to types in additional source files but do not
1024 want them to be compiled, use the [`-implicit`](#option-implicit) option.
1025
1026 **Note**: if you are compiling code for multiple modules, you must always specify
1027 a module source path, and all source files specified on the command line must be
1028 in one of the directories on the module source path, or in a subdirectory thereof.
1029
1030
1031 ### Example of Compiling Multiple Source Files
1032
1033 This example compiles the `Aloha.java`, `GutenTag.java`, `Hello.java`, and
1034 `Hi.java` source files in the `greetings` package.
1035
1036 **Linux and macOS:**
1037
1038 ```
1039 % javac greetings/*.java
1040 % ls greetings
1041 Aloha.class GutenTag.class Hello.class Hi.class
1042 Aloha.java GutenTag.java Hello.java Hi.java
1043 ```
1044
1045 **Windows:**
1046
1047 ```
1048 C:\>javac greetings\*.java
1049 C:\>dir greetings
1050 Aloha.class GutenTag.class Hello.class Hi.class
1051 Aloha.java GutenTag.java Hello.java Hi.java
1052 ```
1053
1054 ### Example of Specifying a User Class Path
1055
1056 After changing one of the source files in the previous example, recompile it:
1057
1058 **Linux and macOS:**
1059
1060 ```
1061 pwd
1062 /examples
1063 javac greetings/Hi.java
1064 ```
1065
1066 **Windows:**
1067
1068 ```
1069 C:\>cd
1070 \examples
1071 C:\>javac greetings\Hi.java
1072 ```
1073
1074 Because `greetings.Hi` refers to other classes in the `greetings` package, the
1075 compiler needs to find these other classes. The previous example works because
1076 the default user class path is the directory that contains the package
1077 directory. If you want to recompile this file without concern for which
1078 directory you are in, then add the examples directory to the user class path by
1079 setting `CLASSPATH`. This example uses the `-classpath` option.
1080
1081 **Linux and macOS:**
1082
1083 > `javac -classpath /examples /examples/greetings/Hi.java`
1084
1085 **Windows:**
1086
1087 > `C:\>javac -classpath \examples \examples\greetings\Hi.java`
1088
1089 If you change `greetings.Hi` to use a banner utility, then that utility also
1090 needs to be accessible through the user class path.
1091
1092 **Linux and macOS:**
1093
1094 ```
1095 javac -classpath /examples:/lib/Banners.jar \
1096 /examples/greetings/Hi.java
1097 ```
1098
1099 **Windows:**
1100
1101 ```
1102 C:\>javac -classpath \examples;\lib\Banners.jar ^
1103 \examples\greetings\Hi.java
1104 ```
1105
1106 To execute a class in the `greetings` package, the program needs access to the
1107 `greetings` package, and to the classes that the `greetings` classes use.
1108
1109 **Linux and macOS:**
1110
1111 > `java -classpath /examples:/lib/Banners.jar greetings.Hi`
1112
1113 **Windows:**
1114
1115 > `C:\>java -classpath \examples;\lib\Banners.jar greetings.Hi`
1116
1117 ## Configuring the Module System
1118
1119 If you want to include additional modules in your compilation, use the
1120 [`--add-modules`](#option-add-modules) option.
1121 This may be necessary when you are compiling code that is not in a module,
1122 or which is in an automatic module, and the code refers to API in the additional
1123 modules.
1124
1125 If you want to restrict the set of modules in your compilation, use the
1126 [`--limit-modules`](#option-limit-modules) option.
1127 This may be useful if you want to ensure that the code you are compiling
1128 is capable of running on a system with a limited set of modules installed.
1129
1130 If you want to break encapsulation and specify that additional packages
1131 should be considered as exported from a module, use the [`--add-exports`](#option-add-exports) option.
1132 This may be useful when performing white-box testing; relying on access
1133 to internal API in production code is strongly discouraged.
1134
1135 If you want to specify that additional packages
1136 should be considered as required by a module, use the [`--add-reads`](#option-add-reads) option.
1137 This may be useful when performing white-box testing; relying on access
1138 to internal API in production code is strongly discouraged.
1139
1140 You can patch additional content into any module using the
1141 [`--patch-module`](#option-patch-module) option. See [Patching a Module] for more details.
1142
1143 ## Searching for Module, Package and Type Declarations
1144
1145 To compile a source file, the compiler often needs information about a module
1146 or type, but the declaration is not in the source files specified on the command
1147 line.
1148
1149 `javac` needs type information for every class or interface used,
1150 extended, or implemented in the source file. This includes classes and
1151 interfaces not explicitly mentioned in the source file, but that provide
1152 information through inheritance.
1153
1154 For example, when you create a subclass of `java.awt.Window`, you are also
1155 using the ancestor classes of `Window`: `java.awt.Container`,
1156 `java.awt.Component`, and `java.lang.Object`.
1157
1158 When compiling code for a module, the compiler also needs to have available
1159 the declaration of that module.
1160
1161 A successful search may produce a class file, a source file, or both. If
1162 both are found, then you can use the [`-Xprefer`](#option-Xprefer) option to instruct the compiler
1163 which to use.
1164
1165 If a search finds and uses a source file, then by default `javac`
1166 compiles that source file. This behavior can be altered with
1167 [`-implicit`](#option-implicit).
1168
1169 The compiler might not discover the need for some type information until after
1170 annotation processing completes. When the type information is found in a source
1171 file and no [`-implicit`](#option-implicit) option is specified, the compiler gives a warning that
1172 the file is being compiled without being subject to annotation processing. To
1173 disable the warning, either specify the file on the command line (so that it
1174 will be subject to annotation processing) or use the [`-implicit`](#option-implicit) option to
1175 specify whether or not class files should be generated for such source files.
1176
1177 The way that `javac` locates the declarations of those types
1178 depends on whether the reference exists within code for a module or not.
1179
1180 ### Searching Package Oriented Paths
1181
1182 When searching for a source or class file on a path composed of package oriented
1183 locations, `javac` will check each location on the path in turn for the
1184 possible presence of the file. The first occurrence of a particular file
1185 shadows (hides) any subsequent occurrences of like-named files. This shadowing
1186 does not affect any search for any files with a different name. This can be
1187 convenient when searching for source files, which may be grouped in different
1188 locations, such as shared code, platform-specific code and generated code.
1189 It can also be useful when injecting alternate versions of a class file into
1190 a package, to debugging or other instrumentation reasons. But, it can also
1191 be dangerous, such as when putting incompatible different versions of a library
1192 on the class path.
1193
1194 ### Searching Module Oriented Paths
1195
1196 Prior to scanning any module paths for any package or type declarations,
1197 `javac` will lazily scan the following paths and locations to determine
1198 the modules that will be used in the compilation.
1199
1200 * The module source path (see the [`--module-source-path`](#option-module-source-path) option)
1201 * The path for upgradeable modules (see the [`--upgrade-module-path`](#option-upgrade-module-path) option)
1202 * The system modules (see the [`--system`](#option-system) option)
1203 * The user module path ( see the [`--module-path`](#option-module-path) option)
1204
1205 For any module, the first occurrence of the module during the scan completely
1206 shadows (hides) any subsequent appearance of a like-named module. While locating
1207 the modules, `javac` is able to determine the packages exported by the module
1208 and to associate with each module a package oriented path for the contents of
1209 the module. For any previously compiled module, this path will typically be a
1210 single entry for either a directory or a file that provides an internal
1211 directory-like hierarchy, such as a JAR file. Thus, when searching for a type
1212 that is in a package that is known to be exported by a module, `javac` can
1213 locate the declaration directly and efficiently.
1214
1215 ### Searching for the Declaration of a Module
1216
1217 If the module has been previously compiled, the module declaration is
1218 located in a file named `module-info.class` in the root of the package hierarchy
1219 for the content of the module.
1220
1221 If the module is one of those currently being compiled, the module declaration
1222 will be either the file named `module-info.class` in the root of the
1223 package hierarchy for the module in the class output directory, or the
1224 file named `module-info.java` in one of the locations on the source path
1225 or one the module source path for the module.
1226
1227 ### Searching for the Declaration of a Type When the Reference is not in a Module
1228
1229 When searching for a type that is referenced in code that is not in a module,
1230 `javac` will look in the following places:
1231
1232 * The platform classes (or the types in exported packages of the platform modules)
1233 (This is for compiled class files only.)
1234
1235 * Types in exported packages of any modules on the module path, if applicable.
1236 (This is for compiled class files only.)
1237
1238 * Types in packages on the class path and/or source path:
1239
1240 * If both are specified, `javac` looks for compiled class files on the class path
1241 and for source files on the source path.
1242
1243 * If the class path is specified, but not source path, `javac` looks for both
1244 compiled class files and source files on the class path.
1245
1246 * If the class path is not specified, it defaults to the current directory.
1247
1248 When looking for a type on the class path and/or source path, if both a compiled
1249 class file and a source file are found, the most recently modified file will
1250 be used by default.
1251 If the source file is newer, it will be compiled and will may override any
1252 previously compiled version of the file. You can use the [`-Xprefer`](#option-Xprefer) option
1253 to override the default behavior.
1254
1255 ### Searching for the Declaration of a Type When the Reference is in a Module
1256
1257 When searching for a type that is referenced in code in a module,
1258 `javac` will examine the declaration of the enclosing module to determine
1259 if the type is in a package that is exported from another module that is
1260 readable by the enclosing module.
1261 If so, `javac` will simply and directly go to the definition of that module
1262 to find the definition of the required type.
1263 Unless the module is another of the modules being compiled, `javac` will
1264 only look for compiled class files files. In other words, `javac` will
1265 not look for source files in platform modules or modules on the module path.
1266
1267 If the type being referenced is not in some other readable module,
1268 `javac` will examine the module being compiled to try and find the
1269 declaration of the type.
1270 `javac` will look for the declaration of the type as follows:
1271
1272 * Source files specified on the command line or on the source path or
1273 module source path.
1274
1275 * Previously compiled files in the output directory.
1276
1277
1278 ## Directory Hierarchies
1279
1280 `javac` generally assumes that source files and compiled class files will be
1281 organized in a file system directory hierarchy or in a type of file that
1282 supports in an internal directory hierarchy, such as a JAR file.
1283 Three different kinds of hierarchy are supported: a _package hierarchy_,
1284 a _module hierarchy_, and a _module source hierarchy_.
1285
1286 While `javac` is fairly relaxed about the organization of source code,
1287 beyond the expectation that source will be organized in one or package
1288 hierarchies, and can generally accommodate organizations prescribed by
1289 development environments and build tools, Java tools in general,
1290 and `javac` and the Java launcher in particular, are more stringent
1291 regarding the organization of compiled class files, and will be
1292 organized in package hierarchies or module hierarchies, as appropriate.
1293
1294 The location of these hierarchies are specified to `javac` with command-line
1295 options, whose names typically end in "path", like [`--source-path`](#option-source-path) or
1296 [`--class-path`](#option-class-path). Also as a general rule, path options whose name includes the
1297 word `module`, like [`--module-path`](#option-module-path), are used to specify module hierarchies,
1298 although some module-related path options allow a package hierarchy to be
1299 specified on a per-module basis. All other path options are used to specify
1300 package hierarchies.
1301
1302 ### Package Hierarchy
1303
1304 In a package hierarchy, directories and subdirectories are used
1305 to represent the component parts of the package name, with the source
1306 file or compiled class file for a type being stored as a file with an
1307 extension of `.java` or `.class` in the most nested directory.
1308
1309 For example, in a package hierarchy, the source file for a class
1310 `com.example.MyClass` will be stored in the file _com/example/MyClass.java_
1311
1312 ### Module Hierarchy
1313
1314 In a module hierarchy, the first level of directories are named
1315 for the modules in the hierarchy; within each of those directories
1316 the contents of the module are organized in package hierarchies.
1317
1318 For example, in a module hierarchy, the compiled class file for a type called
1319 `com.example.MyClass` in a module called `my.library` will be stored in
1320 _my.library/com/example/MyClass.class_.
1321
1322 The various output directories used by `javac` (the class output directory,
1323 the source output directory, and native header output directory)
1324 will all be organized in a module hierarchy when multiple modules are being compiled.
1325
1326 ### Module Source Hierarchy
1327
1328 Although the source for each individual module should always be
1329 organized in a package hierarchy, it may be convenient to group
1330 those hierarchies into a module source hierarchy. This is similar
1331 to a module hierarchy, except that there may be intervening directories
1332 between the directory for the module and the directory that is
1333 the root of the package hierarchy for the source code of the module.
1334
1335 For example, in a module source hierarchy, the source file for a type called
1336 `com.example.MyClass` in a module called `my.library` may be stored in a
1337 file such as
1338 _my.library/src/main/java/com/example/MyClass.java_.
1339
1340 ## The Module Source Path Option
1341
1342 The [`--module-source-path`](#option-module-source-path) option has two forms: a _module-specific form_,
1343 in which a package path is given for each module containing code to be compiled,
1344 and a _module-pattern_ form, in which the source path for each module is specified
1345 by a pattern.
1346 The module-specific form is generally simpler to use when only a small number of
1347 modules are involved; the module-pattern form may be more convenient when the
1348 number of modules is large and the modules are organized in a regular manner that
1349 can be described by a pattern.
1350
1351 Multiple instances of the `--module-source-path` option may be given, each one
1352 using either the module-pattern form or the module-specific form, subject to the
1353 following limitations:
1354
1355 * the module-pattern form may be used at most once
1356 * the module-specific form may be used at most once for any given module
1357
1358 If the module-specific form is used for any module, the associated search path
1359 overrides any path that might otherwise have been inferred from the module-pattern form.
1360
1361 ### Module-specific form
1362
1363 The module-specific form allows an explicit search path to be given for any specific module.
1364 This form is:
1365
1366 * `--module-source-path` *module-name*`=`*file-path* (*path-separator* *file-path*)*
1367
1368 The path separator character is `;` on Windows, and `:` otherwise.
1369
1370 **Note:** this is similar to the form used for the [`--patch-module`](#option-patch-module) option.
1371
1372 ### Module-pattern form
1373
1374 The module-pattern form allows a concise specification of the module source path
1375 for any number of modules organized in regular manner.
1376
1377 * `--module-source-path` *pattern*
1378
1379 The pattern is defined by the following rules, which are applied in order:
1380
1381 * The argument is considered to be a series of segments separated by the path
1382 separator character (`;` on Windows, and `:` otherwise).
1383
1384 * Each segment containing curly braces of the form
1385
1386 string1{alt1 ( ,alt2 )* } string2
1387
1388 is considered to be replaced by a series of segments formed by "expanding" the braces:
1389
1390 string1 alt1 string2
1391 string1 alt2 string2
1392 and so on...
1393
1394 The braces may be nested.
1395
1396 This rule is applied for all such usages of braces.
1397
1398 * Each segment must have at most one asterisk (`*`).
1399 If a segment does not contain an asterisk, it is considered to be as though the
1400 file separator character and an asterisk are appended.
1401
1402 For any module _M_, the source path for that module is formed from the series
1403 of segments obtained by substituting the module name _M_ for the asterisk in
1404 each segment.
1405
1406 **Note**: in this context, the asterisk is just used as a special marker, to
1407 denote the position in the path of the module name. It should not be confused
1408 with the use of `*` as a file name wildcard character, as found on most
1409 operating systems.
1410
1411 ## Patching Modules
1412
1413 javac allows any content, whether in source or compiled form, to be patched
1414 into any module using the [`--patch-module`](#option-patch-module) option.
1415 You may want to do this to compile alternative implementations of a class
1416 to be patched at runtime into a JVM, or to inject additional classes into
1417 the module, such as when testing.
1418
1419 The form of the option is:
1420
1421 * `--patch-module` *module-name*`=`*file-path* (*path-separator* *file-path* )*
1422
1423 The path separator character is `;` on Windows, and `:` otherwise.
1424 The paths given for the module must specify the root of a
1425 package hierarchy for the contents of the module
1426
1427 The option may be given at most once for any given module.
1428 Any content on the path will hide any like-named content later in the path
1429 and in the patched module.
1430
1431 When patching source code into more than one module, the [`--module-source-path`](#option-module-source-path)
1432 must also be used, so that the output directory is organized in a module hierarchy,
1433 and capable of holding the compiled class files for the modules being compiled.
1434
1435 ## Annotation Processing
1436
1437 The `javac` command provides direct support for annotation processing.
1438
1439 The API for annotation processors is defined in the
1440 `javax.annotation.processing` and `javax.lang.model` packages and subpackages.
1441
1442 ### How Annotation Processing Works
1443
1444 Annotation processing is requested by using an option to configure
1445 annotation processing, such as [`-processor`](#option-processor),
1446 [`--processor-path`](#option-processor-path),
1447 [`--processor-module-path`](#option-processor-module-path) or by
1448 explicitly enabling processing with the [`-proc:full`](#option-proc)
1449 or [`-proc:only`](#option-proc) options. Annotation processing is
1450 disabled using the [`-proc:none`](#option-proc) option.
1451
1452 If annotation processing is requested, the compiler searches for any
1453 annotation processors that are available.
1454
1455 The search path can be specified with the
1456 [`-processorpath`](#option-processor-path) option. If no path is
1457 specified, then the user class path is used. Processors are located by
1458 means of service provider-configuration files named
1459 `META-INF/services/javax.annotation.processing.Processor` on the
1460 search path. Such files should contain the names of any
1461 annotationation processors to be used, listed one per
1462 line. Alternatively, processors can be specified explicitly, using the
1463 [`-processor`](#option-processor) option.
1464
1465 After scanning the source files and classes on the command line to determine
1466 what annotations are present, the compiler queries the processors to determine
1467 what annotations they process. When a match is found, the processor is called.
1468 A processor can claim the annotations it processes, in which case no further
1469 attempt is made to find any processors for those annotations. After all of the
1470 annotations are claimed, the compiler does not search for additional
1471 processors.
1472
1473 If any processors generate new source files, then another round of annotation
1474 processing occurs: Any newly generated source files are scanned, and the
1475 annotations processed as before. Any processors called on previous rounds are
1476 also called on all subsequent rounds. This continues until no new source files
1477 are generated.
1478
1479 After a round occurs where no new source files are generated, the annotation
1480 processors are called one last time, to give them a chance to complete any
1481 remaining work. Finally, unless the [`-proc:only`](#option-proc) option is used, the compiler
1482 compiles the original and all generated source files.
1483
1484 If you use an annotation processor that generates additional source
1485 files to be included in the compilation, you can specify a default
1486 module to be used for the newly generated files, for use when a
1487 module declaration is not also generated. In this case, use the
1488 [`--default-module-for-created-files`](#option-default-module-for-created-files) option.
1489
1490 ### Compilation Environment and Runtime Environment.
1491
1492 The declarations in source files and previously compiled class files are analyzed
1493 by `javac` in a _compilation environment_ that is distinct from the
1494 _runtime environment_ used to execute `javac` itself. Although there is a
1495 deliberate similarity between many `javac` options and like-named options for the
1496 Java [launcher](java.html), such as `--class-path`, `--module-path` and so
1497 on, it is important to understand that in general the `javac` options just affect
1498 the environment in which the source files are compiled, and do not affect
1499 the operation of `javac` itself.
1500
1501 The distinction between the compilation environment and runtime environment
1502 is significant when it comes to using annotation processors.
1503 Although annotations processors process elements (declarations) that exist
1504 in the compilation environment, the annotation processor itself is executed
1505 in the runtime environment. If an annotation processor has dependencies on
1506 libraries that are not in modules, the libraries can be placed, along with the
1507 annotation processor itself, on the processor path.
1508 (See the [`--processor-path`](#option-processor-path) option.)
1509 If the annotation processor and its dependencies are in modules, you should
1510 use the processor module path instead.
1511 (See the [`--processor-module-path`](#option-processor-module-path) option.)
1512 When those are insufficient, it may be necessary to provide further
1513 configuration of the runtime environment. This can be done in two ways:
1514
1515 1. If `javac` is invoked from the command line, options can be passed to the
1516 underlying runtime by prefixing the option with [`-J`](#option-J).
1517
1518 2. You can start an instance of a Java Virtual Machine directly and use
1519 command line options and API to configure an environment in which
1520 `javac` can be invoked via one of its [APIs].
1521
1522 ## Compiling for Earlier Releases of the Platform
1523
1524 `javac` can compile code that is to be used on other releases of the platform,
1525 using either the [`--release`](#option-release) option, or the [`--source`](#option-source)/`-source` and
1526 [`--target`](#option-target)/`-target` options, together with additional options to specify the
1527 platform classes.
1528
1529 Depending on the desired platform release, there are some restrictions on some
1530 of the options that can be used.
1531
1532 * When compiling for JDK 8 and earlier releases, you cannot use any option
1533 that is intended for use with the module system.
1534 This includes all of the following options:
1535
1536 * [`--module-source-path`](#option-module-source-path),
1537 [`--upgrade-module-path`](#option-upgrade-module-path),
1538 [`--system`](#option-system),
1539 [`--module-path`](#option-module-path),
1540 [`--add-modules`](#option-add-modules),
1541 [`--add-exports`](#option-add-exports),
1542 `--add-opens`,
1543 [`--add-reads`](#option-add-reads),
1544 [`--limit-modules`](#option-limit-modules),
1545 [`--patch-module`](#option-patch-module)
1546
1547 If you use the `--source`/`-source` or `--target`/`-target` options,
1548 you should also set the appropriate platform classes using the
1549 boot class path family of options.
1550
1551 * When compiling for JDK 9 and later releases, you cannot use any option
1552 that is intended to configure the boot class path.
1553 This includes all of the following options:
1554
1555 * [`-Xbootclasspath/p:`](#option-Xbootclasspath-p),
1556 [`-Xbootclasspath`](#option-Xbootclasspath),
1557 [`-Xbootclasspath/a:`](#option-Xbootclasspath-a),
1558 [`-endorseddirs`](#option-endorseddirs),
1559 [`-Djava.endorsed.dirs`](#option-Djava.endorsed.dirs),
1560 [`-extdirs`](#option-extdirs),
1561 [`-Djava.ext.dirs`](#option-Djava.ext.dirs),
1562 [`-profile`](#option-profile)
1563
1564 If you use the `--source`/`-source` or `--target`/`-target` options,
1565 you should also set the appropriate platform classes using the `--system`
1566 option to give the location of an appropriate installed release of JDK.
1567
1568 When using the `--release` option, only the supported documented API for that
1569 release may be used; you cannot use any options to break encapsulation to
1570 access any internal classes.
1571
1572 ## APIs
1573
1574 The `javac` compiler can be invoked using an API in three different ways:
1575
1576 The [Java Compiler API](../../api/java.compiler/javax/tools/JavaCompiler.html)
1577 : This provides the most flexible way to invoke the compiler,
1578 including the ability to compile source files provided in
1579 memory buffers or other non-standard file systems.
1580
1581 The [ToolProvider API](../../api/java.base/java/util/spi/ToolProvider.html)
1582 : A `ToolProvider` for `javac` can be obtained by calling
1583 `ToolProvider.findFirst("javac")`. This returns an object
1584 with the equivalent functionality of the command-line tool.
1585
1586 **Note**: This API should not be confused with the like-named
1587 API in the [`javax.tools`](../../api/java.compiler/javax/tools/ToolProvider.html)
1588 package.
1589
1590 The `javac` [Legacy API](../../api/jdk.compiler/com/sun/tools/javac/Main.html)
1591 : This API is retained for backward compatibility only.
1592 All new code should use either the Java Compiler API or the ToolProvider API.
1593
1594 **Note:** All other classes and methods found in a package with names that start with
1595 `com.sun.tools.javac` (subpackages of `com.sun.tools.javac`) are strictly
1596 internal and subject to change at any time.
1597
1598 ## Examples of Using -Xlint keys
1599
1600 `cast`
1601 : Warns about unnecessary and redundant casts, for example:
1602
1603 > `String s = (String) "Hello!"`
1604
1605 `classfile`
1606 : Warns about issues related to class file contents.
1607
1608 `deprecation`
1609 : Warns about the use of deprecated items. For example:
1610
1611 ```
1612 java.util.Date myDate = new java.util.Date();
1613 int currentDay = myDate.getDay();
1614 ```
1615
1616 The method `java.util.Date.getDay` has been deprecated since JDK 1.1.
1617
1618 `dep-ann`
1619 : Warns about items that are documented with the `@deprecated` Javadoc
1620 comment, but do not have the `@Deprecated` annotation, for example:
1621
1622 ```
1623 /**
1624 * @deprecated As of Java SE 7, replaced by {@link #newMethod()}
1625 */
1626 public static void deprecatedMethod() { }
1627 public static void newMethod() { }
1628 ```
1629
1630 `divzero`
1631 : Warns about division by the constant integer 0, for example:
1632
1633 > `int divideByZero = 42 / 0;`
1634
1635 `empty`
1636 : Warns about empty statements after `if`statements, for example:
1637
1638 ```
1639 class E {
1640 void m() {
1641 if (true) ;
1642 }
1643 }
1644 ```
1645
1646 `fallthrough`
1647 : Checks the switch blocks for fall-through cases and provides a warning
1648 message for any that are found. Fall-through cases are cases in a switch
1649 block, other than the last case in the block, whose code does not include a
1650 `break` statement, allowing code execution to fall through from that case to
1651 the next case. For example, the code following the case 1 label in this
1652 switch block does not end with a `break` statement:
1653
1654 ```
1655 switch (x) {
1656 case 1:
1657 System.out.println("1");
1658 // No break statement here.
1659 case 2:
1660 System.out.println("2");
1661 }
1662 ```
1663
1664 If the `-Xlint:fallthrough` option was used when compiling this code, then
1665 the compiler emits a warning about possible fall-through into case, with
1666 the line number of the case in question.
1667
1668 `finally`
1669 : Warns about `finally` clauses that cannot be completed normally, for
1670 example:
1671
1672 ```
1673 public static int m() {
1674 try {
1675 throw new NullPointerException();
1676 } catch (NullPointerException e) {
1677 System.err.println("Caught NullPointerException.");
1678 return 1;
1679 } finally {
1680 return 0;
1681 }
1682 }
1683 ```
1684
1685 The compiler generates a warning for the `finally` block in this example.
1686 When the `int` method is called, it returns a value of 0. A `finally` block
1687 executes when the `try` block exits. In this example, when control is
1688 transferred to the `catch` block, the `int` method exits. However, the
1689 `finally` block must execute, so it's executed, even though control was
1690 transferred outside the method.
1691
1692 `options`
1693 : Warns about issues that related to the use of command-line options. See
1694 [Compiling for Earlier Releases of the Platform].
1695
1696 `overrides`
1697 : Warns about issues related to method overrides. For example, consider the
1698 following two classes:
1699
1700 ```
1701 public class ClassWithVarargsMethod {
1702 void varargsMethod(String... s) { }
1703 }
1704
1705 public class ClassWithOverridingMethod extends ClassWithVarargsMethod {
1706 @Override
1707 void varargsMethod(String[] s) { }
1708 }
1709 ```
1710
1711 The compiler generates a warning similar to the following:.
1712
1713 ```
1714 warning: [override] varargsMethod(String[]) in ClassWithOverridingMethod
1715 overrides varargsMethod(String...) in ClassWithVarargsMethod; overriding
1716 method is missing '...'
1717 ```
1718
1719 When the compiler encounters a `varargs` method, it translates the
1720 `varargs` formal parameter into an array. In the method
1721 `ClassWithVarargsMethod.varargsMethod`, the compiler translates the
1722 `varargs` formal parameter `String... s` to the formal parameter
1723 `String[] s`, an array that matches the formal parameter of the method
1724 `ClassWithOverridingMethod.varargsMethod`. Consequently, this example
1725 compiles.
1726
1727 `path`
1728 : Warns about invalid path elements and nonexistent path directories on the
1729 command line (with regard to the class path, the source path, and other
1730 paths). Such warnings cannot be suppressed with the `@SuppressWarnings`
1731 annotation. For example:
1732
1733 - **Linux and macOS:**
1734 `javac -Xlint:path -classpath /nonexistentpath Example.java`
1735
1736 - **Windows:**
1737 `javac -Xlint:path -classpath C:\nonexistentpath Example.java`
1738
1739 `processing`
1740 : Warns about issues related to annotation processing. The compiler generates
1741 this warning when you have a class that has an annotation, and you use an
1742 annotation processor that cannot handle that type of annotation. For example,
1743 the following is a simple annotation processor:
1744
1745 **Source file AnnoProc.java**:
1746
1747 ```
1748 import java.util.*;
1749 import javax.annotation.processing.*;
1750 import javax.lang.model.*;
1751 import javax.lang.model.element.*;
1752
1753 @SupportedAnnotationTypes("NotAnno")
1754 public class AnnoProc extends AbstractProcessor {
1755 public boolean process(Set<? extends TypeElement> elems, RoundEnvironment renv){
1756 return true;
1757 }
1758
1759 public SourceVersion getSupportedSourceVersion() {
1760 return SourceVersion.latest();
1761 }
1762 }
1763 ```
1764
1765 **Source file AnnosWithoutProcessors.java**:
1766
1767 ```
1768 @interface Anno { }
1769
1770 @Anno
1771 class AnnosWithoutProcessors { }
1772 ```
1773
1774 The following commands compile the annotation processor `AnnoProc`, then
1775 run this annotation processor against the source file
1776 `AnnosWithoutProcessors.java`:
1777
1778 ```
1779 javac AnnoProc.java
1780 javac -cp . -Xlint:processing -processor AnnoProc -proc:only AnnosWithoutProcessors.java
1781 ```
1782
1783 When the compiler runs the annotation processor against the source file
1784 `AnnosWithoutProcessors.java`, it generates the following warning:
1785
1786 ```
1787 warning: [processing] No processor claimed any of these annotations: Anno
1788 ```
1789
1790 To resolve this issue, you can rename the annotation defined and used in
1791 the class `AnnosWithoutProcessors` from `Anno` to `NotAnno`.
1792
1793 `rawtypes`
1794 : Warns about unchecked operations on raw types. The following statement
1795 generates a `rawtypes` warning:
1796
1797 > `void countElements(List l) { ... }`
1798
1799 The following example does not generate a `rawtypes` warning:
1800
1801 > `void countElements(List<?> l) { ... }`
1802
1803 `List` is a raw type. However, `List<?>` is an unbounded wildcard
1804 parameterized type. Because `List` is a parameterized interface, always
1805 specify its type argument. In this example, the `List` formal argument is
1806 specified with an unbounded wildcard (`?`) as its formal type parameter,
1807 which means that the `countElements` method can accept any instantiation of
1808 the `List` interface.
1809
1810 `serial`
1811 : Warns about missing `serialVersionUID` definitions on serializable classes.
1812 For example:
1813
1814 ```
1815 public class PersistentTime implements Serializable
1816 {
1817 private Date time;
1818
1819 public PersistentTime() {
1820 time = Calendar.getInstance().getTime();
1821 }
1822
1823 public Date getTime() {
1824 return time;
1825 }
1826 }
1827 ```
1828
1829 The compiler generates the following warning:
1830
1831 ```
1832 warning: [serial] serializable class PersistentTime has no definition of
1833 serialVersionUID
1834 ```
1835
1836 If a serializable class does not explicitly declare a field named
1837 `serialVersionUID`, then the serialization runtime environment calculates a
1838 default `serialVersionUID` value for that class based on various aspects of
1839 the class, as described in the Java Object Serialization Specification.
1840 However, it's strongly recommended that all serializable classes explicitly
1841 declare `serialVersionUID` values because the default process of computing
1842 `serialVersionUID` values is highly sensitive to class details that can
1843 vary depending on compiler implementations. As a result, this might cause
1844 an unexpected `InvalidClassExceptions` during deserialization. To guarantee
1845 a consistent `serialVersionUID` value across different Java compiler
1846 implementations, a serializable class must declare an explicit
1847 `serialVersionUID` value.
1848
1849 `static`
1850 : Warns about issues relating to the use of static variables, for example:
1851
1852 ```
1853 class XLintStatic {
1854 static void m1() { }
1855 void m2() { this.m1(); }
1856 }
1857 ```
1858
1859 The compiler generates the following warning:
1860
1861 ```
1862 warning: [static] static method should be qualified by type name,
1863 XLintStatic, instead of by an expression
1864 ```
1865
1866 To resolve this issue, you can call the `static` method `m1` as follows:
1867
1868 > `XLintStatic.m1();`
1869
1870 Alternately, you can remove the `static` keyword from the declaration of
1871 the method `m1`.
1872
1873 `this-escape`
1874 : Warns about constructors leaking
1875 `this` prior to subclass initialization.
1876 For example, this class:
1877
1878 ```
1879 public class MyClass {
1880 public MyClass() {
1881 System.out.println(this.hashCode());
1882 }
1883 }
1884 ```
1885
1886 generates the following warning:
1887
1888 ```
1889 MyClass.java:3: warning: [this-escape] possible 'this' escape
1890 before subclass is fully initialized
1891 System.out.println(this.hashCode());
1892 ^
1893 ```
1894
1895 A 'this' escape warning is generated when a constructor does something
1896 that might result in a subclass method being invoked before the
1897 constructor returns.
1898 In such cases the subclass method would be operating on an incompletely
1899 initialized instance.
1900 In the above example, a subclass of `MyClass` that overrides
1901 `hashCode()` to incorporate its own fields would likely produce
1902 an incorrect result when invoked as shown.
1903
1904 Warnings are only generated if a subclass could exist that is outside
1905 of the current module (or package, if no module) being compiled.
1906 So, for example, constructors in final and non-public classes do not
1907 generate warnings.
1908
1909 `try`
1910 : Warns about issues relating to the use of `try` blocks, including
1911 try-with-resources statements. For example, a warning is generated for the
1912 following statement because the resource `ac` declared in the `try` block
1913 is not used:
1914
1915 ```
1916 try ( AutoCloseable ac = getResource() ) { // do nothing}
1917 ```
1918
1919 `unchecked`
1920 : Gives more detail for unchecked conversion warnings that are mandated by
1921 the Java Language Specification, for example:
1922
1923 ```
1924 List l = new ArrayList<Number>();
1925 List<String> ls = l; // unchecked warning
1926 ```
1927
1928 During type erasure, the types `ArrayList<Number>` and `List<String>`
1929 become `ArrayList` and `List`, respectively.
1930
1931 The `ls` command has the parameterized type `List<String>`. When the `List`
1932 referenced by `l` is assigned to `ls`, the compiler generates an unchecked
1933 warning. At compile time, the compiler and JVM cannot determine whether `l`
1934 refers to a `List<String>` type. In this case, `l` does not refer to a
1935 `List<String>` type. As a result, heap pollution occurs.
1936
1937 A heap pollution situation occurs when the `List` object `l`, whose static
1938 type is `List<Number>`, is assigned to another `List` object, `ls`, that
1939 has a different static type, `List<String>`. However, the compiler still
1940 allows this assignment. It must allow this assignment to preserve backward
1941 compatibility with releases of Java SE that do not support generics. Because
1942 of type erasure, `List<Number>` and `List<String>` both become `List`.
1943 Consequently, the compiler allows the assignment of the object `l`, which
1944 has a raw type of `List`, to the object `ls`.
1945
1946 `varargs`
1947 : Warns about unsafe use of variable arguments (`varargs`) methods, in
1948 particular, those that contain non-reifiable arguments, for example:
1949
1950 ```
1951 public class ArrayBuilder {
1952 public static <T> void addToList (List<T> listArg, T... elements) {
1953 for (T x : elements) {
1954 listArg.add(x);
1955 }
1956 }
1957 }
1958 ```
1959
1960 A non-reifiable type is a type whose type information is not fully available
1961 at runtime.
1962
1963 The compiler generates the following warning for the definition of the
1964 method `ArrayBuilder.addToList`:
1965
1966 ```
1967 warning: [varargs] Possible heap pollution from parameterized vararg type T
1968 ```
1969
1970 When the compiler encounters a varargs method, it translates the `varargs`
1971 formal parameter into an array. However, the Java programming language
1972 does not permit the creation of arrays of parameterized types. In the method
1973 `ArrayBuilder.addToList`, the compiler translates the `varargs` formal
1974 parameter `T...` elements to the formal parameter `T[]` elements, an array.
1975 However, because of type erasure, the compiler converts the `varargs`
1976 formal parameter to `Object[]` elements. Consequently, there's a
1977 possibility of heap pollution.