1 /*
2 * Copyright (c) 2024, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
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23 */
24
25 #include "cds/aotClassInitializer.hpp"
26 #include "cds/archiveBuilder.hpp"
27 #include "cds/cdsConfig.hpp"
28 #include "dumpTimeClassInfo.inline.hpp"
29 #include "cds/heapShared.hpp"
30 #include "classfile/symbolTable.hpp"
31 #include "classfile/systemDictionaryShared.hpp"
32 #include "classfile/vmSymbols.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "oops/fieldStreams.inline.hpp"
35 #include "oops/instanceKlass.inline.hpp"
36 #include "oops/symbol.hpp"
37 #include "runtime/fieldDescriptor.inline.hpp"
38 #include "runtime/java.hpp"
39 #include "runtime/javaCalls.hpp"
40 #include "runtime/mutexLocker.hpp"
41
42 DEBUG_ONLY(InstanceKlass* _aot_init_class = nullptr;)
43
44 // Detector for class names we wish to handle specially.
45 // It is either an exact string match or a string prefix match.
46 class AOTClassInitializer::AllowedSpec {
47 const char* _class_name;
48 bool _is_prefix;
49 int _len;
50 public:
51 AllowedSpec(const char* class_name, bool is_prefix = false)
52 : _class_name(class_name), _is_prefix(is_prefix)
53 {
54 _len = (class_name == nullptr) ? 0 : (int)strlen(class_name);
55 }
56 const char* class_name() { return _class_name; }
57
58 bool matches(Symbol* name, int len) {
59 assert(_class_name != nullptr, "caller resp.");
60 if (_is_prefix) {
61 return len >= _len && name->starts_with(_class_name);
62 } else {
63 return len == _len && name->equals(_class_name);
64 }
65 }
66 };
67
68
69 // Tell if ik has a name that matches one of the given specs.
70 bool AOTClassInitializer::is_allowed(AllowedSpec* specs, InstanceKlass* ik) {
71 Symbol* name = ik->name();
72 int len = name->utf8_length();
73 for (AllowedSpec* s = specs; s->class_name() != nullptr; s++) {
74 if (s->matches(name, len)) {
75 // If a type is included in the tables inside can_archive_initialized_mirror(), we require that
76 // - all super classes must be included
77 // - all super interfaces that have <clinit> must be included.
78 // This ensures that in the production run, we don't run the <clinit> of a supertype but skips
79 // ik's <clinit>.
80 if (ik->java_super() != nullptr) {
81 DEBUG_ONLY(ResourceMark rm);
82 assert(AOTClassInitializer::can_archive_initialized_mirror(ik->java_super()),
83 "super class %s of %s must be aot-initialized", ik->java_super()->external_name(),
84 ik->external_name());
85 }
86
87 Array<InstanceKlass*>* interfaces = ik->local_interfaces();
88 int len = interfaces->length();
89 for (int i = 0; i < len; i++) {
90 InstanceKlass* intf = interfaces->at(i);
91 if (intf->class_initializer() != nullptr) {
92 assert(AOTClassInitializer::can_archive_initialized_mirror(intf),
93 "super interface %s (which has <clinit>) of %s must be aot-initialized", intf->external_name(),
94 ik->external_name());
95 }
96 }
97
98 return true;
99 }
100 }
101 return false;
102 }
103
104
105 bool AOTClassInitializer::can_archive_initialized_mirror(InstanceKlass* ik) {
106 assert(!ArchiveBuilder::is_active() || !ArchiveBuilder::current()->is_in_buffer_space(ik), "must be source klass");
107 if (!CDSConfig::is_initing_classes_at_dump_time()) {
108 return false;
109 }
110
111 if (!ik->is_initialized() && !ik->is_being_initialized()) {
112 return false;
113 }
114
115 // About "static field that may hold a different value" errors:
116 //
117 // Automatic selection for aot-inited classes
118 // ==========================================
119 //
120 // When CDSConfig::is_initing_classes_at_dump_time is enabled,
121 // AOTArtifactFinder::find_artifacts() finds the classes of all
122 // heap objects that are reachable from HeapShared::_run_time_special_subgraph,
123 // and mark these classes as aot-inited. This preserves the initialized
124 // mirrors of these classes, and their <clinit> methods are NOT executed
125 // at runtime. See aotArtifactFinder.hpp for more info.
126 //
127 // For example, with -XX:+AOTInvokeDynamicLinking, _run_time_special_subgraph
128 // will contain some DirectMethodHandle objects. As a result, the DirectMethodHandle
129 // class is automatically marked as aot-inited.
130 //
131 // When a class is aot-inited, its static fields are already set up
132 // by executing the <clinit> method at AOT assembly time. Later on
133 // in the production run, when the class would normally be
134 // initialized, the VM performs guarding and synchronization as if
135 // it were going to run the <clinit> again, but instead it simply
136 // observes that that class was aot-inited. The VM assumes that, if
137 // it were to run <clinit> again, it would get a semantically
138 // equivalent set of final field values, so it just adopts the
139 // existing field values (from AOT assembly) and skips the call to
140 // <clinit>. There may at that point be fixups performed by ad hoc
141 // code, if the VM recognizes a request in the library.
142 //
143 // It is true that this is not generally correct for all possible
144 // Java code. A <clinit> method might have a side effect beyond
145 // initializing the static fields. It might send an email somewhere
146 // noting the current time of day. In that case, such an email
147 // would have been sent during the AOT assembly phase, and the email
148 // would NOT be sent again during production. This is clearly NOT
149 // what a user would want, if this were a general purpose facility.
150 // But in fact it is only for certain well-behaved classes, which
151 // are known NOT to have such side effects. We know this because
152 // the optimization (of skipping <clinit> for aot-init classes) is
153 // only applied to classes fully defined by the JDK.
154 //
155 // (A day may come when we figure out how to gracefully extend this
156 // optimization to untrusted third parties, but it is not this day.)
157 //
158 // Manual selection
159 // ================
160 //
161 // There are important cases where one aot-init class has a side
162 // effect on another aot-class, a side effect which is not captured
163 // in any static field value in either class. The simplest example
164 // is class A forces the initialization of class B. In that case,
165 // we need to aot-init either both classes or neither. From looking
166 // at the JDK state after AOT assembly is done, it is hard to tell
167 // that A "touched" B and B might escape our notice. Another common
168 // example is A copying a field value from B. We don't know where A
169 // got the value, but it would be wrong to re-initialize B at
170 // startup, while keeping the snapshot of the old B value in A. In
171 // general, if we aot-init A, we need to aot-init every class B that
172 // somehow contributed to A's initial state, and every class C that
173 // was somehow side-effected by A's initialization. We say that the
174 // aot-init of A is "init-coupled" to those of B and C.
175 //
176 // So there are init-coupled classes that cannot be automatically discovered. For
177 // example, DirectMethodHandle::IMPL_NAMES points to MethodHandles::IMPL_NAMES,
178 // but the MethodHandles class is not automatically marked because there are
179 // no archived instances of the MethodHandles type.
180 //
181 // If we aot-initialize DirectMethodHandle, but allow MethodHandles to be
182 // initialized at runtime, MethodHandles::IMPL_NAMES will get a different
183 // value than DirectMethodHandle::IMPL_NAMES. This *may or may not* be a problem,
184 // but to ensure compatibility, we should try to preserve the identity equality
185 // of these two fields.
186 //
187 // To do that, we add MethodHandles to the indy_specs[] table below.
188 //
189 // Luckily we do not need to be all-knowing in order to choose which
190 // items to add to that table. We have tools to help detect couplings.
191 //
192 // Automatic validation
193 // ====================
194 //
195 // CDSHeapVerifier is used to detect potential problems with identity equality.
196 //
197 // A class B is assumed to be init-coupled to some aot-init class if
198 // B has a field which points to a live object X in the AOT heap.
199 // The live object X was created by some other class A which somehow
200 // used B's reference to X, perhaps with the help of an intermediate
201 // class Z. Or, B pulled the reference to X from some other class
202 // Y, and B obtained that reference from Y (or an intermediate Z).
203 // It is not certain how X got into the heap, nor whether B
204 // contributed it, but it is a good heuristic that B is init-coupled
205 // to X's class or some other aot-init class. In any case, B should
206 // be made an aot-init class as well, unless a manual inspection
207 // shows that would be a problem. If there is a problem, then the
208 // JDK code for B and/or X probably needs refactoring. If there is
209 // no problem, we add B to the list. Typically the same scan will
210 // find any other accomplices Y, Z, etc. One failure would be a
211 // class Q whose only initialization action is to scribble a special
212 // value into B, from which the value X is derived and then makes
213 // its way into the heap. In that case, the heuristic does not
214 // identify Q. It is (currently) a human responsibility, of JDK
215 // engineers, not to write such dirty JDK code, or to repair it if
216 // it crops up. Eventually we may have tools, or even a user mode
217 // with design rules and checks, that will vet our code base more
218 // automatically.
219 //
220 // To see how the tool detects the problem with MethodHandles::IMPL_NAMES:
221 //
222 // - Comment out all the lines in indy_specs[] except the {nullptr} line.
223 // - Rebuild the JDK
224 //
225 // Then run the following:
226 // java -XX:AOTMode=record -XX:AOTConfiguration=jc.aotconfig com.sun.tools.javac.Main
227 // java -XX:AOTMode=create -Xlog:cds -XX:AOTCache=jc.aot -XX:AOTConfiguration=jc.aotconfig
228 //
229 // You will see an error like this:
230 //
231 // Archive heap points to a static field that may hold a different value at runtime:
232 // Field: java/lang/invoke/MethodHandles::IMPL_NAMES
233 // Value: java.lang.invoke.MemberName$Factory
234 // {0x000000060e906ae8} - klass: 'java/lang/invoke/MemberName$Factory' - flags:
235 //
236 // - ---- fields (total size 2 words):
237 // --- trace begin ---
238 // [ 0] {0x000000060e8deeb0} java.lang.Class (java.lang.invoke.DirectMethodHandle::IMPL_NAMES)
239 // [ 1] {0x000000060e906ae8} java.lang.invoke.MemberName$Factory
240 // --- trace end ---
241 //
242 // Trouble-shooting
243 // ================
244 //
245 // If you see a "static field that may hold a different value" error, it's probably
246 // because you've made some changes in the JDK core libraries (most likely
247 // java.lang.invoke).
248 //
249 // - Did you add a new static field to a class that could be referenced by
250 // cached object instances of MethodType, MethodHandle, etc? You may need
251 // to add that class to indy_specs[].
252 // - Did you modify the <clinit> of the classes in java.lang.invoke such that
253 // a static field now points to an object that should not be cached (e.g.,
254 // a native resource such as a file descriptior, or a Thread)?
255 //
256 // Note that these potential problems only occur when one class gets
257 // the aot-init treatment, AND another class is init-coupled to it,
258 // AND the coupling is not detected. Currently there are a number
259 // classes that get the aot-init treatment, in java.lang.invoke
260 // because of invokedynamic. They are few enough for now to be
261 // manually tracked. There may be more in the future.
262
263 // IS_PREFIX means that we match all class names that start with a
264 // prefix. Otherwise, it is an exact match, of just one class name.
265 const bool IS_PREFIX = true;
266
267 {
268 static AllowedSpec specs[] = {
269 // everybody's favorite super
270 {"java/lang/Object"},
271
272 {nullptr}
273 };
274 if (is_allowed(specs, ik)) {
275 return true;
276 }
277 }
278
279 if (CDSConfig::is_dumping_method_handles()) {
280 // This table was created with the help of CDSHeapVerifier.
281 // Also, some $Holder classes are needed. E.g., Invokers.<clinit> explicitly
282 // initializes Invokers$Holder. Since Invokers.<clinit> won't be executed
283 // at runtime, we need to make sure Invokers$Holder is also aot-inited.
284 //
285 // We hope we can reduce the size of this list over time, and move
286 // the responsibility for identifying such classes into the JDK
287 // code itself. See tracking RFE JDK-8342481.
288 static AllowedSpec indy_specs[] = {
289 {"java/lang/constant/ConstantDescs"},
290 {"java/lang/constant/DynamicConstantDesc"},
291 {"java/lang/invoke/BoundMethodHandle"},
292 {"java/lang/invoke/BoundMethodHandle$Specializer"},
293 {"java/lang/invoke/BoundMethodHandle$Species_", IS_PREFIX},
294 {"java/lang/invoke/ClassSpecializer"},
295 {"java/lang/invoke/ClassSpecializer$", IS_PREFIX},
296 {"java/lang/invoke/DelegatingMethodHandle"},
297 {"java/lang/invoke/DelegatingMethodHandle$Holder"}, // UNSAFE.ensureClassInitialized()
298 {"java/lang/invoke/DirectMethodHandle"},
299 {"java/lang/invoke/DirectMethodHandle$Constructor"},
300 {"java/lang/invoke/DirectMethodHandle$Holder"}, // UNSAFE.ensureClassInitialized()
301 {"java/lang/invoke/Invokers"},
302 {"java/lang/invoke/Invokers$Holder"}, // UNSAFE.ensureClassInitialized()
303 {"java/lang/invoke/LambdaForm"},
304 {"java/lang/invoke/LambdaForm$Holder"}, // UNSAFE.ensureClassInitialized()
305 {"java/lang/invoke/LambdaForm$NamedFunction"},
306 {"java/lang/invoke/LambdaMetafactory"},
307 {"java/lang/invoke/MethodHandle"},
308 {"java/lang/invoke/MethodHandles"},
309 {"java/lang/invoke/SimpleMethodHandle"},
310 {"java/lang/invoke/StringConcatFactory"},
311 {"java/lang/invoke/VarHandleGuards"},
312 {"java/util/Collections"},
313 {"java/util/stream/Collectors"},
314 {"jdk/internal/constant/ConstantUtils"},
315 {"jdk/internal/constant/PrimitiveClassDescImpl"},
316 {"jdk/internal/constant/ReferenceClassDescImpl"},
317
318 // Can't include this, as it will pull in MethodHandleStatics which has many environment
319 // dependencies (on system properties, etc).
320 // MethodHandleStatics is an example of a class that must NOT get the aot-init treatment,
321 // because of its strong reliance on (a) final fields which are (b) environmentally determined.
322 //{"java/lang/invoke/InvokerBytecodeGenerator"},
323
324 {nullptr}
325 };
326 if (is_allowed(indy_specs, ik)) {
327 return true;
328 }
329 }
330
331 #ifdef ASSERT
332 if (ik == _aot_init_class) {
333 return true;
334 }
335 #endif
336
337 return false;
338 }
339
340 // TODO: currently we have a hard-coded list. We should turn this into
341 // an annotation: @jdk.internal.vm.annotation.RuntimeSetupRequired
342 // See JDK-8342481.
343 bool AOTClassInitializer::is_runtime_setup_required(InstanceKlass* ik) {
344 return ik == vmClasses::Class_klass() ||
345 ik == vmClasses::internal_Unsafe_klass() ||
346 ik == vmClasses::ConcurrentHashMap_klass() ||
347 ik == vmClasses::Reference_klass() ||
348 ik->name()->equals("java/net/URI") ||
349 ik->name()->equals("java/lang/module/ModuleDescriptor");
350 }
351
352 void AOTClassInitializer::call_runtime_setup(JavaThread* current, InstanceKlass* ik) {
353 assert(ik->has_aot_initialized_mirror(), "sanity");
354 if (ik->is_runtime_setup_required()) {
355 if (log_is_enabled(Info, cds, init)) {
356 ResourceMark rm;
357 log_info(cds, init)("Calling %s::runtimeSetup()", ik->external_name());
358 }
359 JavaValue result(T_VOID);
360 JavaCalls::call_static(&result, ik,
361 vmSymbols::runtimeSetup(),
362 vmSymbols::void_method_signature(), current);
363 if (current->has_pending_exception()) {
364 // We cannot continue, as we might have cached instances of ik in the heap, but propagating the
365 // exception would cause ik to be in an error state.
366 AOTLinkedClassBulkLoader::exit_on_exception(current);
367 }
368 }
369 }
370
371 // check_can_be_preinited() is quite costly, so we cache the results inside
372 // DumpTimeClassInfo::_can_be_preinited. See also AOTClassInitializer::reset_preinit_check().
373 bool AOTClassInitializer::check_can_be_preinited(InstanceKlass* ik) {
374 ResourceMark rm;
375
376 if (!SystemDictionaryShared::is_builtin(ik)) {
377 log_info(cds, init)("cannot initialize %s (not built-in loader)", ik->external_name());
378 return false;
379 }
380
381 InstanceKlass* super = ik->java_super();
382 if (super != nullptr && !can_be_preinited_locked(super)) {
383 log_info(cds, init)("cannot initialize %s (super %s not initable)", ik->external_name(), super->external_name());
384 return false;
385 }
386
387 Array<InstanceKlass*>* interfaces = ik->local_interfaces();
388 for (int i = 0; i < interfaces->length(); i++) {
389 if (!can_be_preinited_locked(interfaces->at(i))) {
390 log_info(cds, init)("cannot initialize %s (interface %s not initable)",
391 ik->external_name(), interfaces->at(i)->external_name());
392 return false;
393 }
394 }
395
396 if (HeapShared::is_lambda_form_klass(ik)) {
397 // We allow only these to have <clinit> or non-default static fields
398 return true;
399 }
400
401 if (ik->class_initializer() != nullptr) {
402 log_info(cds, init)("cannot initialize %s (has <clinit>)", ik->external_name());
403 return false;
404 }
405 if (ik->is_initialized() && !has_default_static_fields(ik)) {
406 return false;
407 }
408
409 return true;
410 }
411
412 bool AOTClassInitializer::has_default_static_fields(InstanceKlass* ik) {
413 oop mirror = ik->java_mirror();
414
415 for (JavaFieldStream fs(ik); !fs.done(); fs.next()) {
416 if (fs.access_flags().is_static()) {
417 fieldDescriptor& fd = fs.field_descriptor();
418 int offset = fd.offset();
419 bool is_default = true;
420 bool has_initval = fd.has_initial_value();
421 switch (fd.field_type()) {
422 case T_OBJECT:
423 case T_ARRAY:
424 is_default = mirror->obj_field(offset) == nullptr;
425 break;
426 case T_BOOLEAN:
427 is_default = mirror->bool_field(offset) == (has_initval ? fd.int_initial_value() : 0);
428 break;
429 case T_BYTE:
430 is_default = mirror->byte_field(offset) == (has_initval ? fd.int_initial_value() : 0);
431 break;
432 case T_SHORT:
433 is_default = mirror->short_field(offset) == (has_initval ? fd.int_initial_value() : 0);
434 break;
435 case T_CHAR:
436 is_default = mirror->char_field(offset) == (has_initval ? fd.int_initial_value() : 0);
437 break;
438 case T_INT:
439 is_default = mirror->int_field(offset) == (has_initval ? fd.int_initial_value() : 0);
440 break;
441 case T_LONG:
442 is_default = mirror->long_field(offset) == (has_initval ? fd.long_initial_value() : 0);
443 break;
444 case T_FLOAT:
445 is_default = mirror->float_field(offset) == (has_initval ? fd.float_initial_value() : 0);
446 break;
447 case T_DOUBLE:
448 is_default = mirror->double_field(offset) == (has_initval ? fd.double_initial_value() : 0);
449 break;
450 default:
451 ShouldNotReachHere();
452 }
453
454 if (!is_default) {
455 log_info(cds, init)("cannot initialize %s (static field %s has non-default value)",
456 ik->external_name(), fd.name()->as_C_string());
457 return false;
458 }
459 }
460 }
461
462 return true;
463 }
464
465 bool AOTClassInitializer::can_be_preinited(InstanceKlass* ik) {
466 MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
467 return can_be_preinited_locked(ik);
468 }
469
470 bool AOTClassInitializer::can_be_preinited_locked(InstanceKlass* ik) {
471 if (!CDSConfig::is_initing_classes_at_dump_time()) {
472 return false;
473 }
474
475 assert_lock_strong(DumpTimeTable_lock);
476 DumpTimeClassInfo* info = SystemDictionaryShared::get_info_locked(ik);
477 if (!info->has_done_preinit_check()) {
478 info->set_can_be_preinited(AOTClassInitializer::check_can_be_preinited(ik));
479 }
480 return info->can_be_preinited();
481 }
482
483 // Initialize a class at dump time, if possible.
484 void AOTClassInitializer::maybe_preinit_class(InstanceKlass* ik, TRAPS) {
485 #if 0 // FIXME -- leyden+JEP483 merge
486 if (!ik->is_initialized() && AOTClassInitializer::can_be_preinited(ik)) {
487 if (log_is_enabled(Info, cds, init)) {
488 ResourceMark rm;
489 log_info(cds, init)("preinitializing %s", ik->external_name());
490 }
491 ik->initialize(CHECK);
492 }
493 #endif
494 }
495
496 #ifdef ASSERT
497 void AOTClassInitializer::init_test_class(TRAPS) {
498 // -XX:AOTInitTestClass is used in regression tests for adding additional AOT-initialized classes
499 // and heap objects into the AOT cache. The tests must be carefully written to avoid including
500 // any classes that cannot be AOT-initialized.
501 //
502 // -XX:AOTInitTestClass is NOT a general mechanism for including user-defined objects into
503 // the AOT cache. Therefore, this option is NOT available in product JVM.
504 if (AOTInitTestClass != nullptr && CDSConfig::is_initing_classes_at_dump_time()) {
505 log_info(cds)("Debug build only: force initialization of AOTInitTestClass %s", AOTInitTestClass);
506 TempNewSymbol class_name = SymbolTable::new_symbol(AOTInitTestClass);
507 Handle app_loader(THREAD, SystemDictionary::java_system_loader());
508 Klass* k = SystemDictionary::resolve_or_null(class_name, app_loader, CHECK);
509 if (k == nullptr) {
510 vm_exit_during_initialization("AOTInitTestClass not found", AOTInitTestClass);
511 }
512 if (!k->is_instance_klass()) {
513 vm_exit_during_initialization("Invalid name for AOTInitTestClass", AOTInitTestClass);
514 }
515
516 _aot_init_class = InstanceKlass::cast(k);
517 _aot_init_class->initialize(CHECK);
518 }
519 }
520
521 bool AOTClassInitializer::has_test_class() {
522 return _aot_init_class != nullptr;
523 }
524 #endif