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,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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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/systemDictionaryShared.hpp"
31 #include "classfile/vmSymbols.hpp"
32 #include "memory/resourceArea.hpp"
33 #include "oops/fieldStreams.inline.hpp"
34 #include "oops/instanceKlass.inline.hpp"
35 #include "oops/symbol.hpp"
36 #include "runtime/fieldDescriptor.inline.hpp"
37 #include "runtime/javaCalls.hpp"
38 #include "runtime/mutexLocker.hpp"
39
40 // Detector for class names we wish to handle specially.
41 // It is either an exact string match or a string prefix match.
42 class AOTClassInitializer::AllowedSpec {
43 const char* _class_name;
44 bool _is_prefix;
45 int _len;
46 public:
47 AllowedSpec(const char* class_name, bool is_prefix = false)
48 : _class_name(class_name), _is_prefix(is_prefix)
49 {
50 _len = (class_name == nullptr) ? 0 : (int)strlen(class_name);
51 }
52 const char* class_name() { return _class_name; }
53
54 bool matches(Symbol* name, int len) {
55 assert(_class_name != nullptr, "caller resp.");
56 if (_is_prefix) {
57 return len >= _len && name->starts_with(_class_name);
58 } else {
59 return len == _len && name->equals(_class_name);
60 }
61 }
62 };
63
64
65 // Tell if ik has a name that matches one of the given specs.
66 bool AOTClassInitializer::is_allowed(AllowedSpec* specs, InstanceKlass* ik) {
67 Symbol* name = ik->name();
68 int len = name->utf8_length();
69 for (AllowedSpec* s = specs; s->class_name() != nullptr; s++) {
70 if (s->matches(name, len)) {
71 // If a type is included in the tables inside can_archive_initialized_mirror(), we require that
72 // - all super classes must be included
73 // - all super interfaces that have <clinit> must be included.
74 // This ensures that in the production run, we don't run the <clinit> of a supertype but skips
75 // ik's <clinit>.
76 if (ik->java_super() != nullptr) {
77 DEBUG_ONLY(ResourceMark rm);
78 assert(AOTClassInitializer::can_archive_initialized_mirror(ik->java_super()),
79 "super class %s of %s must be aot-initialized", ik->java_super()->external_name(),
80 ik->external_name());
81 }
82
83 Array<InstanceKlass*>* interfaces = ik->local_interfaces();
84 int len = interfaces->length();
85 for (int i = 0; i < len; i++) {
86 InstanceKlass* intf = interfaces->at(i);
87 if (intf->class_initializer() != nullptr) {
88 assert(AOTClassInitializer::can_archive_initialized_mirror(intf),
89 "super interface %s (which has <clinit>) of %s must be aot-initialized", intf->external_name(),
90 ik->external_name());
91 }
92 }
93
94 return true;
95 }
96 }
97 return false;
98 }
99
100
101 bool AOTClassInitializer::can_archive_initialized_mirror(InstanceKlass* ik) {
102 assert(!ArchiveBuilder::current()->is_in_buffer_space(ik), "must be source klass");
103 if (!CDSConfig::is_initing_classes_at_dump_time()) {
104 return false;
105 }
106
107 if (!ik->is_initialized()) {
108 return false;
109 }
110
111 // About "static field that may hold a different value" errors:
112 //
113 // Automatic selection for aot-inited classes
114 // ==========================================
115 //
116 // When CDSConfig::is_initing_classes_at_dump_time() is enabled,
117 // AOTArtifactFinder::find_artifacts() finds the classes of all
118 // heap objects that are reachable from HeapShared::_run_time_special_subgraph,
119 // and mark these classes as aot-inited. This preserves the initialized
120 // mirrors of these classes, and their <clinit> methods are NOT executed
121 // at runtime. See aotArtifactFinder.hpp for more info.
122 //
123 // For example, with -XX:+AOTInvokeDynamicLinking, _run_time_special_subgraph
124 // will contain some DirectMethodHandle objects. As a result, the DirectMethodHandle
125 // class is automatically marked as aot-inited.
126 //
127 // When a class is aot-inited, its static fields are already set up
128 // by executing the <clinit> method at AOT assembly time. Later on
129 // in the production run, when the class would normally be
130 // initialized, the VM performs guarding and synchronization as if
131 // it were going to run the <clinit> again, but instead it simply
132 // observes that that class was aot-inited. The VM assumes that, if
133 // it were to run <clinit> again, it would get a semantically
134 // equivalent set of final field values, so it just adopts the
135 // existing field values (from AOT assembly) and skips the call to
136 // <clinit>. There may at that point be fixups performed by ad hoc
137 // code, if the VM recognizes a request in the library.
138 //
139 // It is true that this is not generally correct for all possible
140 // Java code. A <clinit> method might have a side effect beyond
141 // initializing the static fields. It might send an email somewhere
142 // noting the current time of day. In that case, such an email
143 // would have been sent during the AOT assembly phase, and the email
144 // would NOT be sent again during production. This is clearly NOT
145 // what a user would want, if this were a general purpose facility.
146 // But in fact it is only for certain well-behaved classes, which
147 // are known NOT to have such side effects. We know this because
148 // the optimization (of skipping <clinit> for aot-init classes) is
149 // only applied to classes fully defined by the JDK.
150 //
151 // (A day may come when we figure out how to gracefully extend this
152 // optimization to untrusted third parties, but it is not this day.)
153 //
154 // Manual selection
155 // ================
156 //
157 // There are important cases where one aot-init class has a side
158 // effect on another aot-class, a side effect which is not captured
159 // in any static field value in either class. The simplest example
160 // is class A forces the initialization of class B. In that case,
161 // we need to aot-init either both classes or neither. From looking
162 // at the JDK state after AOT assembly is done, it is hard to tell
163 // that A "touched" B and B might escape our notice. Another common
164 // example is A copying a field value from B. We don't know where A
165 // got the value, but it would be wrong to re-initialize B at
166 // startup, while keeping the snapshot of the old B value in A. In
167 // general, if we aot-init A, we need to aot-init every class B that
168 // somehow contributed to A's initial state, and every class C that
169 // was somehow side-effected by A's initialization. We say that the
170 // aot-init of A is "init-coupled" to those of B and C.
171 //
172 // So there are init-coupled classes that cannot be automatically discovered. For
173 // example, DirectMethodHandle::IMPL_NAMES points to MethodHandles::IMPL_NAMES,
174 // but the MethodHandles class is not automatically marked because there are
175 // no archived instances of the MethodHandles type.
176 //
177 // If we aot-initialize DirectMethodHandle, but allow MethodHandles to be
178 // initialized at runtime, MethodHandles::IMPL_NAMES will get a different
179 // value than DirectMethodHandle::IMPL_NAMES. This *may or may not* be a problem,
180 // but to ensure compatibility, we should try to preserve the identity equality
181 // of these two fields.
182 //
183 // To do that, we add MethodHandles to the indy_specs[] table below.
184 //
185 // Luckily we do not need to be all-knowing in order to choose which
186 // items to add to that table. We have tools to help detect couplings.
187 //
188 // Automatic validation
189 // ====================
190 //
191 // CDSHeapVerifier is used to detect potential problems with identity equality.
192 //
193 // A class B is assumed to be init-coupled to some aot-init class if
194 // B has a field which points to a live object X in the AOT heap.
195 // The live object X was created by some other class A which somehow
196 // used B's reference to X, perhaps with the help of an intermediate
197 // class Z. Or, B pulled the reference to X from some other class
198 // Y, and B obtained that reference from Y (or an intermediate Z).
199 // It is not certain how X got into the heap, nor whether B
200 // contributed it, but it is a good heuristic that B is init-coupled
201 // to X's class or some other aot-init class. In any case, B should
202 // be made an aot-init class as well, unless a manual inspection
203 // shows that would be a problem. If there is a problem, then the
204 // JDK code for B and/or X probably needs refactoring. If there is
205 // no problem, we add B to the list. Typically the same scan will
206 // find any other accomplices Y, Z, etc. One failure would be a
207 // class Q whose only initialization action is to scribble a special
208 // value into B, from which the value X is derived and then makes
209 // its way into the heap. In that case, the heuristic does not
210 // identify Q. It is (currently) a human responsibility, of JDK
211 // engineers, not to write such dirty JDK code, or to repair it if
212 // it crops up. Eventually we may have tools, or even a user mode
213 // with design rules and checks, that will vet our code base more
214 // automatically.
215 //
216 // To see how the tool detects the problem with MethodHandles::IMPL_NAMES:
217 //
218 // - Comment out all the lines in indy_specs[] except the {nullptr} line.
219 // - Rebuild the JDK
220 //
221 // Then run the following:
222 // java -XX:AOTMode=record -XX:AOTConfiguration=jc.aotconfig com.sun.tools.javac.Main
223 // java -XX:AOTMode=create -Xlog:cds -XX:AOTCache=jc.aot -XX:AOTConfiguration=jc.aotconfig
224 //
225 // You will see an error like this:
226 //
227 // Archive heap points to a static field that may hold a different value at runtime:
228 // Field: java/lang/invoke/MethodHandles::IMPL_NAMES
229 // Value: java.lang.invoke.MemberName$Factory
230 // {0x000000060e906ae8} - klass: 'java/lang/invoke/MemberName$Factory' - flags:
231 //
232 // - ---- fields (total size 2 words):
233 // --- trace begin ---
234 // [ 0] {0x000000060e8deeb0} java.lang.Class (java.lang.invoke.DirectMethodHandle::IMPL_NAMES)
235 // [ 1] {0x000000060e906ae8} java.lang.invoke.MemberName$Factory
236 // --- trace end ---
237 //
238 // Trouble-shooting
239 // ================
240 //
241 // If you see a "static field that may hold a different value" error, it's probably
242 // because you've made some changes in the JDK core libraries (most likely
243 // java.lang.invoke).
244 //
245 // - Did you add a new static field to a class that could be referenced by
246 // cached object instances of MethodType, MethodHandle, etc? You may need
247 // to add that class to indy_specs[].
248 // - Did you modify the <clinit> of the classes in java.lang.invoke such that
249 // a static field now points to an object that should not be cached (e.g.,
250 // a native resource such as a file descriptior, or a Thread)?
251 //
252 // Note that these potential problems only occur when one class gets
253 // the aot-init treatment, AND another class is init-coupled to it,
254 // AND the coupling is not detected. Currently there are a number
255 // classes that get the aot-init treatment, in java.lang.invoke
256 // because of invokedynamic. They are few enough for now to be
257 // manually tracked. There may be more in the future.
258
259 // IS_PREFIX means that we match all class names that start with a
260 // prefix. Otherwise, it is an exact match, of just one class name.
261 const bool IS_PREFIX = true;
262
263 {
264 static AllowedSpec specs[] = {
265 // everybody's favorite super
266 {"java/lang/Object"},
267
268 {nullptr}
269 };
270 if (is_allowed(specs, ik)) {
271 return true;
272 }
273 }
274
275 if (CDSConfig::is_dumping_invokedynamic()) {
276 // This table was created with the help of CDSHeapVerifier.
277 // Also, some $Holder classes are needed. E.g., Invokers.<clinit> explicitly
278 // initializes Invokers$Holder. Since Invokers.<clinit> won't be executed
279 // at runtime, we need to make sure Invokers$Holder is also aot-inited.
280 //
281 // We hope we can reduce the size of this list over time, and move
282 // the responsibility for identifying such classes into the JDK
283 // code itself. See tracking RFE JDK-8342481.
284 static AllowedSpec indy_specs[] = {
285 {"java/lang/constant/ConstantDescs"},
286 {"java/lang/constant/DynamicConstantDesc"},
287 {"java/lang/invoke/BoundMethodHandle"},
288 {"java/lang/invoke/BoundMethodHandle$Specializer"},
289 {"java/lang/invoke/BoundMethodHandle$Species_", IS_PREFIX},
290 {"java/lang/invoke/ClassSpecializer"},
291 {"java/lang/invoke/ClassSpecializer$", IS_PREFIX},
292 {"java/lang/invoke/DelegatingMethodHandle"},
293 {"java/lang/invoke/DelegatingMethodHandle$Holder"}, // UNSAFE.ensureClassInitialized()
294 {"java/lang/invoke/DirectMethodHandle"},
295 {"java/lang/invoke/DirectMethodHandle$Constructor"},
296 {"java/lang/invoke/DirectMethodHandle$Holder"}, // UNSAFE.ensureClassInitialized()
297 {"java/lang/invoke/Invokers"},
298 {"java/lang/invoke/Invokers$Holder"}, // UNSAFE.ensureClassInitialized()
299 {"java/lang/invoke/LambdaForm"},
300 {"java/lang/invoke/LambdaForm$Holder"}, // UNSAFE.ensureClassInitialized()
301 {"java/lang/invoke/LambdaForm$NamedFunction"},
302 {"java/lang/invoke/MethodHandle"},
303 {"java/lang/invoke/MethodHandles"},
304 {"java/lang/invoke/SimpleMethodHandle"},
305 {"java/util/Collections"},
306 {"java/util/stream/Collectors"},
307 {"jdk/internal/constant/ConstantUtils"},
308 {"jdk/internal/constant/PrimitiveClassDescImpl"},
309 {"jdk/internal/constant/ReferenceClassDescImpl"},
310
311 // Can't include this, as it will pull in MethodHandleStatics which has many environment
312 // dependencies (on system properties, etc).
313 // MethodHandleStatics is an example of a class that must NOT get the aot-init treatment,
314 // because of its strong reliance on (a) final fields which are (b) environmentally determined.
315 //{"java/lang/invoke/InvokerBytecodeGenerator"},
316
317 {nullptr}
318 };
319 if (is_allowed(indy_specs, ik)) {
320 return true;
321 }
322 }
323
324 return false;
325 }
326
327 // TODO: currently we have a hard-coded list. We should turn this into
328 // an annotation: @jdk.internal.vm.annotation.RuntimeSetupRequired
329 // See JDK-8342481.
330 bool AOTClassInitializer::is_runtime_setup_required(InstanceKlass* ik) {
331 return ik == vmClasses::Class_klass() ||
332 ik == vmClasses::internal_Unsafe_klass() ||
333 ik == vmClasses::ConcurrentHashMap_klass() ||
334 ik == vmClasses::Reference_klass() ||
335 ik->name()->equals("java/net/URI") ||
336 ik->name()->equals("java/lang/module/ModuleDescriptor");
337 }
338
339 void AOTClassInitializer::call_runtime_setup(JavaThread* current, InstanceKlass* ik) {
340 assert(ik->has_aot_initialized_mirror(), "sanity");
341 if (ik->is_runtime_setup_required()) {
342 if (log_is_enabled(Info, cds, init)) {
343 ResourceMark rm;
344 log_info(cds, init)("Calling %s::runtimeSetup()", ik->external_name());
345 }
346 JavaValue result(T_VOID);
347 JavaCalls::call_static(&result, ik,
348 vmSymbols::runtimeSetup(),
349 vmSymbols::void_method_signature(), current);
350 if (current->has_pending_exception()) {
351 // We cannot continue, as we might have cached instances of ik in the heap, but propagating the
352 // exception would cause ik to be in an error state.
353 AOTLinkedClassBulkLoader::exit_on_exception(current);
354 }
355 }
356 }
357
358 // check_can_be_preinited() is quite costly, so we cache the results inside
359 // DumpTimeClassInfo::_can_be_preinited. See also AOTClassInitializer::reset_preinit_check().
360 bool AOTClassInitializer::check_can_be_preinited(InstanceKlass* ik) {
361 ResourceMark rm;
362
363 if (!SystemDictionaryShared::is_builtin(ik)) {
364 log_info(cds, init)("cannot initialize %s (not built-in loader)", ik->external_name());
365 return false;
366 }
367
368 InstanceKlass* super = ik->java_super();
369 if (super != nullptr && !can_be_preinited_locked(super)) {
370 log_info(cds, init)("cannot initialize %s (super %s not initable)", ik->external_name(), super->external_name());
371 return false;
372 }
373
374 Array<InstanceKlass*>* interfaces = ik->local_interfaces();
375 for (int i = 0; i < interfaces->length(); i++) {
376 if (!can_be_preinited_locked(interfaces->at(i))) {
377 log_info(cds, init)("cannot initialize %s (interface %s not initable)",
378 ik->external_name(), interfaces->at(i)->external_name());
379 return false;
380 }
381 }
382
383 if (HeapShared::is_lambda_form_klass(ik)) {
384 // We allow only these to have <clinit> or non-default static fields
385 return true;
386 }
387
388 if (ik->class_initializer() != nullptr) {
389 log_info(cds, init)("cannot initialize %s (has <clinit>)", ik->external_name());
390 return false;
391 }
392 if (ik->is_initialized() && !has_default_static_fields(ik)) {
393 return false;
394 }
395
396 return true;
397 }
398
399 bool AOTClassInitializer::has_default_static_fields(InstanceKlass* ik) {
400 oop mirror = ik->java_mirror();
401
402 for (JavaFieldStream fs(ik); !fs.done(); fs.next()) {
403 if (fs.access_flags().is_static()) {
404 fieldDescriptor& fd = fs.field_descriptor();
405 int offset = fd.offset();
406 bool is_default = true;
407 bool has_initval = fd.has_initial_value();
408 switch (fd.field_type()) {
409 case T_OBJECT:
410 case T_ARRAY:
411 is_default = mirror->obj_field(offset) == nullptr;
412 break;
413 case T_BOOLEAN:
414 is_default = mirror->bool_field(offset) == (has_initval ? fd.int_initial_value() : 0);
415 break;
416 case T_BYTE:
417 is_default = mirror->byte_field(offset) == (has_initval ? fd.int_initial_value() : 0);
418 break;
419 case T_SHORT:
420 is_default = mirror->short_field(offset) == (has_initval ? fd.int_initial_value() : 0);
421 break;
422 case T_CHAR:
423 is_default = mirror->char_field(offset) == (has_initval ? fd.int_initial_value() : 0);
424 break;
425 case T_INT:
426 is_default = mirror->int_field(offset) == (has_initval ? fd.int_initial_value() : 0);
427 break;
428 case T_LONG:
429 is_default = mirror->long_field(offset) == (has_initval ? fd.long_initial_value() : 0);
430 break;
431 case T_FLOAT:
432 is_default = mirror->float_field(offset) == (has_initval ? fd.float_initial_value() : 0);
433 break;
434 case T_DOUBLE:
435 is_default = mirror->double_field(offset) == (has_initval ? fd.double_initial_value() : 0);
436 break;
437 default:
438 ShouldNotReachHere();
439 }
440
441 if (!is_default) {
442 log_info(cds, init)("cannot initialize %s (static field %s has non-default value)",
443 ik->external_name(), fd.name()->as_C_string());
444 return false;
445 }
446 }
447 }
448
449 return true;
450 }
451
452 bool AOTClassInitializer::can_be_preinited(InstanceKlass* ik) {
453 MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
454 return can_be_preinited_locked(ik);
455 }
456
457 bool AOTClassInitializer::can_be_preinited_locked(InstanceKlass* ik) {
458 if (!CDSConfig::is_initing_classes_at_dump_time()) {
459 return false;
460 }
461
462 assert_lock_strong(DumpTimeTable_lock);
463 DumpTimeClassInfo* info = SystemDictionaryShared::get_info_locked(ik);
464 if (!info->has_done_preinit_check()) {
465 info->set_can_be_preinited(AOTClassInitializer::check_can_be_preinited(ik));
466 }
467 return info->can_be_preinited();
468 }
469
470 // Initialize a class at dump time, if possible.
471 void AOTClassInitializer::maybe_preinit_class(InstanceKlass* ik, TRAPS) {
472 #if 0 // FIXME -- leyden+JEP483 merge
473 if (!ik->is_initialized() && AOTClassInitializer::can_be_preinited(ik)) {
474 if (log_is_enabled(Info, cds, init)) {
475 ResourceMark rm;
476 log_info(cds, init)("preinitializing %s", ik->external_name());
477 }
478 ik->initialize(CHECK);
479 }
480 #endif
481 }