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.
18 *
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.
22 *
23 */
24
25 #include "cds/aotClassInitializer.hpp"
26 #include "cds/archiveBuilder.hpp"
27 #include "cds/cdsConfig.hpp"
28 #include "cds/heapShared.hpp"
29 #include "cds/regeneratedClasses.hpp"
30 #include "classfile/symbolTable.hpp"
31 #include "classfile/systemDictionaryShared.hpp"
32 #include "classfile/vmSymbols.hpp"
33 #include "oops/instanceKlass.inline.hpp"
34 #include "oops/symbol.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/javaCalls.hpp"
37
38 DEBUG_ONLY(InstanceKlass* _aot_init_class = nullptr;)
39
40 bool AOTClassInitializer::can_archive_initialized_mirror(InstanceKlass* ik) {
41 assert(!ArchiveBuilder::is_active() || !ArchiveBuilder::current()->is_in_buffer_space(ik), "must be source klass");
42 if (!CDSConfig::is_initing_classes_at_dump_time()) {
43 return false;
44 }
45
46 if (RegeneratedClasses::is_regenerated_object(ik)) {
47 ik = RegeneratedClasses::get_original_object(ik);
48 }
49
50 if (!ik->is_initialized() && !ik->is_being_initialized()) {
51 return false;
52 }
53
54 // About "static field that may hold a different value" errors:
55 //
56 // Automatic selection for aot-inited classes
57 // ==========================================
58 //
59 // When CDSConfig::is_initing_classes_at_dump_time is enabled,
60 // AOTArtifactFinder::find_artifacts() finds the classes of all
61 // heap objects that are reachable from HeapShared::_run_time_special_subgraph,
62 // and mark these classes as aot-inited. This preserves the initialized
63 // mirrors of these classes, and their <clinit> methods are NOT executed
64 // at runtime. See aotArtifactFinder.hpp for more info.
65 //
66 // For example, with -XX:+AOTInvokeDynamicLinking, _run_time_special_subgraph
67 // will contain some DirectMethodHandle objects. As a result, the DirectMethodHandle
68 // class is automatically marked as aot-inited.
69 //
70 // When a class is aot-inited, its static fields are already set up
71 // by executing the <clinit> method at AOT assembly time. Later on
72 // in the production run, when the class would normally be
73 // initialized, the VM performs guarding and synchronization as if
74 // it were going to run the <clinit> again, but instead it simply
75 // observes that that class was aot-inited. The VM assumes that, if
76 // it were to run <clinit> again, it would get a semantically
77 // equivalent set of final field values, so it just adopts the
78 // existing field values (from AOT assembly) and skips the call to
79 // <clinit>. There may at that point be fixups performed by ad hoc
80 // code, if the VM recognizes a request in the library.
81 //
82 // It is true that this is not generally correct for all possible
83 // Java code. A <clinit> method might have a side effect beyond
84 // initializing the static fields. It might send an email somewhere
85 // noting the current time of day. In that case, such an email
86 // would have been sent during the AOT assembly phase, and the email
87 // would NOT be sent again during production. This is clearly NOT
88 // what a user would want, if this were a general purpose facility.
89 // But in fact it is only for certain well-behaved classes, which
90 // are known NOT to have such side effects. We know this because
91 // the optimization (of skipping <clinit> for aot-init classes) is
92 // only applied to classes fully defined by the JDK.
93 //
94 // (A day may come when we figure out how to gracefully extend this
95 // optimization to untrusted third parties, but it is not this day.)
96 //
97 // Manual selection
98 // ================
99 //
100 // There are important cases where one aot-init class has a side
101 // effect on another aot-class, a side effect which is not captured
102 // in any static field value in either class. The simplest example
103 // is class A forces the initialization of class B. In that case,
104 // we need to aot-init either both classes or neither. From looking
105 // at the JDK state after AOT assembly is done, it is hard to tell
106 // that A "touched" B and B might escape our notice. Another common
107 // example is A copying a field value from B. We don't know where A
108 // got the value, but it would be wrong to re-initialize B at
109 // startup, while keeping the snapshot of the old B value in A. In
110 // general, if we aot-init A, we need to aot-init every class B that
111 // somehow contributed to A's initial state, and every class C that
112 // was somehow side-effected by A's initialization. We say that the
113 // aot-init of A is "init-coupled" to those of B and C.
114 //
115 // So there are init-coupled classes that cannot be automatically discovered. For
116 // example, DirectMethodHandle::IMPL_NAMES points to MethodHandles::IMPL_NAMES,
117 // but the MethodHandles class is not automatically marked because there are
118 // no archived instances of the MethodHandles type.
119 //
120 // If we aot-initialize DirectMethodHandle, but allow MethodHandles to be
121 // initialized at runtime, MethodHandles::IMPL_NAMES will get a different
122 // value than DirectMethodHandle::IMPL_NAMES. This *may or may not* be a problem,
123 // but to ensure compatibility, we should try to preserve the identity equality
124 // of these two fields.
125 //
126 // To do that, we add MethodHandles to the indy_specs[] table below.
127 //
128 // Luckily we do not need to be all-knowing in order to choose which
129 // items to add to that table. We have tools to help detect couplings.
130 //
131 // Automatic validation
132 // ====================
133 //
134 // CDSHeapVerifier is used to detect potential problems with identity equality.
135 //
136 // A class B is assumed to be init-coupled to some aot-init class if
137 // B has a field which points to a live object X in the AOT heap.
138 // The live object X was created by some other class A which somehow
139 // used B's reference to X, perhaps with the help of an intermediate
140 // class Z. Or, B pulled the reference to X from some other class
141 // Y, and B obtained that reference from Y (or an intermediate Z).
142 // It is not certain how X got into the heap, nor whether B
143 // contributed it, but it is a good heuristic that B is init-coupled
144 // to X's class or some other aot-init class. In any case, B should
145 // be made an aot-init class as well, unless a manual inspection
146 // shows that would be a problem. If there is a problem, then the
147 // JDK code for B and/or X probably needs refactoring. If there is
148 // no problem, we add B to the list. Typically the same scan will
149 // find any other accomplices Y, Z, etc. One failure would be a
150 // class Q whose only initialization action is to scribble a special
151 // value into B, from which the value X is derived and then makes
152 // its way into the heap. In that case, the heuristic does not
153 // identify Q. It is (currently) a human responsibility, of JDK
154 // engineers, not to write such dirty JDK code, or to repair it if
155 // it crops up. Eventually we may have tools, or even a user mode
156 // with design rules and checks, that will vet our code base more
157 // automatically.
158 //
159 // To see how the tool detects the problem with MethodHandles::IMPL_NAMES:
160 //
161 // - Comment out all the lines in indy_specs[] except the {nullptr} line.
162 // - Rebuild the JDK
163 //
164 // Then run the following:
165 // java -XX:AOTMode=record -XX:AOTConfiguration=jc.aotconfig com.sun.tools.javac.Main
166 // java -XX:AOTMode=create -Xlog:aot -XX:AOTCache=jc.aot -XX:AOTConfiguration=jc.aotconfig
167 //
168 // You will see an error like this:
169 //
170 // Archive heap points to a static field that may hold a different value at runtime:
171 // Field: java/lang/invoke/MethodHandles::IMPL_NAMES
172 // Value: java.lang.invoke.MemberName$Factory
173 // {0x000000060e906ae8} - klass: 'java/lang/invoke/MemberName$Factory' - flags:
174 //
175 // - ---- fields (total size 2 words):
176 // --- trace begin ---
177 // [ 0] {0x000000060e8deeb0} java.lang.Class (java.lang.invoke.DirectMethodHandle::IMPL_NAMES)
178 // [ 1] {0x000000060e906ae8} java.lang.invoke.MemberName$Factory
179 // --- trace end ---
180 //
181 // Trouble-shooting
182 // ================
183 //
184 // If you see a "static field that may hold a different value" error, it's probably
185 // because you've made some changes in the JDK core libraries (most likely
186 // java.lang.invoke).
187 //
188 // - Did you add a new static field to a class that could be referenced by
189 // cached object instances of MethodType, MethodHandle, etc? You may need
190 // to add that class to indy_specs[].
191 // - Did you modify the <clinit> of the classes in java.lang.invoke such that
192 // a static field now points to an object that should not be cached (e.g.,
193 // a native resource such as a file descriptior, or a Thread)?
194 //
195 // Note that these potential problems only occur when one class gets
196 // the aot-init treatment, AND another class is init-coupled to it,
197 // AND the coupling is not detected. Currently there are a number
198 // classes that get the aot-init treatment, in java.lang.invoke
199 // because of invokedynamic. They are few enough for now to be
200 // manually tracked. There may be more in the future.
201
202 {
203 if (ik == vmClasses::Object_klass()) {
204 // everybody's favorite super
205 return true;
206 }
207 }
208
209 if (CDSConfig::is_dumping_method_handles()) {
210 // The minimal list of @AOTSafeClassInitializer was created with the help of CDSHeapVerifier.
211 // Also, some $Holder classes are needed. E.g., Invokers.<clinit> explicitly
212 // initializes Invokers$Holder. Since Invokers.<clinit> won't be executed
213 // at runtime, we need to make sure Invokers$Holder is also aot-inited.
214 if (ik->has_aot_safe_initializer()) {
215 return true;
216 }
217 }
218
219 #ifdef ASSERT
220 if (ik == _aot_init_class) {
221 return true;
222 }
223 #endif
224
225 return false;
226 }
227
228 void AOTClassInitializer::call_runtime_setup(JavaThread* current, InstanceKlass* ik) {
229 assert(ik->has_aot_initialized_mirror(), "sanity");
230 if (ik->is_runtime_setup_required()) {
231 if (log_is_enabled(Info, aot, init)) {
232 ResourceMark rm;
233 log_info(aot, init)("Calling %s::runtimeSetup()", ik->external_name());
234 }
235 JavaValue result(T_VOID);
236 JavaCalls::call_static(&result, ik,
237 vmSymbols::runtimeSetup(),
238 vmSymbols::void_method_signature(), current);
239 if (current->has_pending_exception()) {
240 // We cannot continue, as we might have cached instances of ik in the heap, but propagating the
241 // exception would cause ik to be in an error state.
242 AOTLinkedClassBulkLoader::exit_on_exception(current);
243 }
244 }
245 }
246
247 #ifdef ASSERT
248 void AOTClassInitializer::init_test_class(TRAPS) {
249 // -XX:AOTInitTestClass is used in regression tests for adding additional AOT-initialized classes
250 // and heap objects into the AOT cache. The tests must be carefully written to avoid including
251 // any classes that cannot be AOT-initialized.
252 //
253 // -XX:AOTInitTestClass is NOT a general mechanism for including user-defined objects into
254 // the AOT cache. Therefore, this option is NOT available in product JVM.
255 if (AOTInitTestClass != nullptr && CDSConfig::is_initing_classes_at_dump_time()) {
256 log_info(aot)("Debug build only: force initialization of AOTInitTestClass %s", AOTInitTestClass);
257 TempNewSymbol class_name = SymbolTable::new_symbol(AOTInitTestClass);
258 Handle app_loader(THREAD, SystemDictionary::java_system_loader());
259 Klass* k = SystemDictionary::resolve_or_null(class_name, app_loader, CHECK);
260 if (k == nullptr) {
261 vm_exit_during_initialization("AOTInitTestClass not found", AOTInitTestClass);
262 }
263 if (!k->is_instance_klass()) {
264 vm_exit_during_initialization("Invalid name for AOTInitTestClass", AOTInitTestClass);
265 }
266
267 _aot_init_class = InstanceKlass::cast(k);
268 _aot_init_class->initialize(CHECK);
269 }
270 }
271
272 bool AOTClassInitializer::has_test_class() {
273 return _aot_init_class != nullptr;
274 }
275 #endif