1 /*
2 * Copyright (c) 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. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package jdk.internal.vm;
26
27 import java.util.Arrays;
28 import java.util.stream.Stream;
29 import jdk.internal.access.JavaLangAccess;
30 import jdk.internal.access.SharedSecrets;
31
32 /**
33 * Represents a snapshot of information about a Thread.
34 */
35 class ThreadSnapshot {
36 private static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();
37 private static final StackTraceElement[] EMPTY_STACK = new StackTraceElement[0];
38 private static final ThreadLock[] EMPTY_LOCKS = new ThreadLock[0];
39 private static final ThreadSnapshot NOT_ALIVE = new ThreadSnapshot();
40
41 // filled by VM
42 private String name;
43 private int threadStatus;
44 private Thread carrierThread;
45 private StackTraceElement[] stackTrace;
46 // owned monitors
47 private ThreadLock[] locks;
48 // an object the thread is blocked/waiting on, converted to ThreadBlocker by ThreadSnapshot.of()
49 private int blockerTypeOrdinal;
50 private Object blockerObject;
51
52 // set by ThreadSnapshot.of()
53 private ThreadBlocker blocker;
54
55 private ThreadSnapshot() {}
56
57 /**
58 * Take a snapshot of a Thread to get all information about the thread.
59 * Return null if the thread is not alive.
60 * @throws UnsupportedOperationException if not supported by VM
61 */
62 static ThreadSnapshot of(Thread thread) {
63 ThreadSnapshot snapshot;
64 if (thread.isVirtual()) {
65 do {
66 // assume unmounted
67 snapshot = JLA.supplyIfUnmounted(thread,
68 () -> create(thread, true), // unmounted and alive
69 () -> NOT_ALIVE); // not alive
70 if (snapshot == NOT_ALIVE) {
71 return null;
72 } else if (snapshot == null) {
73 // not unmounted, retry assuming mounted
74 snapshot = create(thread, false);
75 if (snapshot == null) {
76 // yield before retry
77 Thread.yield();
78 }
79 }
80 } while (snapshot == null);
81 } else {
82 // platform thread
83 snapshot = create(thread, false);
84 if (snapshot == null) {
85 return null; // not alive
86 }
87 }
88
89 if (snapshot.stackTrace == null) {
90 snapshot.stackTrace = EMPTY_STACK;
91 }
92 if (snapshot.locks != null) {
93 Arrays.stream(snapshot.locks).forEach(ThreadLock::finishInit);
94 } else {
95 snapshot.locks = EMPTY_LOCKS;
96 }
97 if (snapshot.blockerObject != null) {
98 snapshot.blocker = new ThreadBlocker(snapshot.blockerTypeOrdinal, snapshot.blockerObject);
99 snapshot.blockerObject = null; // release
100 }
101 return snapshot;
102 }
103
104 /**
105 * Returns the thread name.
106 */
107 String threadName() {
108 return name;
109 }
110
111 /**
112 * Returns the thread state.
113 */
114 Thread.State threadState() {
115 return jdk.internal.misc.VM.toThreadState(threadStatus);
116 }
117
118 /**
119 * Returns the thread stack trace.
120 */
121 StackTraceElement[] stackTrace() {
122 return stackTrace;
123 }
124
125 /**
126 * Returns the thread's parkBlocker.
127 */
128 Object parkBlocker() {
129 return getBlocker(BlockerLockType.PARK_BLOCKER);
130 }
131
132 /**
133 * Returns the object that the thread is blocked on.
134 * @throws IllegalStateException if not in the blocked state
135 */
136 Object blockedOn() {
137 if (threadState() != Thread.State.BLOCKED) {
138 throw new IllegalStateException();
139 }
140 return getBlocker(BlockerLockType.WAITING_TO_LOCK);
141 }
142
143 /**
144 * Returns the object that the thread is waiting on.
145 * @throws IllegalStateException if not in the waiting state
146 */
147 Object waitingOn() {
148 if (threadState() != Thread.State.WAITING
149 && threadState() != Thread.State.TIMED_WAITING) {
150 throw new IllegalStateException();
151 }
152 return getBlocker(BlockerLockType.WAITING_ON);
153 }
154
155 private Object getBlocker(BlockerLockType type) {
156 return (blocker != null && blocker.type == type) ? blocker.obj : null;
157 }
158
159 /**
160 * Returns true if the thread owns any object monitors.
161 */
162 boolean ownsMonitors() {
163 return locks.length > 0;
164 }
165
166 /**
167 * Returns the objects that the thread locked at the given depth. The stream
168 * will contain a null element for a monitor that has been eliminated.
169 */
170 Stream<Object> ownedMonitorsAt(int depth) {
171 return Arrays.stream(locks)
172 .filter(lock -> lock.depth() == depth)
173 .map(lock -> (lock.type == OwnedLockType.LOCKED)
174 ? lock.lockObject()
175 : /*eliminated*/ null);
176 }
177
178 /**
179 * If the thread is a mounted virtual thread then return its carrier.
180 */
181 Thread carrierThread() {
182 return carrierThread;
183 }
184
185 /**
186 * Represents information about a locking operation.
187 */
188 private enum OwnedLockType {
189 LOCKED,
190 // Lock object is a class of the eliminated monitor
191 ELIMINATED,
192 }
193
194 private enum BlockerLockType {
195 // Park blocker
196 PARK_BLOCKER,
197 WAITING_TO_LOCK,
198 // Object.wait()
199 WAITING_ON,
200 }
201
202 /**
203 * Represents a locking operation of a thread at a specific stack depth.
204 */
205 private static class ThreadLock {
206 private static final OwnedLockType[] lockTypeValues = OwnedLockType.values(); // cache
207
208 // set by the VM
209 private int depth;
210 private int typeOrdinal;
211 private Object obj;
212
213 // set by ThreadLock.of()
214 private OwnedLockType type;
215
216 private ThreadLock() {}
217
218 void finishInit() {
219 type = lockTypeValues[typeOrdinal];
220 }
221
222 int depth() {
223 return depth;
224 }
225
226 OwnedLockType type() {
227 return type;
228 }
229
230 Object lockObject() {
231 if (type == OwnedLockType.ELIMINATED) {
232 // we have no lock object, lock contains lock class
233 return null;
234 }
235 return obj;
236 }
237 }
238
239 private record ThreadBlocker(BlockerLockType type, Object obj) {
240 private static final BlockerLockType[] lockTypeValues = BlockerLockType.values(); // cache
241
242 ThreadBlocker(int typeOrdinal, Object obj) {
243 this(lockTypeValues[typeOrdinal], obj);
244 }
245 }
246
247 /**
248 * Return the snapshot of the given thread if alive. If the thread is a virtual
249 * thread this method returns the snapshot if the virtual thread is mounted. If
250 * the virtual thread is unmounted then it must be suspended.
251 * @param thread the target thread
252 * @param suspendedByCaller true if a suspended unmounted virtual thread
253 * @return the snapshot, null if not alive, null if unmounted virtual and not suspended
254 */
255 private static native ThreadSnapshot create(Thread thread, boolean suspendedByCaller);
256 }