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
30 /**
31 * Represents a snapshot of information about a Thread.
32 */
33 class ThreadSnapshot {
34 private static final StackTraceElement[] EMPTY_STACK = new StackTraceElement[0];
35 private static final ThreadLock[] EMPTY_LOCKS = new ThreadLock[0];
36
37 // filled by VM
38 private String name;
39 private int threadStatus;
40 private Thread carrierThread;
41 private StackTraceElement[] stackTrace;
42 // owned monitors
43 private ThreadLock[] locks;
44 // an object the thread is blocked/waiting on, converted to ThreadBlocker by ThreadSnapshot.of()
45 private int blockerTypeOrdinal;
46 private Object blockerObject;
47
48 // set by ThreadSnapshot.of()
49 private ThreadBlocker blocker;
50
51 private ThreadSnapshot() {}
52
53 /**
54 * Take a snapshot of a Thread to get all information about the thread.
55 * Return null if a ThreadSnapshot is not created, for example if the
56 * thread has terminated.
57 * @throws UnsupportedOperationException if not supported by VM
58 */
59 static ThreadSnapshot of(Thread thread) {
60 ThreadSnapshot snapshot = create(thread);
61 if (snapshot == null) {
62 return null; // thread terminated
63 }
64 if (snapshot.stackTrace == null) {
65 snapshot.stackTrace = EMPTY_STACK;
66 }
67 if (snapshot.locks != null) {
68 Arrays.stream(snapshot.locks).forEach(ThreadLock::finishInit);
69 } else {
70 snapshot.locks = EMPTY_LOCKS;
71 }
72 if (snapshot.blockerObject != null) {
73 snapshot.blocker = new ThreadBlocker(snapshot.blockerTypeOrdinal, snapshot.blockerObject);
74 snapshot.blockerObject = null; // release
75 }
76 return snapshot;
77 }
78
79 /**
80 * Returns the thread name.
81 */
82 String threadName() {
83 return name;
84 }
85
86 /**
87 * Returns the thread state.
88 */
89 Thread.State threadState() {
90 return jdk.internal.misc.VM.toThreadState(threadStatus);
91 }
92
93 /**
94 * Returns the thread stack trace.
95 */
96 StackTraceElement[] stackTrace() {
97 return stackTrace;
98 }
99
100 /**
101 * Returns the thread's parkBlocker.
102 */
103 Object parkBlocker() {
104 return getBlocker(BlockerLockType.PARK_BLOCKER);
105 }
106
107 /**
108 * Returns the object that the thread is blocked on.
109 * @throws IllegalStateException if not in the blocked state
110 */
111 Object blockedOn() {
112 if (threadState() != Thread.State.BLOCKED) {
113 throw new IllegalStateException();
114 }
115 return getBlocker(BlockerLockType.WAITING_TO_LOCK);
116 }
117
118 /**
119 * Returns the object that the thread is waiting on.
120 * @throws IllegalStateException if not in the waiting state
121 */
122 Object waitingOn() {
123 if (threadState() != Thread.State.WAITING
124 && threadState() != Thread.State.TIMED_WAITING) {
125 throw new IllegalStateException();
126 }
127 return getBlocker(BlockerLockType.WAITING_ON);
128 }
129
130 private Object getBlocker(BlockerLockType type) {
131 return (blocker != null && blocker.type == type) ? blocker.obj : null;
132 }
133
134 /**
135 * Returns true if the thread owns any object monitors.
136 */
137 boolean ownsMonitors() {
138 return locks.length > 0;
139 }
140
141 /**
142 * Returns the objects that the thread locked at the given depth. The stream
143 * will contain a null element for a monitor that has been eliminated.
144 */
145 Stream<Object> ownedMonitorsAt(int depth) {
146 return Arrays.stream(locks)
147 .filter(lock -> lock.depth() == depth)
148 .map(lock -> (lock.type == OwnedLockType.LOCKED)
149 ? lock.lockObject()
150 : /*eliminated*/ null);
151 }
152
153 /**
154 * If the thread is a mounted virtual thread then return its carrier.
155 */
156 Thread carrierThread() {
157 return carrierThread;
158 }
159
160 /**
161 * Represents information about a locking operation.
162 */
163 private enum OwnedLockType {
164 LOCKED,
165 // Lock object is a class of the eliminated monitor
166 ELIMINATED,
167 }
168
169 private enum BlockerLockType {
170 // Park blocker
171 PARK_BLOCKER,
172 WAITING_TO_LOCK,
173 // Object.wait()
174 WAITING_ON,
175 }
176
177 /**
178 * Represents a locking operation of a thread at a specific stack depth.
179 */
180 private static class ThreadLock {
181 private static final OwnedLockType[] lockTypeValues = OwnedLockType.values(); // cache
182
183 // set by the VM
184 private int depth;
185 private int typeOrdinal;
186 private Object obj;
187
188 // set by ThreadLock.of()
189 private OwnedLockType type;
190
191 private ThreadLock() {}
192
193 void finishInit() {
194 type = lockTypeValues[typeOrdinal];
195 }
196
197 int depth() {
198 return depth;
199 }
200
201 OwnedLockType type() {
202 return type;
203 }
204
205 Object lockObject() {
206 if (type == OwnedLockType.ELIMINATED) {
207 // we have no lock object, lock contains lock class
208 return null;
209 }
210 return obj;
211 }
212 }
213
214 private record ThreadBlocker(BlockerLockType type, Object obj) {
215 private static final BlockerLockType[] lockTypeValues = BlockerLockType.values(); // cache
216
217 ThreadBlocker(int typeOrdinal, Object obj) {
218 this(lockTypeValues[typeOrdinal], obj);
219 }
220 }
221
222 private static native ThreadSnapshot create(Thread thread);
223 }