1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.security.AccessControlContext;
39 import java.security.AccessController;
40 import java.security.PrivilegedAction;
41 import java.security.ProtectionDomain;
42
43 /**
44 * A thread managed by a {@link ForkJoinPool}, which executes
45 * {@link ForkJoinTask}s.
46 * This class is subclassable solely for the sake of adding
47 * functionality -- there are no overridable methods dealing with
48 * scheduling or execution. However, you can override initialization
49 * and termination methods surrounding the main task processing loop.
50 * If you do create such a subclass, you will also need to supply a
51 * custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to
52 * {@linkplain ForkJoinPool#ForkJoinPool use it} in a {@code ForkJoinPool}.
53 *
54 * @since 1.7
55 * @author Doug Lea
56 */
57 public class ForkJoinWorkerThread extends Thread {
58 /*
59 * ForkJoinWorkerThreads are managed by ForkJoinPools and perform
60 * ForkJoinTasks. For explanation, see the internal documentation
61 * of class ForkJoinPool.
62 *
63 * This class just maintains links to its pool and WorkQueue. The
64 * pool field is set immediately upon construction, but the
65 * workQueue field is not set until a call to registerWorker
66 * completes. This leads to a visibility race, that is tolerated
67 * by requiring that the workQueue field is only accessed by the
68 * owning thread.
69 *
70 * Support for (non-public) subclass InnocuousForkJoinWorkerThread
71 * requires that we break quite a lot of encapsulation (via helper
72 * methods in ThreadLocalRandom) both here and in the subclass to
73 * access and set Thread fields.
74 */
75
76 final ForkJoinPool pool; // the pool this thread works in
77 final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
78
79 /**
80 * Creates a ForkJoinWorkerThread operating in the given pool.
81 *
82 * @param pool the pool this thread works in
83 * @throws NullPointerException if pool is null
84 */
85 protected ForkJoinWorkerThread(ForkJoinPool pool) {
86 // Use a placeholder until a useful name can be set in registerWorker
87 super("aForkJoinWorkerThread");
88 this.pool = pool;
89 this.workQueue = pool.registerWorker(this);
90 }
91
92 /**
93 * Version for use by the default pool. Supports setting the
94 * context class loader. This is a separate constructor to avoid
95 * affecting the protected constructor.
96 */
97 ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl) {
98 super("aForkJoinWorkerThread");
99 super.setContextClassLoader(ccl);
100 this.pool = pool;
101 this.workQueue = pool.registerWorker(this);
102 }
103
104 /**
105 * Version for InnocuousForkJoinWorkerThread.
106 */
107 ForkJoinWorkerThread(ForkJoinPool pool,
108 ClassLoader ccl,
109 ThreadGroup threadGroup,
110 AccessControlContext acc) {
111 super(threadGroup, null, "aForkJoinWorkerThread");
112 super.setContextClassLoader(ccl);
113 ThreadLocalRandom.setInheritedAccessControlContext(this, acc);
114 ThreadLocalRandom.eraseThreadLocals(this); // clear before registering
115 this.pool = pool;
116 this.workQueue = pool.registerWorker(this);
117 }
118
119 /**
120 * Returns the pool hosting this thread.
121 *
122 * @return the pool
123 */
124 public ForkJoinPool getPool() {
125 return pool;
126 }
127
128 /**
129 * Returns the unique index number of this thread in its pool.
130 * The returned value ranges from zero to the maximum number of
131 * threads (minus one) that may exist in the pool, and does not
132 * change during the lifetime of the thread. This method may be
133 * useful for applications that track status or collect results
134 * per-worker-thread rather than per-task.
135 *
136 * @return the index number
137 */
138 public int getPoolIndex() {
139 return workQueue.getPoolIndex();
140 }
141
142 /**
143 * Initializes internal state after construction but before
144 * processing any tasks. If you override this method, you must
145 * invoke {@code super.onStart()} at the beginning of the method.
146 * Initialization requires care: Most fields must have legal
147 * default values, to ensure that attempted accesses from other
148 * threads work correctly even before this thread starts
149 * processing tasks.
150 */
151 protected void onStart() {
152 }
153
154 /**
155 * Performs cleanup associated with termination of this worker
156 * thread. If you override this method, you must invoke
157 * {@code super.onTermination} at the end of the overridden method.
158 *
159 * @param exception the exception causing this thread to abort due
160 * to an unrecoverable error, or {@code null} if completed normally
161 */
162 protected void onTermination(Throwable exception) {
163 }
164
165 /**
166 * This method is required to be public, but should never be
167 * called explicitly. It performs the main run loop to execute
168 * {@link ForkJoinTask}s.
169 */
170 public void run() {
171 if (workQueue.array == null) { // only run once
172 Throwable exception = null;
173 try {
174 onStart();
175 pool.runWorker(workQueue);
176 } catch (Throwable ex) {
177 exception = ex;
178 } finally {
179 try {
180 onTermination(exception);
181 } catch (Throwable ex) {
182 if (exception == null)
183 exception = ex;
184 } finally {
185 pool.deregisterWorker(this, exception);
186 }
187 }
188 }
189 }
190
191 /**
192 * Non-public hook method for InnocuousForkJoinWorkerThread.
193 */
194 void afterTopLevelExec() {
195 }
196
197 /**
198 * A worker thread that has no permissions, is not a member of any
199 * user-defined ThreadGroup, uses the system class loader as
200 * thread context class loader, and erases all ThreadLocals after
201 * running each top-level task.
202 */
203 static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
204 /** The ThreadGroup for all InnocuousForkJoinWorkerThreads */
205 private static final ThreadGroup innocuousThreadGroup =
206 AccessController.doPrivileged(new PrivilegedAction<>() {
207 public ThreadGroup run() {
208 ThreadGroup group = Thread.currentThread().getThreadGroup();
209 for (ThreadGroup p; (p = group.getParent()) != null; )
210 group = p;
211 return new ThreadGroup(
212 group, "InnocuousForkJoinWorkerThreadGroup");
213 }});
214
215 /** An AccessControlContext supporting no privileges */
216 private static final AccessControlContext INNOCUOUS_ACC =
217 new AccessControlContext(
218 new ProtectionDomain[] { new ProtectionDomain(null, null) });
219
220 InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
221 super(pool,
222 ClassLoader.getSystemClassLoader(),
223 innocuousThreadGroup,
224 INNOCUOUS_ACC);
225 }
226
227 @Override // to erase ThreadLocals
228 void afterTopLevelExec() {
229 ThreadLocalRandom.eraseThreadLocals(this);
230 }
231
232 @Override // to silently fail
233 public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }
234
235 @Override // paranoically
236 public void setContextClassLoader(ClassLoader cl) {
237 throw new SecurityException("setContextClassLoader");
238 }
239 }
240 }