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(int, ForkJoinWorkerThreadFactory,
53 * UncaughtExceptionHandler, boolean, int, int, int, Predicate, long, TimeUnit)
54 * use it} in a {@code ForkJoinPool}.
55 *
56 * @since 1.7
57 * @author Doug Lea
58 */
59 public class ForkJoinWorkerThread extends Thread {
60 /*
61 * ForkJoinWorkerThreads are managed by ForkJoinPools and perform
62 * ForkJoinTasks. For explanation, see the internal documentation
63 * of class ForkJoinPool.
64 *
65 * This class just maintains links to its pool and WorkQueue. The
66 * pool field is set immediately upon construction, but the
67 * workQueue field is not set until a call to registerWorker
68 * completes. This leads to a visibility race, that is tolerated
69 * by requiring that the workQueue field is only accessed by the
70 * owning thread.
71 *
72 * Support for (non-public) subclass InnocuousForkJoinWorkerThread
73 * requires that we break quite a lot of encapsulation (via helper
74 * methods in ThreadLocalRandom) both here and in the subclass to
75 * access and set Thread fields.
76 */
77
78 final ForkJoinPool pool; // the pool this thread works in
79 final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
80
81 /** An AccessControlContext supporting no privileges */
82 private static final AccessControlContext INNOCUOUS_ACC =
83 new AccessControlContext(
84 new ProtectionDomain[] { new ProtectionDomain(null, null) });
85
86 /**
87 * Creates a ForkJoinWorkerThread operating in the given pool.
88 *
89 * @param pool the pool this thread works in
90 * @throws NullPointerException if pool is null
91 */
92 protected ForkJoinWorkerThread(ForkJoinPool pool) {
93 // Use a placeholder until a useful name can be set in registerWorker
94 super("aForkJoinWorkerThread");
95 this.pool = pool;
96 this.workQueue = pool.registerWorker(this);
97 }
98
99 /**
100 * Version for use by the default pool. Supports setting the
101 * context class loader. This is a separate constructor to avoid
102 * affecting the protected constructor.
103 */
104 ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl) {
105 super("aForkJoinWorkerThread");
106 super.setContextClassLoader(ccl);
107 ThreadLocalRandom.setInheritedAccessControlContext(this, INNOCUOUS_ACC);
108 this.pool = pool;
109 this.workQueue = pool.registerWorker(this);
110 }
111
112 /**
113 * Version for InnocuousForkJoinWorkerThread.
114 */
115 ForkJoinWorkerThread(ForkJoinPool pool,
116 ClassLoader ccl,
117 ThreadGroup threadGroup,
118 AccessControlContext acc) {
119 super(threadGroup, null, "aForkJoinWorkerThread");
120 super.setContextClassLoader(ccl);
121 ThreadLocalRandom.setInheritedAccessControlContext(this, acc);
122 ThreadLocalRandom.eraseThreadLocals(this); // clear before registering
123 this.pool = pool;
124 this.workQueue = pool.registerWorker(this);
125 }
126
127 /**
128 * Returns the pool hosting this thread.
129 *
130 * @return the pool
131 */
132 public ForkJoinPool getPool() {
133 return pool;
134 }
135
136 /**
137 * Returns the unique index number of this thread in its pool.
138 * The returned value ranges from zero to the maximum number of
139 * threads (minus one) that may exist in the pool, and does not
140 * change during the lifetime of the thread. This method may be
141 * useful for applications that track status or collect results
142 * per-worker-thread rather than per-task.
143 *
144 * @return the index number
145 */
146 public int getPoolIndex() {
147 return workQueue.getPoolIndex();
148 }
149
150 /**
151 * Initializes internal state after construction but before
152 * processing any tasks. If you override this method, you must
153 * invoke {@code super.onStart()} at the beginning of the method.
154 * Initialization requires care: Most fields must have legal
155 * default values, to ensure that attempted accesses from other
156 * threads work correctly even before this thread starts
157 * processing tasks.
158 */
159 protected void onStart() {
160 }
161
162 /**
163 * Performs cleanup associated with termination of this worker
164 * thread. If you override this method, you must invoke
165 * {@code super.onTermination} at the end of the overridden method.
166 *
167 * @param exception the exception causing this thread to abort due
168 * to an unrecoverable error, or {@code null} if completed normally
169 */
170 protected void onTermination(Throwable exception) {
171 }
172
173 /**
174 * This method is required to be public, but should never be
175 * called explicitly. It performs the main run loop to execute
176 * {@link ForkJoinTask}s.
177 */
178 public void run() {
179 if (workQueue.array == null) { // only run once
180 Throwable exception = null;
181 try {
182 onStart();
183 pool.runWorker(workQueue);
184 } catch (Throwable ex) {
185 exception = ex;
186 } finally {
187 try {
188 onTermination(exception);
189 } catch (Throwable ex) {
190 if (exception == null)
191 exception = ex;
192 } finally {
193 pool.deregisterWorker(this, exception);
194 }
195 }
196 }
197 }
198
199 /**
200 * Non-public hook method for InnocuousForkJoinWorkerThread.
201 */
202 void afterTopLevelExec() {
203 }
204
205 /**
206 * A worker thread that has no permissions, is not a member of any
207 * user-defined ThreadGroup, uses the system class loader as
208 * thread context class loader, and erases all ThreadLocals after
209 * running each top-level task.
210 */
211 static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
212 /** The ThreadGroup for all InnocuousForkJoinWorkerThreads */
213 private static final ThreadGroup innocuousThreadGroup =
214 AccessController.doPrivileged(new PrivilegedAction<>() {
215 public ThreadGroup run() {
216 ThreadGroup group = Thread.currentThread().getThreadGroup();
217 for (ThreadGroup p; (p = group.getParent()) != null; )
218 group = p;
219 return new ThreadGroup(
220 group, "InnocuousForkJoinWorkerThreadGroup");
221 }});
222
223 InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
224 super(pool,
225 ClassLoader.getSystemClassLoader(),
226 innocuousThreadGroup,
227 INNOCUOUS_ACC);
228 }
229
230 @Override // to erase ThreadLocals
231 void afterTopLevelExec() {
232 ThreadLocalRandom.eraseThreadLocals(this);
233 }
234
235 @Override // to silently fail
236 public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }
237
238 @Override // paranoically
239 public void setContextClassLoader(ClassLoader cl) {
240 if (cl != null && ClassLoader.getSystemClassLoader() != cl)
241 throw new SecurityException("setContextClassLoader");
242 }
243 }
244 }