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