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      */
ForkJoinWorkerThread(ForkJoinPool pool)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      */
ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl)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      */
ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl, ThreadGroup threadGroup, AccessControlContext acc)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      */
getPool()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      */
getPoolIndex()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      */
onStart()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      */
onTermination(Throwable exception)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      */
run()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      */
afterTopLevelExec()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 
InnocuousForkJoinWorkerThread(ForkJoinPool pool)223         InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
224             super(pool,
225                   ClassLoader.getSystemClassLoader(),
226                   innocuousThreadGroup,
227                   INNOCUOUS_ACC);
228         }
229 
230         @Override // to erase ThreadLocals
afterTopLevelExec()231         void afterTopLevelExec() {
232             ThreadLocalRandom.eraseThreadLocals(this);
233         }
234 
235         @Override // to silently fail
setUncaughtExceptionHandler(UncaughtExceptionHandler x)236         public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }
237 
238         @Override // paranoically
setContextClassLoader(ClassLoader cl)239         public void setContextClassLoader(ClassLoader cl) {
240             if (cl != null && ClassLoader.getSystemClassLoader() != cl)
241                 throw new SecurityException("setContextClassLoader");
242         }
243     }
244 }
245