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             if (cl != null && ClassLoader.getSystemClassLoader() != cl)
 240                 throw new SecurityException("setContextClassLoader");
 241         }
 242     }
 243 }