util/concurrent/ForkJoinWorkerThread.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.security.AccessControlContext;
import java.security.ProtectionDomain;

/**
 * A thread managed by a {@link ForkJoinPool}, which executes
 * {@link ForkJoinTask}s.
 * This class is subclassable solely for the sake of adding
 * functionality -- there are no overridable methods dealing with
 * scheduling or execution.  However, you can override initialization
 * and termination methods surrounding the main task processing loop.
 * If you do create such a subclass, you will also need to supply a
 * custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to
 * {@linkplain ForkJoinPool#ForkJoinPool use it} in a {@code ForkJoinPool}.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class ForkJoinWorkerThread extends Thread {
    /*
     * ForkJoinWorkerThreads are managed by ForkJoinPools and perform
     * ForkJoinTasks. For explanation, see the internal documentation
     * of class ForkJoinPool.
     *
     * This class just maintains links to its pool and WorkQueue.  The
     * pool field is set immediately upon construction, but the
     * workQueue field is not set until a call to registerWorker
     * completes. This leads to a visibility race, that is tolerated
     * by requiring that the workQueue field is only accessed by the
     * owning thread.
     *
     * Support for (non-public) subclass InnocuousForkJoinWorkerThread
     * requires that we break quite a lot of encapsulation (via Unsafe)
     * both here and in the subclass to access and set Thread fields.
     */

    final ForkJoinPool pool;                // the pool this thread works in
    final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics

    /**
     * Creates a ForkJoinWorkerThread operating in the given pool.
     *
     * @param pool the pool this thread works in
     * @throws NullPointerException if pool is null
     */
    protected ForkJoinWorkerThread(ForkJoinPool pool) {
        // Use a placeholder until a useful name can be set in registerWorker
        super("aForkJoinWorkerThread");
        this.pool = pool;
        this.workQueue = pool.registerWorker(this);
    }

    /**
     * Version for InnocuousForkJoinWorkerThread
     */
    ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup,
                         AccessControlContext acc) {
        super(threadGroup, null, "aForkJoinWorkerThread");
        U.putOrderedObject(this, INHERITEDACCESSCONTROLCONTEXT, acc);
        eraseThreadLocals(); // clear before registering
        this.pool = pool;
        this.workQueue = pool.registerWorker(this);
    }

    /**
     * Returns the pool hosting this thread.
     *
     * @return the pool
     */
    public ForkJoinPool getPool() {
        return pool;
    }

    /**
     * Returns the unique index number of this thread in its pool.
     * The returned value ranges from zero to the maximum number of
     * threads (minus one) that may exist in the pool, and does not
     * change during the lifetime of the thread.  This method may be
     * useful for applications that track status or collect results
     * per-worker-thread rather than per-task.
     *
     * @return the index number
     */
    public int getPoolIndex() {
        return workQueue.getPoolIndex();
    }

    /**
     * Initializes internal state after construction but before
     * processing any tasks. If you override this method, you must
     * invoke {@code super.onStart()} at the beginning of the method.
     * Initialization requires care: Most fields must have legal
     * default values, to ensure that attempted accesses from other
     * threads work correctly even before this thread starts
     * processing tasks.
     */
    protected void onStart() {
    }

    /**
     * Performs cleanup associated with termination of this worker
     * thread.  If you override this method, you must invoke
     * {@code super.onTermination} at the end of the overridden method.
     *
     * @param exception the exception causing this thread to abort due
     * to an unrecoverable error, or {@code null} if completed normally
     */
    protected void onTermination(Throwable exception) {
    }

    /**
     * This method is required to be public, but should never be
     * called explicitly. It performs the main run loop to execute
     * {@link ForkJoinTask}s.
     */
    public void run() {
        if (workQueue.array == null) { // only run once
            Throwable exception = null;
            try {
                onStart();
                pool.runWorker(workQueue);
            } catch (Throwable ex) {
                exception = ex;
            } finally {
                try {
                    onTermination(exception);
                } catch (Throwable ex) {
                    if (exception == null)
                        exception = ex;
                } finally {
                    pool.deregisterWorker(this, exception);
                }
            }
        }
    }

    /**
     * Erases ThreadLocals by nulling out Thread maps.
     */
    final void eraseThreadLocals() {
        U.putObject(this, THREADLOCALS, null);
        U.putObject(this, INHERITABLETHREADLOCALS, null);
    }

    /**
     * Non-public hook method for InnocuousForkJoinWorkerThread
     */
    void afterTopLevelExec() {
    }

    // Set up to allow setting thread fields in constructor
    private static final sun.misc.Unsafe U;
    private static final long THREADLOCALS;
    private static final long INHERITABLETHREADLOCALS;
    private static final long INHERITEDACCESSCONTROLCONTEXT;
    static {
        try {
            U = sun.misc.Unsafe.getUnsafe();
            Class<?> tk = Thread.class;
            THREADLOCALS = U.objectFieldOffset
                (tk.getDeclaredField("threadLocals"));
            INHERITABLETHREADLOCALS = U.objectFieldOffset
                (tk.getDeclaredField("inheritableThreadLocals"));
            INHERITEDACCESSCONTROLCONTEXT = U.objectFieldOffset
                (tk.getDeclaredField("inheritedAccessControlContext"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }

    /**
     * A worker thread that has no permissions, is not a member of any
     * user-defined ThreadGroup, and erases all ThreadLocals after
     * running each top-level task.
     */
    static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
        /** The ThreadGroup for all InnocuousForkJoinWorkerThreads */
        private static final ThreadGroup innocuousThreadGroup =
            createThreadGroup();

        /** An AccessControlContext supporting no privileges */
        private static final AccessControlContext INNOCUOUS_ACC =
            new AccessControlContext(
                new ProtectionDomain[] {
                    new ProtectionDomain(null, null)
                });

        InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
            super(pool, innocuousThreadGroup, INNOCUOUS_ACC);
        }

        @Override // to erase ThreadLocals
        void afterTopLevelExec() {
            eraseThreadLocals();
        }

        @Override // to always report system loader
        public ClassLoader getContextClassLoader() {
            return ClassLoader.getSystemClassLoader();
        }

        @Override // to silently fail
        public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }

        @Override // paranoically
        public void setContextClassLoader(ClassLoader cl) {
            throw new SecurityException("setContextClassLoader");
        }

        /**
         * Returns a new group with the system ThreadGroup (the
         * topmost, parent-less group) as parent.  Uses Unsafe to
         * traverse Thread.group and ThreadGroup.parent fields.
         */
        private static ThreadGroup createThreadGroup() {
            try {
                sun.misc.Unsafe u = sun.misc.Unsafe.getUnsafe();
                Class<?> tk = Thread.class;
                Class<?> gk = ThreadGroup.class;
                long tg = u.objectFieldOffset(tk.getDeclaredField("group"));
                long gp = u.objectFieldOffset(gk.getDeclaredField("parent"));
                ThreadGroup group = (ThreadGroup)
                    u.getObject(Thread.currentThread(), tg);
                while (group != null) {
                    ThreadGroup parent = (ThreadGroup)u.getObject(group, gp);
                    if (parent == null)
                        return new ThreadGroup(group,
                                               "InnocuousForkJoinWorkerThreadGroup");
                    group = parent;
                }
            } catch (ReflectiveOperationException e) {
                throw new Error(e);
            }
            // fall through if null as cannot-happen safeguard
            throw new Error("Cannot create ThreadGroup");
        }
    }

}
Revision:	1.66
Committed:	Sun Jan 4 01:06:15 2015 UTC (9 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.65:	+2 -3 lines
Log Message:	use ReflectiveOperationException for Unsafe mechanics
#	User	Rev	Content
1	jsr166	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4	jsr166	1.46	* http://creativecommons.org/publicdomain/zero/1.0/
5	jsr166	1.1	*/
6
7			package java.util.concurrent;
8
9	dl	1.60	import java.security.AccessControlContext;
10			import java.security.ProtectionDomain;
11
12	jsr166	1.1	/**
13	jsr166	1.40	* A thread managed by a {@link ForkJoinPool}, which executes
14			* {@link ForkJoinTask}s.
15			* This class is subclassable solely for the sake of adding
16			* functionality -- there are no overridable methods dealing with
17			* scheduling or execution. However, you can override initialization
18			* and termination methods surrounding the main task processing loop.
19			* If you do create such a subclass, you will also need to supply a
20	jsr166	1.57	* custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to
21			* {@linkplain ForkJoinPool#ForkJoinPool use it} in a {@code ForkJoinPool}.
22	jsr166	1.1	*
23			* @since 1.7
24			* @author Doug Lea
25			*/
26			public class ForkJoinWorkerThread extends Thread {
27			/*
28	dl	1.14	* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
29	dl	1.52	* ForkJoinTasks. For explanation, see the internal documentation
30			* of class ForkJoinPool.
31	dl	1.55	*
32			* This class just maintains links to its pool and WorkQueue. The
33	dl	1.56	* pool field is set immediately upon construction, but the
34			* workQueue field is not set until a call to registerWorker
35			* completes. This leads to a visibility race, that is tolerated
36			* by requiring that the workQueue field is only accessed by the
37			* owning thread.
38	dl	1.60	*
39			* Support for (non-public) subclass InnocuousForkJoinWorkerThread
40	jsr166	1.62	* requires that we break quite a lot of encapsulation (via Unsafe)
41	dl	1.60	* both here and in the subclass to access and set Thread fields.
42	jsr166	1.1	*/
43
44	dl	1.52	final ForkJoinPool pool; // the pool this thread works in
45	dl	1.56	final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
46	dl	1.54
47			/**
48	jsr166	1.1	* Creates a ForkJoinWorkerThread operating in the given pool.
49			*
50			* @param pool the pool this thread works in
51			* @throws NullPointerException if pool is null
52			*/
53			protected ForkJoinWorkerThread(ForkJoinPool pool) {
54	dl	1.56	// Use a placeholder until a useful name can be set in registerWorker
55			super("aForkJoinWorkerThread");
56	dl	1.55	this.pool = pool;
57	dl	1.56	this.workQueue = pool.registerWorker(this);
58	dl	1.14	}
59
60	jsr166	1.1	/**
61	dl	1.60	* Version for InnocuousForkJoinWorkerThread
62			*/
63			ForkJoinWorkerThread(ForkJoinPool pool, ThreadGroup threadGroup,
64			AccessControlContext acc) {
65			super(threadGroup, null, "aForkJoinWorkerThread");
66			U.putOrderedObject(this, INHERITEDACCESSCONTROLCONTEXT, acc);
67			eraseThreadLocals(); // clear before registering
68			this.pool = pool;
69			this.workQueue = pool.registerWorker(this);
70			}
71
72			/**
73	jsr166	1.1	* Returns the pool hosting this thread.
74			*
75			* @return the pool
76			*/
77			public ForkJoinPool getPool() {
78			return pool;
79			}
80
81			/**
82	jsr166	1.59	* Returns the unique index number of this thread in its pool.
83			* The returned value ranges from zero to the maximum number of
84			* threads (minus one) that may exist in the pool, and does not
85			* change during the lifetime of the thread. This method may be
86			* useful for applications that track status or collect results
87			* per-worker-thread rather than per-task.
88	jsr166	1.1	*
89			* @return the index number
90			*/
91			public int getPoolIndex() {
92	dl	1.65	return workQueue.getPoolIndex();
93	dl	1.45	}
94
95	jsr166	1.1	/**
96	dl	1.14	* Initializes internal state after construction but before
97			* processing any tasks. If you override this method, you must
98	jsr166	1.39	* invoke {@code super.onStart()} at the beginning of the method.
99	dl	1.14	* Initialization requires care: Most fields must have legal
100			* default values, to ensure that attempted accesses from other
101			* threads work correctly even before this thread starts
102			* processing tasks.
103	jsr166	1.1	*/
104	dl	1.14	protected void onStart() {
105			}
106	jsr166	1.1
107			/**
108	dl	1.14	* Performs cleanup associated with termination of this worker
109			* thread. If you override this method, you must invoke
110			* {@code super.onTermination} at the end of the overridden method.
111	jsr166	1.4	*
112	dl	1.14	* @param exception the exception causing this thread to abort due
113			* to an unrecoverable error, or {@code null} if completed normally
114	jsr166	1.1	*/
115	dl	1.14	protected void onTermination(Throwable exception) {
116	jsr166	1.1	}
117
118			/**
119			* This method is required to be public, but should never be
120			* called explicitly. It performs the main run loop to execute
121	jsr166	1.40	* {@link ForkJoinTask}s.
122	jsr166	1.1	*/
123			public void run() {
124	dl	1.60	if (workQueue.array == null) { // only run once
125			Throwable exception = null;
126	dl	1.52	try {
127	dl	1.60	onStart();
128			pool.runWorker(workQueue);
129	dl	1.52	} catch (Throwable ex) {
130	dl	1.60	exception = ex;
131	dl	1.52	} finally {
132	dl	1.60	try {
133			onTermination(exception);
134			} catch (Throwable ex) {
135			if (exception == null)
136			exception = ex;
137			} finally {
138			pool.deregisterWorker(this, exception);
139			}
140			}
141			}
142			}
143
144			/**
145	jsr166	1.63	* Erases ThreadLocals by nulling out Thread maps.
146	dl	1.60	*/
147			final void eraseThreadLocals() {
148			U.putObject(this, THREADLOCALS, null);
149			U.putObject(this, INHERITABLETHREADLOCALS, null);
150			}
151
152			/**
153			* Non-public hook method for InnocuousForkJoinWorkerThread
154			*/
155			void afterTopLevelExec() {
156			}
157
158			// Set up to allow setting thread fields in constructor
159			private static final sun.misc.Unsafe U;
160			private static final long THREADLOCALS;
161			private static final long INHERITABLETHREADLOCALS;
162			private static final long INHERITEDACCESSCONTROLCONTEXT;
163			static {
164			try {
165			U = sun.misc.Unsafe.getUnsafe();
166			Class<?> tk = Thread.class;
167			THREADLOCALS = U.objectFieldOffset
168			(tk.getDeclaredField("threadLocals"));
169			INHERITABLETHREADLOCALS = U.objectFieldOffset
170			(tk.getDeclaredField("inheritableThreadLocals"));
171			INHERITEDACCESSCONTROLCONTEXT = U.objectFieldOffset
172			(tk.getDeclaredField("inheritedAccessControlContext"));
173	jsr166	1.66	} catch (ReflectiveOperationException e) {
174	dl	1.60	throw new Error(e);
175			}
176			}
177
178			/**
179			* A worker thread that has no permissions, is not a member of any
180			* user-defined ThreadGroup, and erases all ThreadLocals after
181			* running each top-level task.
182			*/
183			static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
184			/** The ThreadGroup for all InnocuousForkJoinWorkerThreads */
185			private static final ThreadGroup innocuousThreadGroup =
186			createThreadGroup();
187
188			/** An AccessControlContext supporting no privileges */
189			private static final AccessControlContext INNOCUOUS_ACC =
190			new AccessControlContext(
191			new ProtectionDomain[] {
192			new ProtectionDomain(null, null)
193			});
194
195			InnocuousForkJoinWorkerThread(ForkJoinPool pool) {
196			super(pool, innocuousThreadGroup, INNOCUOUS_ACC);
197			}
198
199			@Override // to erase ThreadLocals
200			void afterTopLevelExec() {
201			eraseThreadLocals();
202			}
203
204			@Override // to always report system loader
205			public ClassLoader getContextClassLoader() {
206			return ClassLoader.getSystemClassLoader();
207			}
208
209			@Override // to silently fail
210			public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }
211
212			@Override // paranoically
213			public void setContextClassLoader(ClassLoader cl) {
214			throw new SecurityException("setContextClassLoader");
215			}
216
217			/**
218			* Returns a new group with the system ThreadGroup (the
219	jsr166	1.62	* topmost, parent-less group) as parent. Uses Unsafe to
220	jsr166	1.64	* traverse Thread.group and ThreadGroup.parent fields.
221	dl	1.60	*/
222			private static ThreadGroup createThreadGroup() {
223			try {
224			sun.misc.Unsafe u = sun.misc.Unsafe.getUnsafe();
225			Class<?> tk = Thread.class;
226			Class<?> gk = ThreadGroup.class;
227			long tg = u.objectFieldOffset(tk.getDeclaredField("group"));
228			long gp = u.objectFieldOffset(gk.getDeclaredField("parent"));
229			ThreadGroup group = (ThreadGroup)
230			u.getObject(Thread.currentThread(), tg);
231			while (group != null) {
232			ThreadGroup parent = (ThreadGroup)u.getObject(group, gp);
233			if (parent == null)
234			return new ThreadGroup(group,
235			"InnocuousForkJoinWorkerThreadGroup");
236			group = parent;
237			}
238	jsr166	1.66	} catch (ReflectiveOperationException e) {
239	dl	1.60	throw new Error(e);
240	dl	1.45	}
241	dl	1.60	// fall through if null as cannot-happen safeguard
242			throw new Error("Cannot create ThreadGroup");
243	dl	1.45	}
244	jsr166	1.1	}
245	dl	1.60
246	jsr166	1.1	}