/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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.AccessController;

import java.security.PrivilegedAction;

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 helper

     * methods in ThreadLocalRandom) 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 use by the default pool.  Supports setting the

     * context class loader.  This is a separate constructor to avoid

     * affecting the protected constructor.

     */

    ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl) {

        super("aForkJoinWorkerThread");

        super.setContextClassLoader(ccl);

        this.pool = pool;

        this.workQueue = pool.registerWorker(this);

    }

    /**

     * Version for InnocuousForkJoinWorkerThread.

     */

    ForkJoinWorkerThread(ForkJoinPool pool,

                         ClassLoader ccl,

                         ThreadGroup threadGroup,

                         AccessControlContext acc) {

        super(threadGroup, null, "aForkJoinWorkerThread");

        super.setContextClassLoader(ccl);

        ThreadLocalRandom.setInheritedAccessControlContext(this, acc);

        ThreadLocalRandom.eraseThreadLocals(this); // 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);

                }

            }

        }

    }

    /**

     * Non-public hook method for InnocuousForkJoinWorkerThread.

     */

    void afterTopLevelExec() {

    }

    /**

     * A worker thread that has no permissions, is not a member of any

     * user-defined ThreadGroup, uses the system class loader as

     * thread context class loader, 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 =

            java.security.AccessController.doPrivileged(

                new java.security.PrivilegedAction<>() {

                    public ThreadGroup run() {

                        ThreadGroup group = Thread.currentThread().getThreadGroup();

                        for (ThreadGroup p; (p = group.getParent()) != null; )

                            group = p;

                        return new ThreadGroup(group, "InnocuousForkJoinWorkerThreadGroup");

                    }});

        /** An AccessControlContext supporting no privileges */

        private static final AccessControlContext INNOCUOUS_ACC =

            new AccessControlContext(

                new ProtectionDomain[] {

                    new ProtectionDomain(null, null)

                });

        InnocuousForkJoinWorkerThread(ForkJoinPool pool) {

            super(pool,

                  ClassLoader.getSystemClassLoader(),

                  innocuousThreadGroup,

                  INNOCUOUS_ACC);

        }

        @Override // to erase ThreadLocals

        void afterTopLevelExec() {

            ThreadLocalRandom.eraseThreadLocals(this);

        }

        @Override // to silently fail

        public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) { }

        @Override // paranoically

        public void setContextClassLoader(ClassLoader cl) {

            throw new SecurityException("setContextClassLoader");

        }

    }

}