alluxio.concurrent.jsr.ForkJoinWorkerThread Maven / Gradle / Ivy

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/*
 * 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.alluxio.shaded.client.org.publicdomain/zero/1.0/
 */
package alluxio.concurrent.jsr;

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 
 * use it in a {@code ForkJoinPool}.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class ForkJoinWorkerThread extends Thread {
  // CVS rev. 1.76
  /*
   * 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 alluxio.shaded.client.com.letes.
   * 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.
   */

  // A placeholder name until a useful name can be set in registerWorker
  private static final String NAME_PLACEHOLDER = "aForkJoinWorkerThread";

  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(NAME_PLACEHOLDER);
    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(NAME_PLACEHOLDER);
    TLRandom.setContextClassLoader(this, ccl); // java9-concurrent-backport changed
    this.pool = pool;
    this.workQueue = pool.registerWorker(this);
  }

  /**
   * Version for InnocuousForkJoinWorkerThread.
   */
  ForkJoinWorkerThread(ForkJoinPool pool, ClassLoader ccl, ThreadGroup threadGroup,
      AccessControlContext acc) {
    super(threadGroup, NAME_PLACEHOLDER);
    super.setContextClassLoader(ccl);
    TLRandom.setInheritedAccessControlContext(this, acc);
    TLRandom.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 alluxio.shaded.client.com.leted 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 =
        AccessController.doPrivileged(new 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() {
      TLRandom.eraseThreadLocals(this);
    }

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

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