File: WriterPreferenceReadWriteLock.java

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/*
  File: WriterPreferenceReadWriteLock.java

  Originally written by Doug Lea and released into the public domain.
  This may be used for any purposes whatsoever without acknowledgment.
  Thanks for the assistance and support of Sun Microsystems Labs,
  and everyone contributing, testing, and using this code.

  History:
  Date       Who                What
  11Jun1998  dl               Create public version
   5Aug1998  dl               replaced int counters with longs
  25aug1998  dl               record writer thread
   3May1999  dl               add notifications on interrupt/timeout

*/

package EDU.oswego.cs.dl.util.concurrent;

/** 
 * A ReadWriteLock that prefers waiting writers over
 * waiting readers when there is contention. This class
 * is adapted from the versions described in CPJ, improving
 * on the ones there a bit by segregating reader and writer
 * wait queues, which is typically more efficient.
 * <p>
 * The locks are <em>NOT</em> reentrant. In particular,
 * even though it may appear to usually work OK,
 * a thread holding a read lock should not attempt to
 * re-acquire it. Doing so risks lockouts when there are
 * also waiting writers.
 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
 **/

public class WriterPreferenceReadWriteLock implements ReadWriteLock {

  protected long activeReaders_ = 0; 
  protected Thread activeWriter_ = null;
  protected long waitingReaders_ = 0;
  protected long waitingWriters_ = 0;


  protected final ReaderLock readerLock_ = new ReaderLock();
  protected final WriterLock writerLock_ = new WriterLock();

  public Sync writeLock() { return writerLock_; }
  public Sync readLock() { return readerLock_; }

  /*
    A bunch of small synchronized methods are needed
    to allow communication from the Lock objects
    back to this object, that serves as controller
  */


  protected synchronized void cancelledWaitingReader() { --waitingReaders_; }
  protected synchronized void cancelledWaitingWriter() { --waitingWriters_; }


  /** Override this method to change to reader preference **/
  protected boolean allowReader() {
    return activeWriter_ == null && waitingWriters_ == 0;
  }


  protected synchronized boolean startRead() {
    boolean allowRead = allowReader();
    if (allowRead)  ++activeReaders_;
    return allowRead;
  }

  protected synchronized boolean startWrite() {

    // The allowWrite expression cannot be modified without
    // also changing startWrite, so is hard-wired

    boolean allowWrite = (activeWriter_ == null && activeReaders_ == 0);
    if (allowWrite)  activeWriter_ = Thread.currentThread();
    return allowWrite;
   }


  /* 
     Each of these variants is needed to maintain atomicity
     of wait counts during wait loops. They could be
     made faster by manually inlining each other. We hope that
     compilers do this for us though.
  */

  protected synchronized boolean startReadFromNewReader() {
    boolean pass = startRead();
    if (!pass) ++waitingReaders_;
    return pass;
  }

  protected synchronized boolean startWriteFromNewWriter() {
    boolean pass = startWrite();
    if (!pass) ++waitingWriters_;
    return pass;
  }

  protected synchronized boolean startReadFromWaitingReader() {
    boolean pass = startRead();
    if (pass) --waitingReaders_;
    return pass;
  }

  protected synchronized boolean startWriteFromWaitingWriter() {
    boolean pass = startWrite();
    if (pass) --waitingWriters_;
    return pass;
  }

  /**
   * Called upon termination of a read.
   * Returns the object to signal to wake up a waiter, or null if no such
   **/
  protected synchronized Signaller endRead() {
    if (--activeReaders_ == 0 && waitingWriters_ > 0)
      return writerLock_;
    else
      return null;
  }

  
  /**
   * Called upon termination of a write.
   * Returns the object to signal to wake up a waiter, or null if no such
   **/
  protected synchronized Signaller endWrite() {
    activeWriter_ = null;
    if (waitingReaders_ > 0 && allowReader())
      return readerLock_;
    else if (waitingWriters_ > 0)
      return writerLock_;
    else
      return null;
  }


  /**
   * Reader and Writer requests are maintained in two different
   * wait sets, by two different objects. These objects do not
   * know whether the wait sets need notification since they
   * don't know preference rules. So, each supports a
   * method that can be selected by main controlling object
   * to perform the notifications.  This base class simplifies mechanics.
   **/

  protected abstract class Signaller  { // base for ReaderLock and WriterLock
    abstract void signalWaiters();
  }

  protected class ReaderLock extends Signaller implements Sync {

    public  void acquire() throws InterruptedException {
      if (Thread.interrupted()) throw new InterruptedException();
      InterruptedException ie = null;
      synchronized(this) {
        if (!startReadFromNewReader()) {
          for (;;) {
            try { 
              ReaderLock.this.wait();  
              if (startReadFromWaitingReader())
                return;
            }
            catch(InterruptedException ex){
              cancelledWaitingReader();
              ie = ex;
              break;
            }
          }
        }
      }
      if (ie != null) {
        // fall through outside synch on interrupt.
        // This notification is not really needed here, 
        //   but may be in plausible subclasses
        writerLock_.signalWaiters();
        throw ie;
      }
    }


    public void release() {
      Signaller s = endRead();
      if (s != null) s.signalWaiters();
    }


    synchronized void signalWaiters() { ReaderLock.this.notifyAll(); }

    public boolean attempt(long msecs) throws InterruptedException { 
      if (Thread.interrupted()) throw new InterruptedException();
      InterruptedException ie = null;
      synchronized(this) {
        if (msecs <= 0)
          return startRead();
        else if (startReadFromNewReader()) 
          return true;
        else {
          long waitTime = msecs;
          long start = System.currentTimeMillis();
          for (;;) {
            try { ReaderLock.this.wait(waitTime);  }
            catch(InterruptedException ex){
              cancelledWaitingReader();
              ie = ex;
              break;
            }
            if (startReadFromWaitingReader())
              return true;
            else {
              waitTime = msecs - (System.currentTimeMillis() - start);
              if (waitTime <= 0) {
                cancelledWaitingReader();
                break;
              }
            }
          }
        }
      }
      // safeguard on interrupt or timeout:
      writerLock_.signalWaiters();
      if (ie != null) throw ie;
      else return false; // timed out
    }

  }

  protected class WriterLock extends Signaller implements  Sync {

    public void acquire() throws InterruptedException {
      if (Thread.interrupted()) throw new InterruptedException();
      InterruptedException ie = null;
      synchronized(this) {
        if (!startWriteFromNewWriter()) {
          for (;;) {
            try { 
              WriterLock.this.wait();  
              if (startWriteFromWaitingWriter())
                return;
            }
            catch(InterruptedException ex){
              cancelledWaitingWriter();
              WriterLock.this.notify();
              ie = ex;
              break;
            }
          }
        }
      }
      if (ie != null) {
        // Fall through outside synch on interrupt.
        //  On exception, we may need to signal readers.
        //  It is not worth checking here whether it is strictly necessary.
        readerLock_.signalWaiters();
        throw ie;
      }
    }

    public void release(){
      Signaller s = endWrite();
      if (s != null) s.signalWaiters();
    }

    synchronized void signalWaiters() { WriterLock.this.notify(); }

    public boolean attempt(long msecs) throws InterruptedException { 
      if (Thread.interrupted()) throw new InterruptedException();
      InterruptedException ie = null;
      synchronized(this) {
        if (msecs <= 0)
          return startWrite();
        else if (startWriteFromNewWriter()) 
          return true;
        else {
          long waitTime = msecs;
          long start = System.currentTimeMillis();
          for (;;) {
            try { WriterLock.this.wait(waitTime);  }
            catch(InterruptedException ex){
              cancelledWaitingWriter();
              WriterLock.this.notify();
              ie = ex;
              break;
            }
            if (startWriteFromWaitingWriter())
              return true;
            else {
              waitTime = msecs - (System.currentTimeMillis() - start);
              if (waitTime <= 0) {
                cancelledWaitingWriter();
                WriterLock.this.notify();
                break;
              }
            }
          }
        }
      }
      
      readerLock_.signalWaiters();
      if (ie != null) throw ie;
      else return false; // timed out
    }

  }



}