What is java's equivalent of ManualResetEvent?

问题:

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• Java Equivalent of .NET's ManualResetEvent and WaitHandle 4 answers

What is java's equivalent of ManualResetEvent?

回答1:

The closest I know of is the Semaphore. Just use it with a "permit" count of 1, and aquire/release will be pretty much the same as what you know from the ManualResetEvent.

A semaphore initialized to one, and which is used such that it only has at most one permit available, can serve as a mutual exclusion lock. This is more commonly known as a binary semaphore, because it only has two states: one permit available, or zero permits available. When used in this way, the binary semaphore has the property (unlike many Lock implementations), that the "lock" can be released by a thread other than the owner (as semaphores have no notion of ownership). This can be useful in some specialized contexts, such as deadlock recovery.

回答2:

class ManualResetEvent {

  private final Object monitor = new Object();
  private volatile boolean open = false;

  public ManualResetEvent(boolean open) {
    this.open = open;
  }

  public void waitOne() throws InterruptedException {
    synchronized (monitor) {
      while (open==false) {
          monitor.wait();
      }
    }
  }

  public boolean waitOne(long milliseconds) throws InterruptedException {
    synchronized (monitor) {
      if (open) 
        return true;
      monitor.wait(milliseconds);
        return open;
    }
  }

  public void set() {//open start
    synchronized (monitor) {
      open = true;
      monitor.notifyAll();
    }
  }

  public void reset() {//close stop
    open = false;
  }
}

回答3:

Try CountDownLatch with count of one.

CountDownLatch startSignal = new CountDownLatch(1);

回答4:

Based on:

ManualResetEvent allows threads to communicate with each other by signaling. Typically, this communication concerns a task which one thread must complete before other threads can proceed.

from here:

http://msdn.microsoft.com/en-us/library/system.threading.manualresetevent.aspx

you possibly want to look at the Barriers in the Java concurrency package - specifically CyclicBarrier I believe:

http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/CyclicBarrier.html

It blocks a fixed number of threads until a particular event has occured. All the threads must come together at a barrier point.

回答5:

I believe the crux of the .NET MRE is thread affinity and its ability to let all waiting threads go through when Set is called. I found the use of the Semaphore works well. However, if I get 10 or 15 threads waiting, then I run into another issue. Specifically, it occurs when Set is called. In .Net, all waiting threads are released. Using a semphore does not release all. So I wrapped it in a class. NOTE: I am very familiar with .NET threading. I am relatively new to Java threading and synchronization. Nevertheless, I am willing to jump in and get some real feedback. Here's my implementation with assumptions that a Java novice would make:

public class ManualEvent {
private final static int MAX_WAIT = 1000;
private final static String TAG = "ManualEvent"; 
private Semaphore semaphore = new Semaphore(MAX_WAIT, false);

private volatile boolean signaled = false;
public ManualEvent(boolean signaled) {
    this.signaled = signaled; 
    if (!signaled) {
        semaphore.drainPermits();
    }
}

public boolean WaitOne() {
    return WaitOne(Long.MAX_VALUE);
}

private volatile int count = 0;
public boolean WaitOne(long millis) {
    boolean bRc = true;
    if (signaled)
        return true;

    try {
        ++count;
        if (count > MAX_WAIT) {
            Log.w(TAG, "More requests than waits: " + String.valueOf(count));
        }

        Log.d(TAG, "ManualEvent WaitOne Entered");
        bRc = semaphore.tryAcquire(millis, TimeUnit.MILLISECONDS);
        Log.d(TAG, "ManualEvent WaitOne=" + String.valueOf(bRc));
    }
    catch (InterruptedException e) {
        bRc = false;
    }
    finally {
        --count;
    }

    Log.d(TAG, "ManualEvent WaitOne Exit");
    return bRc;
}

public void Set() {
    Log.d(TAG, "ManualEvent Set");
    signaled = true;
    semaphore.release(MAX_WAIT);
}

public void Reset() {
    signaled = false;
    //stop any new requests
    int count = semaphore.drainPermits();
    Log.d(TAG, "ManualEvent Reset: Permits drained=" + String.valueOf(count));
}

}

Also note that I am basically betting that there's no more than a 1000 requests waiting for a release at any given time. By releasing and aquiring in batches, I am attempting to release any waiting threads. Note the call to WaitOne is working 1 permit at a time.