Conditional wait

Question

I have the following class using synchronized in an attempt to prevent access to a key until it has been generated:

public class KeyManager {

  private String key;

  public KeyManager() {
      genKey();
  }

  private void genKey() {
    new Thread(new Runnable() { 
      synchronized(KeyManager.this) {
        public void run() {
          key = operationThatTakesALongTime();
        }
      }
    }).start();
  }

  synchronized public String getKey() {
    return key;
  }

}

The problem is that getKey() sometimes gets called before the inner thread and it grabs the lock first.

What I really need is a wait in getKey() that waits only if the key is null. How to do this?

Answer 1

I would use a CountDownLatch instead of reimplementing the logic.

I would also probably avoid starting a new Thread from the constructor, because it can lead to subtle concurrency bugs - instead you can simply make the genKey method public and add the relevant javadoc (this method must be called first blabla):

public class KeyManager {

  private volatile boolean genStarted = false;
  private final CountDownLatch keyGenerated = new CountDownLatch(1);
  private String key;

  public void genKey() {
    genStarted = true;
    new Thread(new Runnable() { 
      public void run() {
        key = operationThatTakesALongTime();
        keyGenerated.countDown();
      }
    }).start();
  }

  public String getKey() throws InterruptedException {
    if (!genStarted) throw new IllegalStateException("you must run genKey first");
    keyGenerated.await();
    return key;
  }
}

Answer 2

Mhm

synchronized public long getKey() 
{
    while (key == null) 
    { 
        try 
        { 
            wait(); 
        } 
        catch(InterruptedException e) 
        { 
            /* handle here */ 
        } 
    }
    return key;
}

wait will let the thread to release the lock, then could take it again when notify is called. If key is still null it will release the lock again.

But i think it's not the best way to do this.

It throws InterruptedException.