Doing Two or More Tasks At Once: Threads

Using the notifyAll and wait Methods

The CubbyHole stores its value in a private member variable called contents. CubbyHole has another private member variable, available, that is a boolean. available is true when the value has just been put but not yet gotten and is false when the value has been gotten but not yet put. So, here's one possible implementation for the put and get methods:

public synchronized int get() {    // won't work!
    if (available == true) {
        available = false;
        return contents;
    }
}
public synchronized int put(int value) {    // won't work!
    if (available == false) {
        available = true;
        value = ;
    }
}

As implemented, these two methods won't work. Look at the get method. What happens if the Producer hasn't put anything in the CubbyHole and available isn't true? get does nothing. Similarly, if the Producer calls put before the Consumer got the value, put doesn't do anything.

You really want the Consumer to wait until the Producer puts something in the CubbyHole and the Producer must notify the Consumer when it's done so. Similarly, the Producer must wait until the Consumer takes a value (and notifies the Producer of its activities) before replacing it with a new value. The two threads must coordinate more fully and can use Object's wait and notifyAll methods to do so.

Here are the new implementations of get and put that wait on and notify each other of their activities:

public synchronized int get() {
    while (available == false) {
        try {
            // wait for Producer to put value
            wait();
        } catch (InterruptedException e) {
        }
    }
    available = false;
    // notify Producer that value has been retrieved
    notifyAll();
    return contents;
}
public synchronized void put(int value) {
    while (available == true) {
        try {
            // wait for Consumer to get value
            wait();
        } catch (InterruptedException e) {
        }
    }
    contents = value;
    available = true;
    // notify Consumer that value has been set
    notifyAll();
} 

The code in the get method loops until the Producer has produced a new value. Each time through the loop, get calls the wait method. The wait method relinquishes the lock held by the Consumer on the CubbyHole (thereby allowing the Producer to get the lock and update the CubbyHole) and then waits for notification from the Producer. When the Producer puts something in the CubbyHole, it notifies the Consumer by calling notifyAll. The Consumer then comes out of the wait state, available is now true, the loop exits, and the get method returns the value in the CubbyHole.

The put method works in a similar fashion, waiting for the Consumer thread to consume the current value before allowing the Producer to produce a new one.

The notifyAll method wakes up all threads waiting on the object in question (in this case, the CubbyHole). The awakened threads compete for the lock. One thread gets it, and the others go back to waiting. The Object class also defines the notify method, which arbitrarily wakes up one of the threads waiting on this object.

The Object class contains not only the version of wait that is used in the producer/consumer example and which waits indefinitely for notification, but also two other versions of the wait method:

wait(long timeout)

Waits for notification or until the timeout period has elapsed. timeout is measured in milliseconds.

wait(long timeout, int nanos)

Waits for notification or until timeout milliseconds plus nanos nanoseconds have elapsed.

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Note: Besides using these timed wait methods to synchronize threads, you also can use them in place of sleep. Both wait and sleep delay for the requested amount of time, but you can easily wake up wait with a notify but a sleeping thread cannot be awoken prematurely. This doesn't matter too much for threads that don't sleep for long, but it could be important for threads that sleep for minutes at a time.

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Doing Two or More Tasks At Once: Threads