Threads of Control

Synchronizing Threads

So far, this lesson has contained examples with independent, asynchronous threads. That is, each thread contained all of the data and methods required for its execution and didn't require any outside resources or methods. In addition, the threads in those examples ran at their own pace without concern over the state or activities of any other concurrently running threads.

However, there are many interesting situations where separate, concurrently running threads do share data and must consider the state and activities of other threads. One such set of programming situations are known as producer/consumer scenarios where the producer generates a stream of data which then is consumed by a consumer.

For example, imagine a Java application where one thread (the producer) writes data to a file while a second thread (the consumer) reads data from the same file. Or, as you type characters on the keyboard, the producer thread places key events in an event queue and the consumer thread reads the events from the same queue. Both of these examples use concurrent threads that share a common resource: the first shares a file, the second shares an event queue. Because the threads share a common resource, they must be synchronized in some way.

This lesson teaches you about Java thread synchronization through a simple producer/consumer example.

Producer/Consumer Example

The Producer generates an integer between 0 and 9 (inclusive), stores it in a CubbyHole object, and prints the generated number. To make the synchronization problem more interesting, the Producer sleeps for a random amount of time between 0 and 100 milliseconds before repeating the number generating cycle:

class Producer extends Thread {
    private CubbyHole cubbyhole;
    private int number;

    public Producer(CubbyHole c, int number) {
        cubbyhole = c;
        this.number = number;
    }

    public void run() {
        for (int i = 0; i < 10; i++) {
            cubbyhole.put(i);
            System.out.println("Producer #" + this.number + " put: " + i);
            try {
                sleep((int)(Math.random() * 100));
            } catch (InterruptedException e) {
            }
        }
    }
}

The Consumer, being ravenous, consumes all integers from the CubbyHole (the exact same object into which the Producer put the integers in the first place) as quickly as they become available.

class Consumer extends Thread {
    private CubbyHole cubbyhole;
    private int number;

    public Consumer(CubbyHole c, int number) {
        cubbyhole = c;
        this.number = number;
    }

    public void run() {
        int value = 0;
        for (int i = 0; i < 10; i++) {
            value = cubbyhole.get();
            System.out.println("Consumer #" + this.number + " got: " + value);
        }
    }
}

The Producer and Consumer in this example share data through a common CubbyHole object. And you will note that neither the Producer nor the Consumer makes any effort whatsoever to ensure that the Consumer is getting each value produced once and only once. The synchronization between these two threads actually occurs at a lower level, within the get and put methods of the CubbyHole object. However, let's assume for a moment that these two threads make no arrangements for synchronization and talk about the potential problems that might arise in that situation.

One problem arises when the Producer is quicker than the Consumer and generates two numbers before the Consumer has a chance to consume the first one. Thus the Consumer would skip a number. Part of the output might look like this:

    . . .

Consumer #1 got: 3
Producer #1 put: 4
Producer #1 put: 5
Consumer #1 got: 5

    . . .

Another problem that might arise is when the Consumer is quicker than the Producer and consumes the same value twice. In this situation, the Consumer would print the same value twice and might produce output that looked like this:

    . . .

Producer #1 put: 4
Consumer #1 got: 4
Consumer #1 got: 4
Producer #1 put: 5

    . . .

Either way, the result is wrong. You want the Consumer to get each integer produced by the Producer exactly once. Problems such as those just described are called race conditions. They arise from multiple, asynchronously executing threads trying to access a single object at the same time and getting the wrong result.

To prevent race conditions in our producer/consumer example, the storage of a new integer into the CubbyHole by the Producer must be synchronized with the retrieval of an integer from the CubbyHole by the Consumer. The Consumer must consume each integer exactly once. The producer/consumer program uses two different mechanisms to synchronize the Producer thread and the Consumer thread: monitors, and the notifyAll and wait methods.

Monitors

Objects such as the CubbyHole that are shared between two threads and whose accesses must be synchronized are called condition variables. The Java language allows you to synchronize threads around a condition variable through the use of monitors. Monitors prevent two threads from simultaneously accessing the same variable.

The notifyAll and wait Methods

At a higher level, the producer/consumer example uses Object's notifyAll and wait methods to coordinate the Producer and Consumer's activity. The CubbyHole uses notifyAll and wait to ensure that each value placed in the CubbyHole by the Producer is retrieved once and only once by the Consumer.

The Main Program

Here's a small stand-alone Java application that creates a CubbyHole object, a Producer, a Consumer, and then starts both the Producer and the Consumer.

class ProducerConsumerTest {
    public static void main(String[] args) {
        CubbyHole c = new CubbyHole();
        Producer p1 = new Producer(c, 1);
        Consumer c1 = new Consumer(c, 1);

        p1.start();
        c1.start();
    }
}

The Output

Here's the output of ProducerConsumerTest.

Producer #1 put: 0
Consumer #1 got: 0
Producer #1 put: 1
Consumer #1 got: 1
Producer #1 put: 2
Consumer #1 got: 2
Producer #1 put: 3
Consumer #1 got: 3
Producer #1 put: 4
Consumer #1 got: 4
Producer #1 put: 5
Consumer #1 got: 5
Producer #1 put: 6
Consumer #1 got: 6
Producer #1 put: 7
Consumer #1 got: 7
Producer #1 put: 8
Consumer #1 got: 8
Producer #1 put: 9
Consumer #1 got: 9

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Threads of Control