Threads - synchronized kewyord

• threads often need to share a common resource ie a file, with one thread reading from the file while another thread writes to the file
• this is an example of a producer/consumer relationship

Race conditions

• race conditions occur when multiple, asynchronously executing threads access the same object returning unexpected (wrong) results
• they can be avoided by synchronizing the methods which access the shared resource
• the Sun Thread tutorial has an example which uses a Producer class, and a Consumer class which respectively write and read integers from a CubbyHole class. If the CubbyHole class is unsynchronized, as in the following code:

public class CubbyHole {
    private int contents;

    public int get() {
        return contents;
    }

    public synchronized void put(int value) {
        contents = value;
    }
}

Example output from an unsynchronized Producer/Consumer

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

• results are unpredictable; a number may be read before a number has been produced or multiple numbers may be produced with only one or two being read
• adding synchronization ensures that a number is first produced, then read in the correct order

public class CubbyHole {
    private int contents;
    private boolean available = false;

    public synchronized int get() {
        while (available == false) {
            try {
                wait();
            } catch (InterruptedException e) { }
        }
        available = false;
        notifyAll();
        return contents;
    }

    public synchronized void put(int value) {
        while (available == true) {
            try {
                wait();
            } catch (InterruptedException e) { }
        }
        contents = value;
        available = true;
        notifyAll();
    }
}

• the keyword synchronized is added to the method declarations
• the Object methods wait() and notifyAll() are used to communicate between executing threads

Output after code is synchronized

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

Source for synchronized version

Other examples using synchronized

• An example of using synchronized methods and object locks Thread3
• An example using a synchronized statement on a common object Thread4
• An example of synchronizing access to variables Account
• An example of a museum which uses Walkmen radios for tours: WalkmanHire uses Museum, Counter, and Visitors classes.

Also see

Sun Tech Tip: Using Synchronized Statements
Acquire multiple locks in a fixed, global order to avoid deadlock