The synchronized Statement

The Synchronized Statement

While creating synchronized methods within classes that you create is an easy and effective means of achieving synchronization, it will not work in all cases. To understand why, consider the following. Imagine that you want to synchronize access to objects of a class that was not designed for multithreaded access. That is, the class does not use synchronized methods. Further, this class was not created by you, but by a third party and you do not have access to the source code. Thus, you can't add synchronized to the appropriate methods within the class. How can access to an object of this class be synchronized? Fortunately, the solution to this problem is quite easy: You simply put calls to the methods defined by this class inside a synchronized block.

 

This is the general form of the synchronized statement:

 

synchronized(object) {

// statements to be synchronized

}

 

Here, object is a reference to the object being synchronized. A synchronized block ensures that a call to a method that is a member of object occurs only after the current thread has successfully entered object's monitor.

 

Here is an alternative version of the preceding example, using a synchronized block within the run( ) method:

 

// This program uses a synchronized block.

class Callme {

void call(String msg) {

System.out.print("[" + msg);

try {

Thread.sleep(1000);

} catch (InterruptedException e) {

System.out.println("Interrupted");

}

System.out.println("]");

}

}

class Caller implements Runnable {

String msg;

Callme target;

Thread t;

public Caller(Callme targ, String s) {

target = targ;

msg = s;

t = new Thread(this);

t.start();

}

// synchronize calls to call()

public void run() {

synchronized(target) { // synchronized block

target.call(msg);

}

}

}

class Synch1 {

public static void main(String args[]) {

Callme target = new Callme();

Caller ob1 = new Caller(target, "Hello");

Caller ob2 = new Caller(target, "Synchronized");

Caller ob3 = new Caller(target, "World");

// wait for threads to end

try {

ob1.t.join();

ob2.t.join();

ob3.t.join();

} catch(InterruptedException e) {

296  

System.out.println("Interrupted");

}

}

}

 

Here, the call( ) method is not modified by synchronized. Instead, the synchronized statement is used inside Caller's run( ) method. This causes the same correct output as the preceding example, because each thread waits for the prior one to finish before proceeding.