You know threads allow multiple activities to proceed concurrently. While execution multiple threads may share common resources. Thus multiple threads compete for the common resources, called race condition. However, there are critical situations, where it is desirable that only one thread at a time has an access to a shared resource to avoid the race condition. The process, by which this is achieved is called synchronization.

Synchronization works with locks. Every object in Java has a built-in lock, that only comes into play when the object has synchronized method or synchronized block. As the synchronized method or block executes, a lock is imposed on instance, consisting of the synchronized code. We can also use term monitor for the locked object. There is only one lock per object, if one thread picked the lock, no other thread can pick it until the first thread releases the lock. In other words, no other thread can enter the synchronized code. Releasing the lock means, the thread which was entered in synchronized code (method or block) exits from it.

Implementing Synchronized Method

Use keyword synchronized in method header, as shown below:

Synchronized methods are useful when methods can manipulate the state of an object in ways that can corrupt the state, if executed concurrently.

For Example:

class Resource
	  synchronized void printTable(int tbl) //synchronized method
			for(int i=1;i<=5;i++)
		  	     int value=tbl*i;
			     try{ Thread.sleep(100);}
			     catch(Exception ex){ex.printStackTrace();}
class User implements Runnable
		Resource res;
		Thread  th;
		int tbl;
	User(Resource res, int tbl)
			th=new Thread(this);
	 public void run()
public class SynchronizedMethod 
	public static void main(String[] args)
	   Resource res=new Resource(); //creation of object to be shared
	    User thrd1=new User(res, 5); // object shared with thrd1
	    User thrd2=new User(res, 10); //object shared with thrd2

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Rajesh K. Bansal (SCJP-Sun Certified Java Programmer)
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