Synchronization in Java - Synchronized keyword

Synchronized method and Synchronized block

We have learnt how to create thread in Java.In this post, we are going to see how to use synchronization in Java.
Synchronization is a very important concept in a multithreading environment.Synchronization is used when we want the resource to be accessed by a single thread at a time.Thus synchronization is useful to avoid the concurrency issues.

Problem without  Synchronization 

Let's write a program to understand the problem without Synchronization.In below program, we have created two thread using anonymous class and both are trying to increment the counter value by calling a method.

• Thread.sleep() is used to simulate the real time process which takes some time.
• join() is used to print counter value once both thread complete the task.

If we run below program we will get different output every time because both threads are trying to update the counter without waiting for each other thus leading to the race condition.A race condition occurs when two or more thread can access shared data and try to change it at the same time.

Program:

package com.javainstance;

/**
 * @author javainstance
 *
 */
public class MultiThread {


private int counter=0;

public void countIncrementor(){

counter=counter+1;
}

public void TaskManager(){


Thread threadOne=new Thread(new Runnable() {

@Override
public void run() {
for(int i=0;i<100;i++){
  try {
Thread.sleep(5);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}  

countIncrementor();
}


}
});

Thread threadTwo=new Thread(new Runnable() {

@Override
public void run() {
for(int i=0;i<100;i++){
  try {
Thread.sleep(5);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}  
countIncrementor();
}


}
});

threadOne.start();
threadTwo.start();
try {
        threadOne.join();
threadTwo.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

public static void main(String[] args) {

MultiThread multiThread=new MultiThread();
multiThread.TaskManager();

System.out.println("Counter is:"+ multiThread.counter);



}


}


Output:

Counter is:186
Counter is:185
Counter is:179

Synchronization using Synchronized method

In below above program, we just need to modify countIncrementor() method to synchronized method.

public synchronized  void countIncrementor(){

counter=counter+1;

}

Now if we run the program every time we will get the same output.

Output:


Counter is:200

How Synchronization method works?

Every object in Java has a monitor lock.So whenever a thread calls any synchronized method it acquires the monitor lock on that object.Only one thread at a time can acquire an object lock when method releases the lock after execution then only other thread can acquire the lock.Thus in the multithreaded environment, we can avoid concurrency issue.

Synchronization using Synchronized block

We can also use synchronized block to obtain synchronization in Java.To use synchronized block we have to modify countIncrementor() method as below.In synchronized block, we have to pass a parameter object of which monitor lock will be acquired by the thread.Here this keyword denotes MultiThread class object.

public void countIncrementor() {

synchronized (this) {

counter = counter + 1;

}

}

Which is better synchronized method or synchronized block ?

It depends on the code or task which you are going to define inside the synchronized method or synchronized block.However synchronized block has an advantage over synchronized method that it locks only critical part of the code which actually needed to be thread safe instead of the full method.Thus it is more efficient in most of the cases.

Disadvantage of synchronized keyword

The major disadvantage of synchronized keyword is that it avoid the concurrent read also from multiple thread thus leads the performance issue.

Volatile keyword in Java

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