Synchronization in Java

Multi-threaded programs may often come to a situation where multiple threads try to access the same resources and finally produce erroneous and unforeseen results. 

Why use Java Synchronization?

Java Synchronization is used to make sure by some synchronization method that only one thread can access the resource at a given point in time. 

Java Synchronized Blocks

Java provides a way of creating threads and synchronizing their tasks using synchronized blocks. 

A synchronized block in Java is synchronized on some object. All synchronized blocks synchronize on the same object and can only have one thread executed inside them at a time. All other threads attempting to enter the synchronized block are blocked until the thread inside the synchronized block exits the block.

Note: Synchronized blocks in Java are marked with the synchronized keyword. 

General Form of Synchronized Block

// Only one thread can execute at a time. 
// sync_object is a reference to an object
// whose lock associates with the monitor. 
// The code is said to be synchronized on
// the monitor object
synchronized(sync_object)
{
   // Access shared variables and other
   // shared resources
}

This synchronization is implemented in Java with a concept called monitors or locks. Only one thread can own a monitor at a given time. When a thread acquires a lock, it is said to have entered the monitor. All other threads attempting to enter the locked monitor will be suspended until the first thread exits the monitor.

Types of Synchronization

There are two synchronizations in Java mentioned below:

1. Process Synchronization
2. Thread Synchronization

1. Process Synchronization in Java

Process Synchronization is a technique used to coordinate the execution of multiple processes. It ensures that the shared resources are safe and in order.

2. Thread Synchronization in Java

Thread Synchronization is used to coordinate and ordering of the execution of the threads in a multi-threaded program. There are two types of thread synchronization are mentioned below:

• Mutual Exclusive
• Cooperation (Inter-thread communication in Java)

Mutual Exclusive

Mutual Exclusive helps keep threads from interfering with one another while sharing data. There are three types of Mutual Exclusive mentioned below:

• Synchronized method.
• Synchronized block.
• Static synchronization.

Example of Synchronization

Below is the implementation of the Java Synchronization:

Sending     Hi 

 Hi Sent
Sending     Bye 

 Bye Sent

The output is the same every time we run the program.

Explanation

In the above example, we choose to synchronize the Sender object inside the run() method of the ThreadedSend class. Alternately, we could define the whole send() block as synchronized, producing the same result. Then we don’t have to synchronize the Message object inside the run() method in ThreadedSend class. 

// An alternate implementation to demonstrate
// that we can use synchronized with method also.

class Sender {
   public synchronized void send(String msg)
   {
       System.out.println("Sending\t" + msg);
       try {
           Thread.sleep(1000);
       }
       catch (Exception e) {
           System.out.println("Thread interrupted.");
       }
       System.out.println("\n" + msg + "Sent");
   }
}

We do not always have to synchronize a whole method. Sometimes it is preferable to synchronize only part of a method. Java synchronized blocks inside methods make this possible.

// One more alternate implementation to demonstrate
// that synchronized can be used with only a part of  
// method

class Sender  
{
   public void send(String msg)
   {
       synchronized(this)
       {
           System.out.println("Sending\t" + msg );
           try  
           {
               Thread.sleep(1000);
           }  
           catch (Exception e)  
           {
               System.out.println("Thread interrupted.");
           }
           System.out.println("\n" + msg + "Sent");
       }
   }
}

Example of the synchronized method by using an anonymous class

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This article is contributed by Souradeep Barua. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above