Java Synchronized Block

The synchronized block in Java allows for synchronization to be applied to a particular resource within a method.

Imagine a scenario where a method consists of 50 lines of code, but there is a need to synchronize only a specific portion, say 5 lines. In such situations, a synchronized block can be employed.

Placing all method code inside a synchronized block achieves the same synchronization effect as using a synchronized method.

Points to Remember

✅ Synchronized block is used to lock an object for any shared resource.
✅ Scope of synchronized block is smaller than the method.
✅ A Java synchronized block doesn't allow more than one JVM, to provide access control to a shared resource.
✅ The system performance may degrade because of the slower working of synchronized keyword.
✅ Java synchronized block is more efficient than Java synchronized method.

Syntax

Example


synchronized (object reference expression) {   
  //code block   
}

Example of Synchronized Block

Let's examine a basic illustration of a synchronized block.

Example


//Creating a class which containse synchronized block within method
class Table  
{      
 void printTable(int n){    
   synchronized(this){//synchronized block    
     for(int i=1;i<=5;i++){    
      System.out.println(n*i);    
      try{    
       Thread.sleep(400);    
      }catch(Exception e){System.out.println(e);}    
     }    
   }    
 }//end of the method    
}    

//Creating Thread Class to call method
class MyThread1 extends Thread{    
Table t;    
MyThread1(Table t){    
this.t=t;    
}    
public void run(){    
t.printTable(5);    
}    
    
}    
class MyThread2 extends Thread{    
Table t;    
MyThread2(Table t){    
this.t=t;    
}    
public void run(){    
t.printTable(100);    
}    
}    
//Creating Main class to start threads
public class Main{    
public static void main(String args[]){    
Table obj = new Table();//only one object    
MyThread1 t1=new MyThread1(obj);    
MyThread2 t2=new MyThread2(obj);    
t1.start();    
t2.start();    
}    
}

Output:

Output

Synchronized Block Example Using Anonymous Class

Consider the following illustration demonstrating a synchronized block in which threads are generated utilizing an anonymous class.

Example


//Creating a class which containse synchronized block within method
class Table  
{      
 void printTable(int n){    
   synchronized(this){//synchronized block    
     for(int i=1;i<=5;i++){    
      System.out.println(n*i);    
      try{    
       Thread.sleep(400);    
      }catch(Exception e){System.out.println(e);}    
     }    
   }    
 }//end of the method    
}    

//Creating Main class to create and start threads
public class Main{    
 public static void main(String args[]){    
  final Table obj = new Table();//only one object    
  Thread t1=new Thread(){
    public void run(){    
      obj.printTable(5);    
    } 
  };
  Thread t2=new Thread(){
    public void run(){    
      obj.printTable(100);    
    } 
  };
  t1.start();    
  t2.start();    
 }    
}

Output:

Output

Message Passing Example using Synchronized Block

Let's explore another instance of a synchronized block where a message is being passed to the sender thread.

Example


// A Sender class  
class Sender   
{   
  public void SenderMsg(String msg)  
  {   
    System.out.println("Sending a Message: "  + msg);  
    try  
    {   
      Thread.sleep(800);   
    }   
    catch (Exception e)   
    {   
      System.out.println("Thread interrupted.");   
    }   
    System.out.println(msg+ "Sent");  
  }  
}   
// A Sender class for sending a message using Threads   
class SenderThread extends Thread   
{   
  private String msg;   
  Sender sd;   
  
  // Receiver method to receive a message object and a message to be sent   
  SenderThread(String m, Sender obj)  
  {   
    msg = m;  
    sd = obj;   
  }   
  
  public void run()   
  {   
    // Checks that only one thread sends a message at a time.   
    synchronized(sd)   
    {   
      // synchronizing the sender object   
      sd.SenderMsg(msg);  
    }   
  }   
}   
// Creating a main class   
public class Main
{   
  public static void main(String args[])   
  {   
    Sender sender = new Sender();   
    SenderThread sender1 = new SenderThread("Hola " , sender);  
    SenderThread sender2 =  new SenderThread("Welcome to C# Programming website ", sender);  
  
    sender1.start();   
    sender2.start();   
  
    // wait for threads to end   
    try  
    {   
      sender1.join();   
      sender2.join();   
    }   
    catch(Exception e)   
    {   
      System.out.println("Interrupted");   
    }   
  }   
}

Output:

Output


Sending a Message: Hola 
Hola Sent
Sending a Message: Welcome to C# Programming website 
Welcome to C# Programming website Sent

Points to Remember

Syntax

Example of Synchronized Block

Example

Synchronized Block Example Using Anonymous Class

Example

Message Passing Example using Synchronized Block

Example

Input Required