Reentrant Monitors in Java: Understanding Thread Synchronization

Discover reentrant monitors in Java, which allow a thread to reuse the same monitor for different synchronized methods. Learn how this feature helps avoid deadlocks by enabling a thread to reacquire a lock it already holds, improving thread synchronization and efficiency.

Reentrant Monitor in Java

In Java, monitors are reentrant, which means a thread can reuse the same monitor for different synchronized methods if the methods are called from within each other.

Advantage of Reentrant Monitor

One major advantage of reentrant monitors is that they eliminate the possibility of a single thread deadlocking when trying to reacquire a lock it already holds.

Understanding Reentrant Monitor with Example

Let's illustrate the concept of reentrant monitors with the following example:

class Reentrant {  
public synchronized void m() {  
n();  
System.out.println("This is m() method");  
}  

public synchronized void n() {  
System.out.println("This is n() method");  
}  
}  

In the Reentrant class above, both m() and n() methods are synchronized. The m() method internally calls the n() method.

public class ReentrantExample {  
public static void main(String args[]) {  
final Reentrant r = new Reentrant();  

Thread t1 = new Thread() {  
public void run() {  
r.m(); // calling method of Reentrant class  
}  
};  

t1.start();  
}  
}  

This is n() method
This is m() method

Explanation

When t1.start() is executed, it creates a new thread that calls the m() method of the Reentrant class. Inside m(), it calls the n() method. Both methods are synchronized, and since they are called from the same thread, the thread can reacquire the lock on the monitor it already holds.