Monitor in Java

A monitor in Java is a synchronization mechanism that controls concurrent access to an object. It ensures that only one thread can execute critical section (a block of code that accesses shared resources) at a time, while other threads wait. When a thread enters a synchronized block:

• It requests the monitor of the object.
• If the monitor is available, it acquires it and executes the synchronized code.
• If another thread already holds the monitor, the new thread waits until the monitor is released.
• After execution, the thread releases the monitor, allowing other threads to acquire it

Key Concepts of Monitors

1. Mutual Exclusion

Mutual Exclusion ensures that only one thread can execute a synchronized section of code at a time. Example: Only one thread can update a shared variable at a time.

2. Thread Coordination

Thread Coordination allows threads to communicate and coordinate with each other using:

• wait() -> Thread releases the monitor and waits.
• notify() -> Wakes up one waiting thread.
• notifyAll() -> Wakes up all waiting threads.

Example: Implementing a Monitor Using the synchronized Keyword

class SharedResource{
    
    void printTable(int n) {
        
        // Monitor lock on current object
        synchronized(this) { 
            for (int i = 1; i <= 2; i++) {
                System.out.println(n * i);
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    System.out.println(e.getMessage());
                }
            }
        }
    }
}

class MyThread1 extends Thread {
    SharedResource resource;
    MyThread1(SharedResource resource) {
        this.resource = resource;
    }
    public void run() {
        resource.printTable(5);
    }
}

class MyThread2 extends Thread {
    SharedResource resource;
    MyThread2(SharedResource resource) {
        this.resource = resource;
    }
    public void run() {
        resource.printTable(10);
    }
}

public class GFG{
    
    public static void main(String[] args) {
        SharedResource obj = new SharedResource();
        MyThread1 t1 = new MyThread1(obj);
        MyThread2 t2 = new MyThread2(obj);
        t1.start();
        t2.start();
    }
}

Output:

Explanation

• The synchronized (this) block acquires the monitor lock of the current object.
• When one thread executes the printTable() method, the other thread must wait until the first one releases the monitor.
• This ensures mutual exclusion, preventing data inconsistency and race conditions.

Note: The output order may vary depending on thread scheduling, but due to synchronization, one thread completes its entire loop before the other begins.

Monitor vs Lock