How do I create a thread synchronized block in Java?

The objective of thread synchronization is to ensure that when several threads want access to a single resource, only one thread can access it at any given time.
You can manage synchronization of your program at method level (synchronized method) or at block level (synchronized block). To make a block of code synchronized you can use the synchronizedkeyword.
The example below show how you can use the synchronized keyword.

public class Incrementor {
    private int count;

    //
    // A synchronized method example.
    //
    public synchronized void increment(int value) {
        count += value;
        System.out.println(Thread.currentThread().getName() +
                ": inc >>> " + count);
    }

    public void decrement(int value) {
        //
        // A synchronized block example the use the current object instance
        // as the monitor object.
        //
        synchronized (this) {
            count -= value;
            System.out.println(Thread.currentThread().getName() +
                    ": dec >>> " + count);
        }
    }
}


Another class

public class IncrementThread implements Runnable {
    private Incrementor incrementor;

    public IncrementThread(Incrementor incrementor) {
        this.incrementor = incrementor;
    }

    public void run() {
        for (int i = 1; i <= 5; i++) {
            incrementor.increment(i * 10);
            incrementor.decrement(i * 2);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

another class

public class IncrementorDemo {
    public static void main(String[] args) {
        Incrementor incrementor = new Incrementor();

        Thread t1 = new Thread(new IncrementThread(incrementor), "T1");
        Thread t2 = new Thread(new IncrementThread(incrementor), "T2");

        t1.start();
        t2.start();
    }
}


Below is the output..

T1: inc >>> 10
T1: dec >>> 8
T2: inc >>> 18
T2: dec >>> 16
T1: inc >>> 36
T1: dec >>> 32
T2: inc >>> 52
T2: dec >>> 48
T1: inc >>> 78
T1: dec >>> 72
T2: inc >>> 102
T2: dec >>> 96
T1: inc >>> 136
T1: dec >>> 128
T2: inc >>> 168
T2: dec >>> 160
T1: inc >>> 210
T1: dec >>> 200
T2: inc >>> 250
T2: dec >>> 240