Synchronized block not working

4. The synchronized block of code will obtain a lock on the StringBuffer object from step 3.

Well, you're not doing that, are you?

synchronized(this) {

You're obtaining a lock on the instance of MySyncBlockTest on which that run() method is being called. That ... isn't going to do anything. There's no contention for that resource; each Thread has its own instance of MySyncBlockTest.

You should lock on the StringBuffer object

 synchronized(sb) {            for (int i=0; i<10; i++) {                System.out.print(sb);            }

I was confused too. The answer provided by Brian is correct

synchronized (this){

is for getting the lock on an instance. It would be useful when there is a single instance of a class and multiple threads accessing it.

I wrote the following program to demonstrate this:

package com.threads.chapter9;public class TestSunchronizedBlocksUsingRunnable implements Runnable {StringBuffer s;@Overridepublic void run() {    synchronized (this) {        for (int i = 1; i <= 100; i++) {            System.out.println(i);        }        char c = s.charAt(0);        c++;        s.setCharAt(0, c);    }}TestSunchronizedBlocksUsingRunnable(StringBuffer s) {    this.s = s;}public static void main(String[] args) {    StringBuffer s = new StringBuffer("A");    TestSunchronizedBlocksUsingRunnable instance1 = new TestSunchronizedBlocksUsingRunnable(s);    Thread thread1 = new Thread(instance1);    Thread thread2 = new Thread(instance1);    Thread thread3 = new Thread(instance1);    thread1.start();    thread2.start();    thread3.start();}}

The above code will display the same output but the scenario is completely different. So what you use inside synchronized block is really crucial.