How the Synchronization works

[Shiva Mohan]

I am trying to understand how the locks will work in synchronization. It seems like theoretically I got how locks would work. But I couldn�t get how the synchronization code works in a coding way.

    
class JavaThreads extends Thread{
    StringBuffer letter;
    
    public Java_threads(StringBuffer letter){
        this.letter=letter;
    }
    
    public void run(){
       synchronized(letter){//---------------line-9
            for(int i=1;i<=100;++i){
                System.out.print(letter);
            }
             System.out.println();
             //Increment the letter in StringBuffer
             char temp=letter.charAt(0);
             
             ++temp;
             letter.setCharAt(0, temp);
       }//--------------------------------line-18
    }
    public static void main(String[] args){
        StringBuffer sb=new StringBuffer("A");
        new JavaThreads (sb).start();
         new JavaThreads (sb).start();
          new JavaThreads (sb).start();
    }
}

We have started three threads on one object (sb).Can anyone please explain the synchronization block for me?

[prarthana reddy]

What happens when you compile the code ? Synchronization has to be done on an object not on an instance variable ( letter ---line 9 ) .

[Surendra Kumar]

Hi Prarthana,

synchronization can be done on any object, and in this code the variable 'letter' is an object of StringBuffer.

[Shiva Mohan]

When run the code (without line -9 and line-18) also the output is same like with synchronization on. Can anyone please explain execution steps of synchronized block and without that?

[Nilesh Patel]

Hi,
when you run code without synchronization block ( w/o line 9 & 18) it will give the series of "A" in output or my be depend on platform...

if you use synchronozation block output will be series of "A" "B" "C"

Means w/o sync all three thread use letter object same time and using sync block on letter object it is use one by one...

you can verift it using below line on line No. 11

System.out.print(Thread.currentThread().getName()+letter);

[Vepa Sritej]

In this case, all the three threads have the same StringBuffer Instance. That is their respective letter references point to the same StringBuffer object so synchronisation works.Because lock is obtained on the same object.

Try changing the constructor code to
public JavaThread(StringBuffer b)
{
sb= new StringBuffer(b.toString());
}
In this case synchronisation will not occur as each thread has its own sb object.