CODEWITHSUNDEEP
javamultithreadingsynchronized
hermit
hermit

Reputation: 1107

synchronized block showing weird results

I am trying to run a simple multi-threaded program in java that uses a synchronized block inside a non-static method.I have a class TestThread which has a field string which is a StringBuffer variable. I have created two threads one and two and each of their string variables initialized to StringBuffer b which contains the value A . The first thread that goes to running state has to display the value A hundred times and after that increment it by one so that the next thread running will display the incremented value B hundred times too.I have used the current object denoted by this inside the synchronized. But unfortunately I am not getting the expected output. The first thread is displaying A more than hundred times and second thread is displaying B less than 100. And each time I run it, I am getting different outputs.So I think that the mutual exclusion is not achieved. What I am doing wrong here? 

    public class TestThread extends Thread{

        StringBuffer string;
        public void run(){
            synchronized(this){
                for(int i=0;i<100;i++){
                    System.out.print(this);
                }
                System.out.println();
                string.setCharAt(0,(char)(string.charAt(0)+1));
            }
        }

        public TestThread(StringBuffer string){
            this.string=string;
        }

        public String toString(){
            return string.toString();
        }

        public static void main(String args[]){
            StringBuffer b=new StringBuffer("A");
            TestThread one=new TestThread(b);
            TestThread two=new TestThread(b);
            one.start();
            two.start();
    }
    }

Upvotes: 0

Views: 50

Answers (1)

TheLostMind
TheLostMind

Reputation: 36304

You are locking on the current object i.e, this. Thus you are locking on 2 different objects. Use a common lock and then try the same example.

synchronized(this) ==> synchronized(someGlobalObject)

Upvotes: 2

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