Locking String Objects

[Jared Folino]

I have a class that has two synchronized methods. One sets the Sring, and then prints it 3 times. The next method changes the String to "concurrency error" but that is the end of the method. Now as they are locking the same object the "concurreny error" and that is the end of the method. Theorectically as they lock the same object "concurrency error" should never print as the other synchronized method changes it back. This is not the case. Can somneone please help me?

public class JavaLocks2 implements Runnable{
    
    String name = "";
 
    public JavaLocks2() {
    }
    
    public void run() {
        for (int i = 0; i < 10; i++) {
            go();
            go2();
        }
    }
    
    public void go2(){
        synchronized(this.name){
            name = "--------------------------Concurreny Error:";
        }
    }
    
    
    public void go(){
        synchronized(this.name) {
            try {
                name = Thread.currentThread().getName();
                System.out.println("The current thread is: " + name);
                Thread.sleep(88);
                name += "1";
                System.out.println("The current thread is: " + name);
                Thread.sleep(55);
                name+= "2";
                System.out.println("The current thread is: " + name);
            } catch (Exception e){
                
            }
        }
    }
    
    public static void main(String[] args) {
        JavaLocks2 j = new JavaLocks2();
        Thread a = new Thread(j);
        a.setName("R");
        Thread b = new Thread(j);
        b.setName("Wiggy");
        a.start();
        b.start();
    }
}

[Henry: Added Code Tags]
[ April 29, 2007: Message edited by: Henry Wong ]

[Henry Wong]

I have a class that has two synchronized methods. One sets the Sring, and then prints it 3 times. The next method changes the String to "concurrency error" but that is the end of the method. Now as they are locking the same object the "concurreny error" and that is the end of the method. Theorectically as they lock the same object "concurrency error" should never print as the other synchronized method changes it back. This is not the case. Can somneone please help me?

First, [DELETED: oh, I see what you are trying to describe. Ignore this point. See the next point for your answer.]


Second, synchronization is based on objects, not references... This...

synchronized(this.name){
    name = "--------------------------Concurreny Error:";
}   

is not a good idea. As any other thread that calls the same block will lock on a different object, as you have changed it. Same goes for stuff like...

name += "1";
 
 -- and --
 
name += "2";   

Basically, since you are constantly changing the objects, the threads are locking on different objects. Hence, it is not threadsafe.

Henry
[ April 29, 2007: Message edited by: Henry Wong ]

[Burkhard Hassel]

Howdy, cowboys!

What Henry forgot to mention (presumably because he thinks everybody knows it):

Strings are immutable.

Ever heard?

Use a StringBuffer instead.
I tried it out, it works.


Yours,
Bu.