Question

I’m trying to understand thread basics, and as a first example I create two thread that write a String on the stdout. As I know the scheduler allows to execute the threads using a round robin schedule. Thats why I got:

PING
PING
pong
pong
pong
PING
PING
PING
pong
pong

Now I want to use a shared variable, so every thread will know if its your turn:

public class PingPongThread extends Thread {
private String msg;
private static String turn;

public PingPongThread(String msg){
    this.msg = msg;
}
@Override
public void run() {
    while(true) {
        playTurn();
    }

}
public synchronized void playTurn(){
    if (!msg.equals(turn)){
        turn=msg;
        System.out.println(msg);
    }
}
}

Main class:

public class ThreadTest {
    public static void main(String[] args) {
        PingPongThread thread1 = new PingPongThread("PING");
        PingPongThread thread2 = new PingPongThread("pong");
        thread1.start();
        thread2.start();
    }
}

I synchronized the “turn manager” but I still get something like:

PING
PING
pong
pong
pong
PING
PING
PING
pong
pong

Can someone explains what I am missing, and Why I’m not getting Ping pong… ping pong.
Thanks!

Answer 1

This line:

public synchronized void playTurn(){
    //code
}

is equivalent in behavior to

public void playTurn() {
    synchronized(this) {
         //code
    }
}

that’s why no synchronization is occuring, because as Brian Agnew pointed out, the threads are synchronizing on two different objects (thread1, thread2), each on it’s own instance resulting in no effective synchronization.

If you would use your turn variable for synchronization, e.g.:

private static String turn = ""; // must initialize or you ll get an NPE

public void playTurn() {
    synchronized(turn) {
         //...
         turn = msg; // (1)
         //...
    }
}

then the situation is a lot better (run multiple times to verify), but there is also no 100% synchronization. In the beggining (mostly) you get a double ping and double pong, and afterwards they look synchronized, but you still can get double pings/pongs.

The synchronized block locks upon value (see this great answer) and not the reference to that value. (see EDIT)

So let’s take a look at one possible scenario:

thread1 locks on ""
thread2 blocks on ""
thread1 changes the value of turn variable to "PING" - thread2 can continue since "" is no longer locked 

To verify that I tried putting

try {
    Thread.currentThread().sleep(1000); // try with 10, 100 also multiple times
 } 
 catch (InterruptedException ex) {}

before and after

turn = msg;

and it looks synchronized?! But, if you put

 try {
    Thread.yield();
    Thread.currentThread().sleep(1000); //  also  try multiple times
 } 
 catch (InterruptedException ex) {}

after few seconds you’ll see double pings/pongs. Thread.yield() essenitally means “I’m done with the processor, put some else thread to work”. This is obviously system thread scheduler implementation on my OS.

So, to synchronize correctly we must remove line

    turn = msg;

so that threads could always synchronize on the same value – not really 🙂 As explained in the great answer given above – Strings (immutable objects) are dangerous as locks – because if you create String “A” on 100 places in your program all 100 references(variables) will point to the same “A” in memory – so you could oversynchronize.

So, to answer your original question, modify your code like this:

 public void playTurn() {
    synchronized(PingPongThread.class) {
         //code
    }
}

and the parallel PingPong example will be 100% correctly implemented (see EDIT^2).

The above code is equivalent to:

 public static synchronized void playTurn() {
     //code
 }

The PingPongThread.class is a Class object, e.g. on every instance you can call getClass() which always has only one instance.

Also you could do like this

 public static Object lock = new Object();

 public void playTurn() {
    synchronized(lock) {
         //code
    }
}

Also, read and program examples(running multiple times whenever neccessary) this tutorial.

EDIT:

To be technically correct:

The synchronized method is the same as synchronized statement locking upon this. Let’s call the argument of the synchronized statement “lock” – as Marko pointed out, “lock” is a variable storing a reference to an object/instance of a class. To quote the spec:

The synchronized statement computes a reference to an object; it then attempts to perform a lock action on that object’s monitor..

So the synchronizaton is not acutally made upon value – the object/class instance, but upon the object monitor associated with that instance/value. Because

Each object in Java is associated with a monitor..

the effect remains the same.

EDIT^2:

Following up on the comments remarks:
“and the parallel PingPong example will be 100% correctly implemented” – meaning, the desired behavior is achieved (without error).

IMHO, a solution is correct if the result is correct. There are many ways of solving the problem, so the next criteria would be simplicity/elegance of the solution – the phaser solution is better approach, because as Marko said in other words in some comment there is a lot lesser chance of making error using phaser object than using synchronized mechanism – which can be seen from all the (non)solution variants in this post. Notable to see is also comparison of code size and overall clarity.

To conclude, this sort of constructs should be used whenever they are applicable to the problem in question.