Tavo Tavo - 27 days ago 7
Java Question

Synchronized method blocks execution

I'm implementing a dummy program with three classes to have a more visual reference of how

Future
s work. My problem is that sometimes the program will get locked at the synchronized method and it won't go on. And I can't find a reason why. Can someone spot the reason why none of the fighters will print the "I won" line?

My current output when it blocks:


Fighter1

Fighter1 has entered the sync method

Fighter2

Fighter2 has entered the sync method


The code is as follows.

Main class:

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class TmpTest {
private static final ExecutorService executorService =
Executors.newFixedThreadPool(2);

public static void main(final String... args) {

final Fighter fighter1 = new Fighter("Fighter1");
final Fighter fighter2 = new Fighter("Fighter2");

final Future<String> submitFighter1 = executorService.submit(fighter1);
final Future<String> submitFighter2 = executorService.submit(fighter2);

while (!submitFighter1.isDone() || !submitFighter2.isDone()) {
if (submitFighter1.isDone()) {
System.out.println("Fighter 1 wins!");
submitFighter2.cancel(true);
executorService.shutdown();
} else if (submitFighter2.isDone()) {
submitFighter1.cancel(true);
System.out.println("Fighter 2 wins!");
executorService.shutdown();
}
}
}
}


Fighter class:

class Fighter implements Callable<String> {

private final String fighterName;
private final ClassWithSyncMethod classWithSyncMethod;

public Fighter(final String fighterName) {
this.fighterName = fighterName;
classWithSyncMethod = new ClassWithSyncMethod(fighterName);
}

@Override
public String call() throws Exception {
return classWithSyncMethod.syncMethod();
}
}


Dummy class with synchronized method:

class ClassWithSyncMethod {

private final String fighterName;

public ClassWithSyncMethod(final String fighterName) {
this.fighterName = fighterName;
}

public synchronized String syncMethod() {
System.out.println(fighterName + " has entered the sync method");
try {
Thread.sleep(1000);
} catch (final InterruptedException e) {
System.out.println("Exception trying to sleep the fighter " + fighterName + ";" + e);
}
return fighterName + " shouts: I won!";
}
}

Answer Source

This has nothing to do with synchronization. Each fighter synchronizes on a different object, so they don't interfere with each other.

The reason you don't see the "I won" line is much simpler - you never print it. The ClassWithSyncMethod#syncMethod() method (which is returned by Fighter#call()) doesn't print anything, it just returns a value. If you want to print it, you'd have to do so yourself from your main.

E.g.:

while (!submitFighter1.isDone() || !submitFighter2.isDone()) {
    if (submitFighter1.isDone()) {
        System.out.println("Fighter 1 wins!");
        System.out.println(submitFighter1.get()); // Here!
        submitFighter2.cancel(true);
        executorService.shutdown();
    } else if (submitFighter2.isDone()) {
        submitFighter1.cancel(true);
        System.out.println("Fighter 2 wins!");
        System.out.println(submitFighter2.get()); // And here!
        executorService.shutdown();
    }
}