Why is synchronized block better than synchronized method?

Question

I have started learning synchronization in threading.

Synchronized method:

public class Counter {

   private static int count = 0;

   public stati         


        

Answer 1

Difference between synchronized block and synchronized method are following:

1. synchronized block reduce scope of lock, but synchronized method's scope of lock is whole method.
2. synchronized block has better performance as only the critical section is locked but synchronized method has poor performance than block.
3. synchronized block provide granular control over lock but synchronized method lock either on current object represented by this or class level lock.
4. synchronized block can throw NullPointerException but synchronized method doesn't throw.

5. synchronized block: synchronized(this){}

   synchronized method: public synchronized void fun(){}

Answer 2

Because lock is expensive, when you are using synchronized block you lock only if _instance == null, and after _instance finally initialized you'll never lock. But when you synchronize on method you lock unconditionally, even after the _instance is initialized. This is the idea behind double-checked locking optimization pattern http://en.wikipedia.org/wiki/Double-checked_locking.

Answer 3

synchronized should only be used when you want your class to be Thread safe. In fact most of the classes should not use synchronized anyways. synchronized method would only provide a lock on this object and only for the duration of its execution. if you really wanna to make your classes thread safe, you should consider making your variables volatile or synchronize the access.

one of the issues of using synchronized method is that all of the members of the class would use the same lock which will make your program slower. In your case synchronized method and block would execute no different. what I'd would recommend is to use a dedicated lock and use a synchronized block something like this.

public class AClass {
private int x;
private final Object lock = new Object();     //it must be final!

 public void setX() {
    synchronized(lock) {
        x++;
    }
 }
}

Answer 4

One classic difference between Synchronized block and Synchronized method is that Synchronized method locks the entire object. Synchronized block just locks the code within the block.

Synchronized method: Basically these 2 sync methods disable multithreading. So one thread completes the method1() and the another thread waits for the Thread1 completion.

class SyncExerciseWithSyncMethod {

public synchronized void method1() {
    try {
        System.out.println("In Method 1");
        Thread.sleep(5000);
    } catch (Exception e) {
        System.out.println("Catch of method 1");
    } finally {
        System.out.println("Finally of method 1");
    }

}

public synchronized void method2() {
    try {
        for (int i = 1; i < 10; i++) {
            System.out.println("Method 2 " + i);
            Thread.sleep(1000);
        }
    } catch (Exception e) {
        System.out.println("Catch of method 2");
    } finally {
        System.out.println("Finally of method 2");
    }
}

}

Output

In Method 1

Finally of method 1

Method 2 1

Method 2 2

Method 2 3

Method 2 4

Method 2 5

Method 2 6

Method 2 7

Method 2 8

Method 2 9

Finally of method 2

---

Synchronized block: Enables multiple threads to access the same object at same time [Enables multi-threading].

class SyncExerciseWithSyncBlock {

public Object lock1 = new Object();
public Object lock2 = new Object();

public void method1() {
    synchronized (lock1) {
        try {
            System.out.println("In Method 1");
            Thread.sleep(5000);
        } catch (Exception e) {
            System.out.println("Catch of method 1");
        } finally {
            System.out.println("Finally of method 1");
        }
    }

}

public void method2() {

    synchronized (lock2) {
        try {
            for (int i = 1; i < 10; i++) {
                System.out.println("Method 2 " + i);
                Thread.sleep(1000);
            }
        } catch (Exception e) {
            System.out.println("Catch of method 2");
        } finally {
            System.out.println("Finally of method 2");
        }
    }
}

}

Output

In Method 1

Method 2 1

Method 2 2

Method 2 3

Method 2 4

Method 2 5

Finally of method 1

Method 2 6

Method 2 7

Method 2 8

Method 2 9

Finally of method 2