Two Synchronized blocks execution in Java

Question

Why is it that two synchronized blocks can't be executed simultaneously by two different threads in Java.

EDIT

public class JavaApplication4 {

    public static void main(String[] args) {
        new JavaApplication4();
    }

    public JavaApplication4() {
        Thread t1 = new Thread() {

            @Override
            public void run() {
                if (Thread.currentThread().getName().equals("Thread-1")) {
                    test(Thread.currentThread().getName());
                } else {
                    test1(Thread.currentThread().getName());
                }
            }
        };
        Thread t2 = new Thread(t1);
        t2.start();
        t1.start();

    }

    public synchronized void test(String msg) {
        for (int i = 0; i < 10; i++) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException ex) {
            }
            System.out.println(msg);
        }
    }

    public synchronized void test1(String msg) {
        for (int i = 0; i < 10; i++) {
            try {
                Thread.sleep(100);
            } catch (InterruptedException ex) {
            }
            System.out.println(msg + " from test1");
        }
    }
}

Answer 1

Your statement is false. Any number of synchronized blocks can execute in parallel as long as they don't contend for the same lock.

But if your question is about blocks contending for the same lock, then it is wrong to ask "why is it so" because that is the purpose of the whole concept. Programmers need a mutual exclusion mechanism and they get it from Java through synchronized.

Finally, you may be asking "Why would we ever need to mutually exclude code segments from executing in parallel". The answer to that would be that there are many data structures that only make sense when they are organized in a certain way and when a thread updates the structure, it necessarily does it part by part, so the structure is in a "broken" state while it's doing the update. If another thread were to come along at that point and try to read the structure, or even worse, update it on its own, the whole thing would fall apart.

EDIT

I saw your example and your comments and now it's obvious what is troubling you: the semantics of the synchronized modifier of a method. That means that the method will contend for a lock on this's monitor. All synchronized methods of the same object will contend for the same lock.

Answer 2

That is the whole concept of synchronization, if you are taking a lock on an object (or a class), none of the other threads can access any synchronized blocks.

Example

Class A{

public void method1()
{
 synchronized(this)//Block 1 taking lock on Object
 {
  //do something
 }
}

public void method2()
{
 synchronized(this)//Block 2 taking lock on Object
 {
  //do something
 }
}
}

If one thread of an Object enters any of the synchronized blocks, all others threads of the same object will have to wait for that thread to come out of the synchronized block to enter any of the synchronized blocks. If there are N number of such blocks, only one thread of the Object can access only one block at a time. Please note my emphasis on Threads of same Object. The concept will not apply if we are dealing with threads from different objects.

Let me also add that if you are taking a lock on class, the above concept will get expanded to any object of the class. So if instead of saying synchronized(this), I would have used synchronized(A.class), code will instruct JVM, that irrespective of the Object that thread belongs to, make it wait for other thread to finish the synchronized block execution.

Edit: Please understand that when you are taking a lock (by using synchronized keyword), you are not just taking lock on one block. You are taking lock on the object. That means you are telling JVM "hey, this thread is doing some critical work which might change the state of the object (or class), so don't let any other thread do any other critical work" . Critical work, here refers to all the code in synchronized blocks which take lock on that particular Object (or class), and not only in one synchronized block.