锁定类的静态成员

Question

According to my understanding, the following piece of code should result in a deadlock. The reason being, when thread t1 locks static object firstData, he has acquired a lock on the class. So, when he tries to lock another static object secondData, the request should block.

However, the program runs fine and prints *** Successfully acquired both the locks

What is it about locking static objects that im missing here?

public class Deadlock {
    public static void main(String[] args) {

        Thread t1 = new Thread(new DeadlockRunnable());
        t1.start();
    }
} 

 class DeadlockRunnable implements Runnable {
    static  Object firstData = new Object();
    static  Object secondData = new Object();

    public void run() {
        synchronized(firstData) {
            synchronized(secondData) {
                System.out.println("*** Successfully acquired both the locks");
            }
        }
    }

}

For all those who answered that the locks are on object, instead of class, please take a look at this

Answer 1

Firstly, you have a mistake here:

The reason being, when thread t1 locks static object firstData, he has acquired a lock on the class.

Locking a static object locks only that object, not the class. You are locking two separate objects.

The question you refered to is about synchronized methods not synchronized statements . These two related constructs work in slightly different ways.

---

Secondly, even if you were locking on the same object, your code would still not deadlock ( ideone ). Intrinsic locks are reentrant . This means that a thread does not deadlock itself if it tries to take the same lock twice.

Reentrant Synchronization

Recall that a thread cannot acquire a lock owned by another thread. But a thread can acquire a lock that it already owns. Allowing a thread to acquire the same lock more than once enables reentrant synchronization. This describes a situation where synchronized code, directly or indirectly, invokes a method that also contains synchronized code, and both sets of code use the same lock. Without reentrant synchronization, synchronized code would have to take many additional precautions to avoid having a thread cause itself to block.

Source

Answer 2

"when thread t1 locks static object firstData, he has acquired a lock on the class"
Not sure why you think so. t1 acquires a lock on firstData, not on the containing class. There is no possible deadlock in your code as it is.

EDIT
Following your comment, the link is about the difference between those 2 declarations:

public synchronized method() // lock on the instance (this)
public static synchronized method() // lock on the class (Myclass.class)

But there is no link with deadlocks.