java - If a synchronized method calls another non-synchronized method, is there a lock on the non-synchronized method

Question

In Java, if a synchronized method contains a call to a non-synchronized, can another method still access the non-synchronized method at the same time? Basically what I'm asking is everything in the synchronized method have a lock on it (including calls to other synchronized methods)?

Answer 1

If a synchronized method calls another non-synchronized method, is there a lock on the non-synchronized method

The answer depends on the context.

If you are in a synchronized method for an object, then calls by other threads to other methods of the same object instance that are also synchronized are locked. However calls by other threads to non-synchronized methods are not locked – anyone can call them at the same time.

public synchronized void someSynchronizedMethod() {
    ...
    someNonSynchronizedMethod();
    ...
}

// anyone can call this method even if the someSynchronizedMethod() method has
// been called and the lock has been locked
public void someNonSynchronizedMethod() {
   ...
}

Also, if you call someSynchronizedMethod() but happen to be within the someNonSynchronizedMethod() method, you still hold the lock. The lock is enabled when you enter a synchronized method (or block) and is disabled when you exit that method. You can call all sorts of other unsynchronized methods and they will still be locked.

But you are asking two different things in your question:

In Java, if a synchronized method contains a call to a non-synchronized, can another method still access the non-synchronized method at the same time?

Yes. Other methods can access non-synchronized methods.

Basically what I'm asking is everything in the synchronized method have a lock on it (including calls to other synchronized methods)?

Uh, yes. Other calls to synchronized methods are locked. But non-synchronized methods are not locked.

Also, remember that if the method is static then the lock is on the Class object in the ClassLoader.

// this locks on the Class object in the ClassLoader
public static synchronized void someStaticMethod() {

If the method is an instance method then the lock is on the instance of the class.

// this locks on the instance object that contains the method
public synchronized void someInstanceMethod() {

There are 2 different locks in those 2 cases.

Lastly, when you are dealing with synchronized instance methods, each instance of the class is what is locked. This means that two threads could be in the same synchronized method at the same time with different instances. But if 2 threads try to operate on synchronized methods on the same instance, one will block until the other one exits the method.