Java synchronized method

First of all, forget about synchronized methods. A so-called synchronized method...

synchronized AnyType foobar(...) {    doSomething();}

Is nothing but a shortcut way of writing this:

AnyType foobar(...) {    synchronized(this) {        doSomething();    }}

There is nothing special about the method in either case. What is special is the synchronized block, and what a synchronized block does is very simple. When the JVM executes this:

synchronized(foo) {    doSomething();}

It first evaluates the expression foo. The result must be an object reference. Then it locks the object, performs the body of the synchronized block, and then it unlocks the object.

But what does locked mean? It may mean less than you think. It does not prevent other threads from using the object. It doesn't prevent them from accessing the object's fields or, from updating its fields. The only thing that locking an object prevents is, it prevents other threads from locking the same object at the same time.

If thread A tries to enter synchronized(foo) {...} while thread B already has foo locked (either in the same synchronized block, or in a different one), then thread A will be forced to wait until thread B releases the lock.

You use synchronized blocks to protect data.

Suppose your program has some collection of objects that can be in different states. Suppose that some states make sense, but there are other states that don't make sense—invalid states.

Suppose that it is not possible for a thread to change the data from one valid state to another valid state without temporarily creating an invalid state.

If you put the code that changes the state in a synchronized(foo) block, and you put every block of code that can see the state into a synchronized block that locks the same object, foo, then you will prevent other threads from seeing the temporary invalid state.

Yes, other threads can access the objects used in the method; the synchronized keyword guarantees that no more than one thread at the time can execute the code of the method.

From https://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html:

• First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads. Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.

In this context, synchronized simultaneously locks this method and any other method similarly marked as synchronized in your class.