Question

I’ve come across code like the following several times

class Foo {
   private Object lock = new Object();

   public void doSomething() {
      synchronized(lock) {
         ...

What I’m interested in is why a lock object is created instead of writing synchronized(this)? Is it there to enable sharing the lock? I vaguely remember reading that it is an optimization. Is that true? Also, does it, in some context, make sense to have the lock declared as final?

Answer 1

1. Synchronizing on this is discouraged because if the object itself is used as a lock from the outside it would break the internal synchronization. Also, remember that synchronized methods are also using this as a lock, which may cause an unwanted effect.
2. Declaring the lock final is recommended to prevent a situation in which a lock object is reassigned inside a synchronized block, thus causing different threads to see different lock objects. See this other question: Locking on a mutable object – Why is it considered a bad practice?