Item 66: Synchronize access to shared mutable data

The synchronized keyword ensures that only a single thread can execute a method or block at one time. Many programmers think of synchronization solely as a means of mutual exclusion, to prevent an object from being observed in an inconsistent state while it’s being modified by another thread. In this view, an object is created in a consistent state (Item 15) and locked by the methods that access it. These methods observe the state and optionally cause a state transition, transforming the object from one consistent state to another. Proper use of synchronization guarantees that no method will ever observe the object in an inconsistent state.

This view is correct, but it’s only half the story. Without synchronization, one thread’s changes might not be visible to other threads. Not only does synchronization prevent a thread from observing an object in an inconsistent state, but it ensures that each thread entering a synchronized method or block sees the effects of all previous modifications that were guarded by the same lock.