J.15. Overview of Thread Synchronization

When multiple threads share an object and it’s modified by one or more of them, indeterminate results may occur unless access to the shared object is managed properly. If one thread is in the process of updating a shared object and another thread also tries to update it, it’s unclear which thread’s update takes effect. When this happens, the program’s behavior cannot be trusted—sometimes the program will produce the correct results, and sometimes it won’t. In either case, there’ll be no indication that the shared object was manipulated incorrectly.

The problem can be solved by giving only one thread at a time exclusive access to code that manipulates the shared object. During that time, other threads desiring to manipulate the object are kept waiting. When the thread with exclusive access to the object finishes manipulating it, one of the threads that was waiting is allowed to proceed. This process, called thread synchronization, coordinates access to shared data by multiple concurrent threads. By synchronizing threads in this manner, you can ensure that each thread accessing a shared object excludes all other threads from doing so simultaneously—this is called mutual exclusion.