Cooperating

Since you can't be sure whether you'll be working with a cooperative or preemptive model, it's important not to assume preemption is available. CPU-intensive threads should yield control at periodic intervals. There are four different ways a thread can give up control and allow other threads to run.

• It can block.
• It can call Thread.yield().
• It can go to sleep.
• It can be suspended.

Blocking occurs when a thread has to wait for an operation to complete. Most commonly this is an I/O operation, particularly one involving a network connection. It's also possible for a call to block while waiting for user input. Placing I/O and user input in separate, high priority threads is often a good idea because it allows the computer to be used more efficiently. Other CPU intensive threads can get a lot of work done while waiting for data to come in over the network or the user to type a character or two.

When a program calls Thread.yield(), it's signifying that the current thread, the one which called Thread.yield(), is willing to step aside in favor of another thread. The VM looks to see if any other threads of the same priority are ready to run. If any are, it pauses the currently executing thread and passes control to the next thread in line. If no other threads of the same or higher priority are ready to run, control returns to the thread that yielded. Thus Thread.yield() only signals a willingness to give up control. It does not guarantee that the thread will actually stop. That depends completely on what other threads exist and what their status is.

If a thread definitely wants to give up control for a period of time, whether or not there are any other threads of equal or higher priority ready to run, then it can call sleep(). The sleep() methods put a thread to sleep for a certain amount of time during which even lower priority threads may have an opportunity to run.

public static void sleep(long milliseconds) throws InterruptedException
public static void sleep(long milliseconds, int nanoseconds) 
  throws InterruptedException

Finally, a thread can be suspended. When a thread calls suspend() (or more commonly when a different thread invokes the thread's suspend() method), it is paused indefinitely until some other thread starts it running again by invoking its resume() method.

public final void suspend()
public final void resume()