Concurrency - SCJP Sun Certified Programmer for Java Platform SE6 - page 353

The two threads have the same priority and politely yield to each other. As you can see
by the output, each thread is getting equal time on the CPU. In a real-world scenario, you
might not call yield quite this often, but in general it is a good method to call whenever
you want your threads to get along with other threads and not hog the CPU unnecessarily.

Blocked Threads

Threads access shared memory in a process and therefore need to be synchronized. The
upcoming section “Thread Synchronization” discusses the synchronized keyword and the
details you need to know about thread synchronization. Synchronization in Java is done at
the object level, where threads ask for an object's lock before entering synchronized code.
If a thread asks for a lock and the lock is already in use by another thread, it becomes a
blocked thread . A blocked thread stays blocked until the requested lock becomes available,
at which point it returns to the runnable state, as Figure 5.4 shows.

FIGURE 5.4 A thread goes from RUNNABLE to BLOCKED when a synchronized lock is

unavailable.

RUNNABLE CPU

scheduled

lock
becomes
available

synchronized

BLOCKED

The getState method of a blocked thread returns the BLOCKED value of the Thread
.State enumeration.

Waiting and Timed-Waiting Threads

The Java language has built-in threading capabilities, as demonstrated by the wait and
notify methods of the Object class. The wait method invoked on an Object causes the
thread to wait until another thread calls notify on the same Object . We discuss the details
in the section “The wait , notify , and notifyAll Methods,” but for now let's see the effect
these methods have on the state of a thread.
The wait method in Object has three overloaded versions:

public final void wait() throws InterruptedException
public final void wait(long timeout) throws InterruptedException
public final void wait(long timeout, int nanos) throws InterruptedException

The wait method causes the thread to wait for either notify or notifyAll to be invoked
on the same object or wait for the specifi ed time to elapse. The wait and notify methods
cannot be invoked unless the thread has the object's lock. When wait is invoked, the lock