Threads - Beginning Java 8 Language Features - page 223

Thread started...
Going to sleep...
Going to sleep...
Going to sleep...
Suspended...
Resumed...
Going to sleep...
Going to sleep...
Going to sleep...
Thread stopped...
Note that you have two instance variables for the StopSuspendResume class. The suspended instance variable is
not declared volatile . It is not necessary to declare it volatile because it is always accessed inside a synchronized
method/block. The following code in the run() method is used to implement the suspend and resume features:
synchronized (this) {
while (suspended) {
System.out.println("Suspended...");
this.wait();
System.out.println("Resumed...");
}
}
When the suspended instance variable is set to true , the thread calls the wait() method on itself to wait. Note
the use of the synchronized block. It uses this as the object to synchronize. This is the reason that you can call
this.wait() inside the synchronized block because you have obtained the lock on this object before entering the
synchronized block. Once the this.wait() method is called, the thread releases the lock on this object and keeps
waiting in the wait set until another thread calls the resumeThread() method to notify it. I also use the this.notify()
method call inside the stopThread() method because if the thread is suspended when the stopThread() method is
called, the thread will not stop; rather, it will remain suspended.
The thread in this example sleeps for only one second in its run() method. Suppose your thread sleeps for an
extended period. In such a case, calling the stopThread() method will not stop the thread immediately because the
thread will stop only when it wakes up and checks its keepRunning instance variable value in its next loop iteration.
In such cases, you can use the interrupt() method inside the stopThread() method to interrupt sleeping/waiting
threads, and when InterruptedException is thrown, you need to handle it appropriately.
If you use the technique used in Listing 6-25 to stop a thread, you may run into problems in some situations. The
while-loop inside the run() method depends on the keepRunning instance variable, which is set in the stopThread()
method. The example in this listing is simple. It is just meant to demonstrate the concept of how to stop, suspend,
and resume a thread. Suppose inside the run() method, your code waits for other resources like calling a method
someBlockingMethodCall() as shown:
while (keepRunning) {
try {
someBlockingMethodCall();
}
catch (InterruptedException e) {
e.printStackTrace();
}
}