Java Threads, Second Edition

wait() and sleep()

The Object class also overloads the wait() method to allow it to take a timeout specified in milliseconds (though, as we mentioned in Chapter 2, the timeout resolution may not be as precise as one millisecond):

void wait(long timeout)

Waits for a condition to occur. However, if the notification has not occurred in timeout milliseconds, it returns anyway. This is a method of the Object class and must be called from a synchronized block or method.

void wait(long timeout, int nanos)

Waits for a condition to occur. However, if the notification has not occurred in timeout milliseconds and nanos nanoseconds, it returns anyway. This is a method of the Object class and must be called from a synchronized block or method.

These methods are provided to support external events. In cases where we are only concerned with a notification arriving, we normally do not use these methods. However, notifications can be dependent on external conditions, in which case we are also concerned with when a notification arrives. A timeout may be needed in case those conditions do not occur. As an example, we might write a program that connects to a stock feed server. The program may be willing to wait 30 seconds to connect to the server (that is, to satisfy the condition of being connected); if the connection does not occur within 30 seconds, the program may try to contact a backup server. We’d accomplish this by calling the wait(30000) method in our program.

We may still add a timeout when ...