Guarded Suspension

Origin:

Lea 97

Reason:

A method should wait to execute until a precondition holds.

Synopsis:

Use an object's methods wait and notify.

Example:

A queue is used by two threads to exchange objects. A thread can take an object from the queue only if the queue contains elements.

Solution:

In its simplest form, the method guarded by a precondition looks like:

synchronized void guardedMethod () {
    while (! precondition) { wait (); }
        ... // code requiring precondition
}

Any method that might change the precondition looks like:

synchronized void stateChangingMethod () {
    ... // code possibly changing precondition
    notify ();
}

Both methods are synchronized on the same object.