Building Blocks - Java Concurrency in Practice - page 109

A synchronizer is any object that coordinates the control flow of threads based on its state.
Blocking queues can act as synchronizers; other types of synchronizers include semaphores,
barriers, and latches. There are a number of synchronizer classes in the platform library; if
these do not meet your needs, you can also create your own using the mechanisms described
in Chapter 14 .

All synchronizers share certain structural properties: they encapsulate state that determines
whether threads arriving at the synchronizer should be allowed to pass or forced to wait,
provide methods to manipulate that state, and provide methods to wait efficiently for the syn-
chronizer to enter the desired state.

5.5.1. Latches

A latch is a synchronizer that can delay the progress of threads until it reaches its terminal
state [CPJ 3.4.2]. A latch acts as a gate: until the latch reaches the terminal state the gate is
closed and no thread can pass, and in the terminal state the gate opens, allowing all threads
to pass. Once the latch reaches the terminal state, it cannot change state again, so it remains
open forever. Latches can be used to ensure that certain activities do not proceed until other
one-time activities complete, such as:
• Ensuring that a computation does not proceed until resources it needs have been ini-

tialized. A simple binary (two-state) latch could be used to indicate “Resource R has
been initialized”, and any activity that requires R would wait first on this latch.

• Ensuring that a service does not start until other services on which it depends have

started. Each service would have an associated binary latch; starting service S would
involve first waiting on the latches for other services on which S depends, and then
releasing the S latch after startup completes so any services that depend on S can then
proceed.

• Waiting until all the parties involved in an activity, for instance the players in a multi-

player game, are ready to proceed. In this case, the latch reaches the terminal state
after all the players are ready.

CountDownLatch is a flexible latch implementation that can be used in any of these situ-
ations; it allows one or more threads to wait for a set of events to occur. The latch state con-
sists of a counter initialized to a positive number, representing the number of events to wait
for. The countDown method decrements the counter, indicating that an event has occurred,
and the await methods wait for the counter to reach zero, which happens when all the events
have occurred. If the counter is nonzero on entry, await blocks until the counter reaches
zero, the waiting thread is interrupted, or the wait times out.