Java The Complete Reference, Seventh Edition

such as its synchronizers, callable threads, and executors, that are applicable to a wide variety

of situations. For these reasons, this chapter presents an overview of the concurrency utilities

and shows several examples of their use.

The Concurrent API Packages

The concurrency utilities are contained in thejava.util.concurrentpackage and in its two

subpackages,java.util.concurrent.atomicandjava.util.concurrent.locks. A brief overview

of their contents is given here.

java.util.concurrent

java.util.concurrentdefines the core features that support alternatives to the built-in approaches

to synchronization and interthread communication. It defines the following key features:

Synchronizers

Executors

Concurrent collections

Synchronizers offer high-level ways of synchronizing the interactions between multiple

threads. The synchronizer classes defined byjava.util.concurrentare

Semaphore Implements the classic semaphore

CountDownLatch Waits until a specified number of events have occurred

CyclicBarrier Enables a group of threads to wait at a predefined execution point

Exchanger Exchanges data between two threads

Notice that each synchronizer provides a solution to a specific type of synchronization

problem. This enables each synchronizer to be optimized for its intended use. In the past,

these types of synchronization objects had to be crafted by hand. The concurrent API

standardizes them and makes them available to all Java programmers.

Executors manage thread execution. At the top of the executor hierarchy is theExecutor

interface, which is used to initiate a thread.ExecutorServiceextendsExecutorand provides

methods that manage execution. There are two implementations ofExecutorService:

ThreadPoolExecutorandScheduledThreadPoolExecutor.java.util.concurrentalso

defines theExecutorsutility class, which includes a number of static methods that simplify

the creation of various executors.

Related to executors are theFutureandCallableinterfaces. AFuturecontains a value

that is returned by a thread after it executes. Thus, its value becomes defined “in the future,”

when the thread terminates.Callabledefines a thread that returns a value.

java.util.concurrentdefines several concurrent collection classes, including

ConcurrentHashMap,ConcurrentLinkedQueue, andCopyOnWriteArraylist. These offer

concurrent alternatives to their related classes defined by the Collections Framework.

Finally, to better handle thread timing,java.util.concurrentdefines theTimeUnit

enumeration.

788 Part II: The Java Library