在Android中，主线程不能执行耗时的操作，否则可能会导致ANR。那么，耗时操作应该在其它线程中执行。线程的创建和销毁都会有性能开销，创建过多的线程也会由于互相抢占系统资源而导致阻塞的现象。这个时候，就需要使用线程池。
线程池的优点可以概括为以下几点:
1、重用线程池中的线程，避免线程创建、销毁带来的性能开销；
2、能有效地控制线程池的最大并发数，避免大量的线程之间因互相抢占系统资源导致的阻塞现象；
3、能够对线程进行简单的管理。
以上线程池的优点引用自《Android开发艺术探索》
线程池的具体实现类为ThreadPoolExecutor，ThreadPoolExecutor继承自AbstractExecutorService,AbstractExecutorService又实现了ExecutorService接口，ExecutorService继承自Executor。
ThreadPoolExecutor有四个重载的构造方法：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}最终都调用到了:
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}对应这个方法中的参数:
corePoolSize:核心线程数。如果没有设置allowCoreThreadTimeOut为true，则核心线程空闲时也不会销毁。如果设置allowCoreThreadTimeOut为true,则受keepAliveTime控制，空闲时间超过keepAliveTime，会被回收。
maximumPoolSize:最大线程数。
keepAliveTime:非核心线程的空闲超时时长。超过这个时间，非核心线程会被回收。核心线程如果allowCoreThreadTimeOut为true,则在空闲超过这个时间也会被回收。
unit:超时的单位。
workQueue:线程池中的任务队列。通过线程池的execute()方法提交的Runnable任务会被放入任务队列中。
threadFactory:线程工厂。
handler:饱和策略。当任务队列和线程池都满后，对新提交的任务的处理策略。
ThreadPoolExecutor执行任务的规则:
1、如果线程池中的线程数量未达到核心线程数量，则开启一个新的核心线程来执行任务；
2、如果线程池中的线程数量已经大于等于核心线程数量，则会把新的任务放入任务队列中；
3、如果任务队列已满，并且线程池中的线程未满，则开启非核心线程来处理新的任务；
4、如果任务队列和线程池都已满，则会交给handler饱和策略来处理。
下面通过一个简单的案例来验证以上规则：
static class WorkThread implements Runnable {
private String name;
public WorkThread(String name) {
this.name = name;
}
public void run() {
try {
Thread.sleep(5000);
System.out.println("Thread: " + name + " work finish");
} catch (Exception e) {
e.printStackTrace();
}
}
};
Executor executor = new ThreadPoolExecutor(5, 100, 60, TimeUnit.SECONDS, new LinkedBlockingQueue<>(5));
for (int i = 0; i < 20; i++) {
WorkThread thread = new WorkThread("" + i);
executor.execute(thread);
}定义了一个线程池，核心线程池数量为5，线程数量为100，超时时间为60秒，任务队列为5。在子线程中，sleep 5秒来模拟耗时的操作。然后开启了20个线程，并放入线程池中执行。执行的结果如下：
Thread: 0 work finish
Thread: 1 work finish
Thread: 13 work finish
Thread: 10 work finish
Thread: 3 work finish
Thread: 11 work finish
Thread: 4 work finish
Thread: 2 work finish
Thread: 19 work finish
Thread: 18 work finish
Thread: 17 work finish
Thread: 15 work finish
Thread: 16 work finish
Thread: 14 work finish
Thread: 12 work finish
Thread: 5 work finish
Thread: 9 work finish
Thread: 8 work finish
Thread: 7 work finish
Thread: 6 work finish前五个线程在核心线程中执行，第6-10个线程由于核心线程已满，因此在任务队列中等待执行，第11-20个线程，由于核心线程和队列都已满，而线程池中还可以开启线程，因此在非核心线程中执行。从结果来看，线程0-4，10-19会先执行完，然后任务队列中的线程5-9才执行，验证了以上的线程池任务执行规则。
为便于使用线程池，线程池还有几种简便的定义方法：
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}通过Executors的static方法来创建。其中:
FixedThreadPool:线程数量固定的线程池。从方法的定义来看，这种线程池里的线程全都是核心线程，并且没有超时时间,任务队列也是没有限制的。
CachedThreadPool:这种线程池没有核心线程，全是非核心线程，并且超时时间为60秒，任务队列没有限制。这种线程适合执行大量的耗时较短的任务。
SingleThreadExecutor:只有一个核心线程，没有超时时间，任务队列没有限制。可以确保任务按顺序执行。
ScheduledThreadPool:核心线程数量固定。非核心线程没有限制。非核心线程闲置时会被立即回收。这类线程池适合执行定时任务和具有固定周期的重复任务。