android 中的线程池
线程池的优点：
1 重用线程池中的线程，避免了线程的创建和销毁带来的性能开销
2 能有效的控制最大并发数，避免大量线程之间因为喜欢抢资源而导致阻塞
3 能够对线程进行简单的管理,提供定时执行以及指定间隔时间循环执行等
android 中的线程池源自java 中的Executor，Executor是一个接口，正真的实现是ThreadPoolExecutor。
ThreadPoolExecutor 提供参数配置线程池。
下面是一个常用的构造方法：
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
corePoolSize线程池的核心线程数
maximumPoolSize线程池中能容纳的最大线程数，当活动线程达到这个数之后，后续的任务会被阻塞
keepAliveTime 非核心线程闲置时的超时时长，超过这个时长，非核心线程就会被回收
unit 用于指定keepAliveTime参数的时间单位。
workQueue 线程池中的任务队列
threadFactory 线程工厂
ThreadPoolExecutor执行任务时大致遵循的规则：
1 如果线程池中的线程数量未达到核心线程的数量，那么会直接启动一个核心线程来执行任务。
2 如果线程池中的线程数量已达到或者超过核心线程的数量，那么任务会被插入到任务队列中排队等待执行
3 如果2中无法将任务插入到任务队列，这往往是由于任务队列已满，这个时候如果线程数量未达到线程池规定的最大值，那么会立刻启动一个非核心线程执行任务
4 如果3中线程数量达到线程池规定的最大值，那么就拒绝执行此任务ThreadPoolExecutor会调用rejectedExecution的rejectedExecution 来通知调用者
这个是AsyncTask的线程池的配置：
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;//核心线程数等于CPU核数+1
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;//线程池的最大线程数为CPU核数的2倍+1
private static final int KEEP_ALIVE = 1;//核心线程无超时机制，非核心线程超时1秒 private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//任务任务队列容量128
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128); public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
下面来看线程池的分类
Executors类创建线程池：
FixedThreadPool
Executors类newFixedThreadPool创建：
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
CachedThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(),
threadFactory);
}
ScheduledThreadPool
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
SingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1, threadFactory));
}