性能优化中，其中一块是线程池的使用，初探线程池，从简单了解到源码分析，深入理解才能合理运用。
（一）线程池的来源及优点
单个异步任务，每次都需创建线程、销毁线程，当任务处理的时间短而请求数却巨大时，将导致资源消耗过多。
比如，数据库连接，需消耗大量资源，建立线程池能重用数据库访问线程。
使用线程池的优点：
1.降低资源消耗。重用现有线程，减少线程创建销毁开销。
2.提高响应速度。任务启动，无需经历线程创建，速度提升。
3.线程控制性强。如定时、定期、单线程设置等。
（二）线程池类别
四种线程池类型：
1.newCachedThreadPool
有缓存的线程池，适用于大量短期任务
2.newFixedThreadPool
定长的线程池，可控制最大线程并发数
3.newScheduledThreadPool
定长的线程池，可定时和周期性执行线程
4.newSingleThreadExecutor
单线程的线程池，保证所有任务按顺序执行
（三）线程池类别实例分析
1.newCachedThreadPool
private void newCacheThreadPool() {
ExecutorService threadPool = Executors.newCachedThreadPool();
for (int i = 0; i < 5; i++) {
final int index = i;
try {
if (i == 1 || i == 3) {
Thread.sleep(100);
}
}
catch (InterruptedException e) {
e.printStackTrace();
}
threadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("new cached thread pool " + index + ":" +
Thread.currentThread().getId());
}
});
}
}
运行结果：
new cached thread pool 0:9 new cached thread pool 1:9 new cached thread pool 2:10 new cached thread pool 3:10 new cached thread pool 4:9例子分析：因为1和3线程休息了一下，等待其他线程执行完，所以重用了旧有的线程。
2.newFixedThreadPool
private void newFixedThreadPool() {
ExecutorService threadPool = Executors.newFixedThreadPool(2);
for (int i = 0; i < 5; i++) {
final int index = i;
threadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("new fixed thread pool " + index + ":" +
Thread.currentThread().getId());
}
});
}
}
运行结果：
new fixed thread pool 0:9 new fixed thread pool 1:10 new fixed thread pool 2:9 new fixed thread pool 4:9 new fixed thread pool 3:10例子分析：定长为2，其他线程进入队列，等待前俩线程执行完才运行。
3.newScheduledThreadPool
private void newScheduledThreadPool() {
ScheduledExecutorService threadPool = Executors.newScheduledThreadPool(2);
try {
Thread.sleep(100);
}
catch (InterruptedException e) {
e.printStackTrace();
}
threadPool.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
System.out.println("new scheduled thread pool :" + Thread.currentThread().getId());
}
}, 1, 3, TimeUnit.SECONDS);
}
运行结果：
new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9 new scheduled thread pool :9例子分析：延迟1秒开始，每隔3秒运行一次。
4.newSingleThreadExecutor
private void newSingleThreadPool() {
ExecutorService threadPool = Executors.newSingleThreadExecutor();
for (int i = 0; i < 5; i++) {
final int index = i;
threadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("new single thread pool " + index + ":" +
Thread.currentThread().getId());
}
});
}
}
运行结果：
new single thread pool 0:9 new single thread pool 1:9 new single thread pool 2:9 new single thread pool 3:9 new single thread pool 4:9例子分析：只有单线程，所以，每次只能执行一个线程，其余任务等待状态。
线程池源码分析请看java-线程池（二）