Android 线程池ThreadPool的基本使用_android 线程池使用

此文章是参考其它文章写的，因为本人之前没怎么接触过线程池，所以可能与其它文章相似。在这里只是做一个记录，以后继续深入学习！

我在写异步时，经常这样写：

new Thread(new Runnable() {
    @Override
    public void run() {
        // TODO
    }
}).start();

这样new出来的匿名对象会存在一些问题：

1. 由于是匿名的，无法对它进行管理
2. 如果需要多次执行这个操作就new多次，可能创建多个，占用系统资源
3. 无法执行更多的操作

所以，使用线程池的好处：

1. 可以重复利用存在的线程，减少系统的开销
2. 利用线程池可以执行定时、并发数的控制

原理：根据系统的环境情况，可以自动或手动设置线程数量，达到运行的最佳效果；少了浪费了系统资源，多了造成系统拥挤效率不高。用线程池控制线程数量，其他线程排队等候。一个任务执行完毕，再从队列的中取最前面的任务开始执行。若队列中没有等待进程，线程池的这一资源处于等待。当一个新任务需要运行时，如果线程池中有等待的工作线程，就可以开始运行了；否则进入等待队列。

四种线程池

（1）newCachedThreadPool：

创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。

（2）newFixedThreadPool：

创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。

（3）newScheduledThreadPool：

创建一个定长线程池，支持定时及周期性任务执行。

（4）newSingleThreadExecutor：

创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。

1.newCachedThreadPool

/*
     * 可以缓存线程池
     */
    public static void Function1() {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (int i = 0; i < 30; i++) {//30次
            final int index = i;
            try {
                Thread.sleep(100); // 休眠时间越短创建的线程数越多
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    System.out.println("active count = " + Thread.activeCount()
                            + " index = " + index);
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            });
        }
    }

打印结果：

active count = 2 index = 0
active count = 3 index = 1
active count = 4 index = 2
active count = 5 index = 3
active count = 6 index = 4
active count = 7 index = 5
active count = 8 index = 6
active count = 9 index = 7
active count = 10 index = 8
active count = 11 index = 9
active count = 11 index = 10
active count = 11 index = 11
active count = 11 index = 12
active count = 11 index = 13
active count = 11 index = 14
active count = 11 index = 15
active count = 11 index = 16
active count = 11 index = 17
active count = 11 index = 18
active count = 11 index = 19
active count = 11 index = 20
active count = 11 index = 21
active count = 11 index = 22
active count = 11 index = 23
active count = 11 index = 24
active count = 11 index = 25
active count = 11 index = 26
active count = 11 index = 27
active count = 11 index = 28
active count = 11 index = 29
active count = 10 index = 30

从打印消息来看开始线程数在增加，后来稳定，可以修改休眠时间，休眠时间越短创建的线程数就越多，因为前面的还没执行完，线程池中没有可以执行的就需要创建；如果把休眠时间加大，创建的线程数就会少

2.newFixedThreadPool

根据传入的参数创建线程数目

     /**
         * 定长线程池
         */
        public static void Function2 () {
            ExecutorService executorService = Executors.newFixedThreadPool(3);
            for (int i = 0; i < 30; i++) {
                final int index = i;
                executorService.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            System.out.println("index = " + index
                                    + "  thread count = " + Thread.activeCount());
                            Thread.sleep(2000);
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }
                    }
                });
            }
        }

3.newScheduledThreadPool

/**
         * 定长线程池，可做延时
         */
        public static void Function3 () {
            ScheduledExecutorService executorService = Executors
                    .newScheduledThreadPool(5);
            executorService.schedule(new Runnable() {
                @Override
                public void run() {
                    System.out.println("delay 3 seconds" + "  thread count = "
                            + Thread.activeCount());
                }
            }, 3, TimeUnit.SECONDS);
        }
        /**
         * 定期执行，可以用来做定时器
         */
        public static void Function4 () {
            ScheduledExecutorService executorService = Executors
                    .newScheduledThreadPool(3);
            executorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    System.out
                            .println("delay 1 seconds, and excute every 3 seconds"
                                    + "  thread count = " + Thread.activeCount());
                }
            }, 1, 3, TimeUnit.SECONDS);
        }

打印结果:

delay 1 seconds, and excute every 3 seconds  thread count = 2
delay 1 seconds, and excute every 3 seconds  thread count = 3
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4
delay 1 seconds, and excute every 3 seconds  thread count = 4

4.newSingleThreadExecutor这个最简单

/**
     * 单例线程
     */
    public static void Function5() {
        ExecutorService singleThreadExecutor = Executors
                .newSingleThreadExecutor();
        for (int i = 0; i < 5; i++) {
            final int index = i;
            singleThreadExecutor.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        System.out.println("index = " + index
                                + "  thread count = " + Thread.activeCount());
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            });
        }
    }

打印结果：

index = 0  thread count = 2
index = 1  thread count = 2
index = 2  thread count = 2
index = 3  thread count = 2
index = 4  thread count = 2

只创建了一个线程。