Android性能优化之使用线程池

在写程序时有些异步程序只执行一遍就不需要了，为了方便经常会写下面的代码

new Thread(new Runnable() {     @Override    public void run() {        // TODO Auto-generated method stub    }}).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
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

Java的线程池对Android也是适用的
线程池的作用：
线程池作用就是限制系统中执行线程的数量。
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。
为什么要用线程池:
1.减少了创建和销毁线程的次数，每个工作线程都可以被重复利用，可执行多个任务。
2.可以根据系统的承受能力，调整线程池中工作线线程的数目，防止因为消耗过多的内存，而把服务器累趴下(每个线程需要大约1MB内存，线程开的越多，消耗的内存也就越大，最后死机)。

Java通过Executors提供四种线程池，分别为：
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。
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。
newScheduledThreadPool 创建一个定长线程池，支持定时及周期性任务执行。
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 < 50; i++) {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 blocke.printStackTrace();}executorService.execute(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("active count = " + Thread.activeCount()+ " index = " + index);try {Thread.sleep(1000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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 = 11 index = 30
active count = 11 index = 31
active count = 11 index = 32
active count = 11 index = 33
active count = 11 index = 34
active count = 11 index = 35
active count = 11 index = 36
active count = 11 index = 37
active count = 11 index = 38
active count = 11 index = 39
active count = 11 index = 40
active count = 11 index = 41
active count = 11 index = 42
active count = 11 index = 43
active count = 11 index = 44
active count = 11 index = 45
active count = 11 index = 46
active count = 11 index = 47
active count = 11 index = 48
active count = 10 index = 49
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() {@Overridepublic void run() {try {System.out.println("index = " + index+ "  thread count = " + Thread.activeCount());Thread.sleep(2000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.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() {@Overridepublic 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() {@Overridepublic 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 = 2delay 1 seconds, and excute every 3 seconds  thread count = 3delay 1 seconds, and excute every 3 seconds  thread count = 4delay 1 seconds, and excute every 3 seconds  thread count = 4delay 1 seconds, and excute every 3 seconds  thread count = 4delay 1 seconds, and excute every 3 seconds  thread count = 4delay 1 seconds, and excute every 3 seconds  thread count = 4delay 1 seconds, and excute every 3 seconds  thread count = 4delay 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() {@Overridepublic void run() {try {System.out.println("index = " + index+ "  thread count = " + Thread.activeCount());Thread.sleep(1000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}});}}

打印结果：

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

只创建了一个线程