java和android中让多个runnable按顺序依次执行

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让线程依次执行的难点就是如何判断前一个线程执行结束。runnable对象一个特性就是如果是用runnable.run();启动，那么这个runnable对象还是在启动它的线程中执行的，并不是在独立线程中执行，所以我们可以把多个runnable对象在一个线程中执行，那么这些runnable就是一个一个执行的，既然是按顺序执行的，那么保留顺序信息就要用到queue对象了，我们建立一个queue对象保存这个顺序信息。请看示例代码

首先创建一个类，这个类包含了一个queue对象

class SerialExecutor {final Queue<Runnable> tasks = new LinkedBlockingQueue<Runnable>();final Executor executor;Runnable active;SerialExecutor(Executor executor) {this.executor = executor;}public void addrun(Runnable r) {tasks.add(r);}public void execute(final Runnable r) {try {r.run();} finally {scheduleNext();}}protected void scheduleNext() {if ((active = tasks.poll()) != null) {this.execute(active);}}}

主类

public class Demo {public static void main(String[] args) {final Executor executor = new Executor() {@Overridepublic void execute(Runnable command) {// TODO Auto-generated method stub// new Thread(command).start();command.run();}};Runnable runnable1 = new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("Runnable1");}};Runnable runnable2 = new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("Runnable2");}};Runnable runnable3 = new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("Runnable3");}};Runnable runnable4 = new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("Runnable4");}};Runnable runnable5 = new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("Runnable5");}};SerialExecutor se = new SerialExecutor(executor);se.addrun(runnable1);se.addrun(runnable2);se.addrun(runnable3);se.addrun(runnable4);se.addrun(runnable5);se.scheduleNext();   ｝｝

这个流程就是在main类中创建5个Runnable对象，这5个Runnable对象分别加入SerialExecutor对象中的队列中，然后从队列中取出第一个对象，没有则结束方法。这个程序运行会按照12345的顺序打印Runnable。这个按顺序执行的原因就是因为Executor对象中的execute方法中是command.run(0)而不是new Thread(command).start()，这样每一个要执行的Runnable都会被阻塞在这一个一个执行。这个方法要注意不要在主线程中阻塞。

还有一个方法就是用到synchronized关键字的特性，把执行语句写在用synchronized修饰的方法中，取出队列中的对象执行

代码如下

class SerialExecutor {final Queue<Runnable> tasks = new LinkedBlockingQueue<Runnable>();final Executor executor;Runnable active;SerialExecutor(Executor executor) {this.executor = executor;}public void addrun(Runnable r) {tasks.add(r);}public synchronized void execute(final Runnable r) {try {r.run();} finally {scheduleNext();}}protected void scheduleNext() {if ((active = tasks.poll()) != null) {this.execute(active);}}}

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