Android多线程研究（1）——线程基础及源码剖析

从今天起我们来看一下Android中的多线程的知识，Android入门容易，但是要完成一个完善的产品却不容易，让我们从线程开始一步步深入Android内部。

一、线程基础回顾

package com.maso.test;public class TraditionalThread {public static void main(String[] args) {/* * 线程的第一种创建方式 */Thread thread1 = new Thread(){@Overridepublic void run() {try {sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while(true){System.out.println(Thread.currentThread().getName());}}};thread1.start();/* *线程的第二种创建方式  */Thread thread2 = new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while (true) {System.out.println(Thread.currentThread().getName());}}});thread2.start();/* * 线程的调用优先级 */new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while(true){System.out.println("Runnable");}}}){public void run() {try {sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while(true){System.out.println("Thread");}};}.start();}}

上面代码中是我们都很熟悉的线程的两种创建方式，如果对这些还感到陌生请先看Java线程基础。

打开Thread类的源码可以看到Thread类有8个构造函数，我们先看看上面的两种构造函数的源码。

    public Thread() {        init(null, null, "Thread-" + nextThreadNum(), 0);    }

在构造的时候直接调用了init方法

    private void init(ThreadGroup g, Runnable target, String name,                      long stackSize) {        if (name == null) {            throw new NullPointerException("name cannot be null");        }        Thread parent = currentThread();        SecurityManager security = System.getSecurityManager();        if (g == null) {            /* Determine if it's an applet or not */            /* If there is a security manager, ask the security manager               what to do. */            if (security != null) {                g = security.getThreadGroup();            }            /* If the security doesn't have a strong opinion of the matter               use the parent thread group. */            if (g == null) {                g = parent.getThreadGroup();            }        }        /* checkAccess regardless of whether or not threadgroup is           explicitly passed in. */        g.checkAccess();        /*         * Do we have the required permissions?         */        if (security != null) {            if (isCCLOverridden(getClass())) {                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);            }        }        g.addUnstarted();        this.group = g;        this.daemon = parent.isDaemon();        this.priority = parent.getPriority();        this.name = name.toCharArray();        if (security == null || isCCLOverridden(parent.getClass()))            this.contextClassLoader = parent.getContextClassLoader();        else            this.contextClassLoader = parent.contextClassLoader;        this.inheritedAccessControlContext = AccessController.getContext();        this.target = target;        setPriority(priority);        if (parent.inheritableThreadLocals != null)            this.inheritableThreadLocals =                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);        /* Stash the specified stack size in case the VM cares */        this.stackSize = stackSize;        /* Set thread ID */        tid = nextThreadID();    }

里面的东西比较多，但是我们可以看到会初始化一个变量Runnable target;

下面我们再来看看run方法中是个什么东东？

    @Override    public void run() {        if (target != null) {            target.run();        }    }

原来run方法中会先判断是否初始化了Runnable target变量，如果没有则空实现，如果target不为空则先执行Runnable接口中的run方法。有的朋友可能会猜想下面的代码会先调用Runnable接口中的run方法，然后才调用Thread实现类中的run方法。

/* * 线程的调用优先级 */new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while(true){System.out.println("Runnable");}}}){public void run() {try {sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}while(true){System.out.println("Thread");}};}.start();

其实事实并非如此，因为上面代码中是一个匿名内部类，实际上是一种从Thread的继承和实现，所以下面的run方法覆盖了Thread中的run方法，所以Runnable中的run方法根本不会执行。

下面再看看Runnable接口的源代码

publicinterface Runnable {    /**     * When an object implementing interface <code>Runnable</code> is used     * to create a thread, starting the thread causes the object's     * <code>run</code> method to be called in that separately executing     * thread.     * <p>     * The general contract of the method <code>run</code> is that it may     * take any action whatsoever.     *     * @see     java.lang.Thread#run()     */    public abstract void run();}

发现Runnable接口只有一个抽象的run方法。

为什么要搞一个Runnable接口来实现多线程呢？从Thread继承不是更方便吗？Runnable接口有如下优势，所以我们常常会选择实现Runnable接口：

1、适合多个程序代码的线程去处理同一个资源。

public class ThreadTest1 extends Thread {private int count = 5;     public void run() {        for (int i = 0; i < 7; i++) {            if (count > 0) {                System.out.println("count= " + count--);            }        }    }     public static void main(String[] args) {    //这样实际上是创建了三个互不影响的线程实例        ThreadTest1 t1 = new ThreadTest1();        ThreadTest1 t2 = new ThreadTest1();        ThreadTest1 t3 = new ThreadTest1();        t1.start();        t2.start();        t3.start();    }}

public class ThreadTest1{         public static void main(String [] args) {        MyThread my = new MyThread();        //开启了三个线程，但是操作的是同一个run方法        new Thread(my, "1号窗口").start();        new Thread(my, "2号窗口").start();        new Thread(my, "3号窗口").start();    } }class MyThread implements Runnable{     private int ticket = 5;  //5张票     public void run() {        for (int i=0; i<=20; i++) {            if (this.ticket > 0) {                System.out.println(Thread.currentThread().getName()+ "正在卖票"+this.ticket--);            }        }    }}

2、避免Java特性中的单根继承的限制。

3、可以保持代码和数据的分离（创建线程数和数据无关）。

4、更能体现Java面向对象的设计特点。

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