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(){
			@Override
			public 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() {

			@Override
			public 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() {

			@Override
			public 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() {

			@Override
			public 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

public
interface 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。