ThreadLocal

往下看之前，需要了解一下Java的ThreadLocal类，可参考博文：

解密ThreadLocal

Looper、Handler和MessageQueue

我们分析一下之前的这段代码，查看一下Handler机制中，Handle、Looper和MessageQueue之间到底是什么关系

class WorkerThread extends Thread {
        @Override
        public void run() {
            super.run();
            Looper.prepare();
            handler = new Handler() {
                @Override
                public void handleMessage(Message msg) {
                    super.handleMessage(msg);
                    Log.d("TTTT", "handleMessage:" + Thread.currentThread().getName());
                    int i = msg.what;
                    Log.d("TTTT", "收到了消息对象:" + i);
                }
            };
            Looper.loop();
        }
    }

1. 首先执行 Looper.prepare() ;

查看Looper的源代码，路径为：sources\android-19\android\os\Looper.java

首先，Looper类的几个成员变量

    // sThreadLocal.get() will return null unless you‘ve called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    private static Looper sMainLooper;  // guarded by Looper.class
    final MessageQueue mQueue;
    final Thread mThread;

这就表明，每一个Looper对象，都与一个消息队列（MessageQueue）相对应，同时也与一个线程（Thread）相对应。

当我们调用Looper的prepare()方法的时候，执行如下代码：

public static void prepare() {
    prepare(true);
}
private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

从上面的代码可以看出，当执行了Looper.prepare()之后，会先调用ThreadLocal的get方法，判断是否已经存在以当前线程为键的键值对，如果已经存在，则抛出异常 “Only one Looper may be created per thread” 。如果不存在，则执行 sThreadLocal.set(new Looper(quitAllowed)) ;新建一个Looper对象，存入ThreadLocal中。

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

可以看出，创建Looper对象的同时，新建了一个MessageQueue对象，赋值给Looper对象的mQueue属性，将当前线程，赋值给Looper的mThread属性。

2. 接下来执行

    handler = new Handler() {
                @Override
                public void handleMessage(Message msg) {
                    super.handleMessage(msg);
                    Log.d("TTTT", "handleMessage:" + Thread.currentThread().getName());
                    int i = msg.what;
                    Log.d("TTTT", "收到了消息对象:" + i);
                }
            };

实例化一个Handler对象，执行其handleMessage方法，查看Handler的源码，路径：sources\android-19\android\os\Hnadler.java

可以看到Handler类有如下成员变量

    final MessageQueue mQueue;
    final Looper mLooper;
    final Callback mCallback;
    final boolean mAsynchronous;
    IMessenger mMessenger;

找到其Handler的构造方法

    public Handler() {
        this(null, false);
    }
        public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can‘t create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

调用了Looper的 myLooper() 方法赋值给mLooper

    mLooper = Looper.myLooper();

我们查看Looper.java

    public static Looper myLooper() {
        return sThreadLocal.get();
    }

发现将当前线程为键的Looper对象从ThreadLocal中取出并返回，所以，在创建Handler对象的时候，Handler对象的拿到了之前创建的Looper对象

　　if (mLooper == null) {
　　　　throw new RuntimeException(
                "Can‘t create handler inside thread that has not called Looper.prepare()");
　　}

如果拿到的Looper对象为空，则抛出异常 “Can‘t create handler inside thread that has not called Looper.prepare()” 告知用户在没有调用Looper.preapre()之前，不能创建Handler对象。

    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;

然后将Looper对象的mQueue给Handler的mQueue。

这样，就将Thread、Handler、Looper、MessageQueue联系起来了。