什么是Looper

android源码上Looper类的注释为：

• 用于为线程运行消息循环的类，在默认情况下线程是没有与其关联的消息循环的;可以通过在线程中调用Looper.prepare()创建一个与线程绑定的消息循环,让其处理消息，直到循环停止。
• 大多数情况下与Looper（消息循环） 的交互都是通过 Handler类进行的
• 下述代码展示了一个典型的线程创建Looper的实例：

class LooperThread extends Thread {public Handler mHandler;public void run() {Looper.prepare();mHandler = new Handler() {public void handleMessage(Message msg) {//处理传入进来的消息}}Looper.loop();}
}
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Looper作用

• 此类用于准备线程的消息循环类（prepare(boolean quitAllowed)） 、开始消息循环(loop())、消息循环的退出(quit() /quitSafely())。

Looper源码解析

在Looper作用中提到了其三个作用和对应的方法,下面分别来看这些方法的实现：

1. prepare()

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) );}
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上述代码调用了 ThreadLocal.set() 保存了Looper实例

private Looper(boolean quitAllowed) {mQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();}
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Looper的构造方法中对MessageQueue和Thread进行了实例化。

1. loop()

/*** Run the message queue in this thread. Be sure to call* {@link #quit()} to end the loop.*/public static void loop() {final Looper me = myLooper();if (me == null) {throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");}final MessageQueue queue = me.mQueue;for (;;) {Message msg = queue.next(); // might blockif (msg == null) {// No message indicates that the message queue is quitting.return;}try {msg.target.dispatchMessage(msg); } msg.recycleUnchecked();}}
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上述代码是经过删减留下的主要逻辑代码，其中可以看到：

• 获取当前的Looper实例进行非空判断。
• 获取当前Looper中的MessageQueue。
• 通过一个死循环调用queue.next方法取出Message
• 当Message为空时表明MessageQueue正在退出，则跳出循环
• 否则继续往下通过Message.target获取Handler，调用其dispatchMessage将Message分发给Handler处理
• 最后调用Message的 recycleUnchecked() 方法将其内容清除并放入消息池中循环使用(节省了Message创建以及销毁的时间，并提高了内存的使用率);

1. quit() /quitSafely()

/*** Quits the looper.* <p>* Causes the {@link #loop} method to terminate without processing any* more messages in the message queue.* </p><p>* Any attempt to post messages to the queue after the looper is asked to quit will fail.* For example, the {@link Handler#sendMessage(Message)} method will return false.* </p><p class="note">* Using this method may be unsafe because some messages may not be delivered* before the looper terminates.  Consider using {@link #quitSafely} instead to ensure* that all pending work is completed in an orderly manner.* </p>** @see #quitSafely*/public void quit() {mQueue.quit(false);}
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  /*** Quits the looper safely.* <p>* Causes the {@link #loop} method to terminate as soon as all remaining messages* in the message queue that are already due to be delivered have been handled.* However pending delayed messages with due times in the future will not be* delivered before the loop terminates.* </p><p>* Any attempt to post messages to the queue after the looper is asked to quit will fail.* For example, the {@link Handler#sendMessage(Message)} method will return false.* </p>*/public void quitSafely() {mQueue.quit(true);}
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上述quit()方法会将消息队列中的所有消息移除；而quitSafely()则是将消息队列中所有的延迟消息移除，及时消息继续处理结束后再退出。上述两个方法在调用过后，再调用Handler的sendMessage则会返回false，也就是不能再发送消息了。

1. myLooper()

/*** Return the Looper object associated with the current thread.  Returns* null if the calling thread is not associated with a Looper.*/public static @Nullable Looper myLooper() {return sThreadLocal.get();}
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上述方法通过ThreadLocal获取当前线程的Looper。

1. ThreadLocal

   在perpare() 中我们看到调用了ThreadLocal的set方法保存了当前线程的Looper实例，下面展示set的代码

/*** Sets the current thread`s copy of this thread-local variable* to the specified value.  Most subclasses will have no need to* override this method, relying solely on the {@link #initialValue}* method to set the values of thread-locals.** @param value the value to be stored in the current thread`s copy of*        this thread-local.*/public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}
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可以看出会先获取当前线程的ThreadLocalMap（此类为ThreadLocal的静态内部类），然后根据判断结果通过不同的方法保存value值;下面展示ThreadLocalMap的set方法

/*** 该数组根据需要调整大小.* 该数组长度总是为2的幂数.*/private Entry[] table;private void set(ThreadLocal<?> key, Object value) {// We don`t use a fast path as with get() because it is at// least as common to use set() to create new entries as// it is to replace existing ones, in which case, a fast// path would fail more often than not.Entry[] tab = table;int len = tab.length;int i = key.threadLocalHashCode & (len-1);for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]) {ThreadLocal<?> k = e.get();if (k == key) {e.value = value;return;}if (k == null) {replaceStaleEntry(key, value, i);return;}}tab[i] = new Entry(key, value);int sz = ++size;if (!cleanSomeSlots(i, sz) && sz >= threshold)rehash();}
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• 首先循环Entry数组（Entry为继承弱引用的类）
  • 判断是否有与key相等的值，若有则替换value值并返回；
  • 判断是k值是非为空，若为空则将其替换为key并返回
• new个新的Entry值赋给tab[i]
• 上述Entry结构为

        static class Entry extends WeakReference<ThreadLocal<?>> {/*** The value associated with this ThreadLocal.*/Object value;Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}}
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在myLooper() 中我们看到调用了ThreadLocal的get方法获取了当前线程的Looper实例，下面展示get的代码

public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T) e.value;return result;}}return setInitialValue();}
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可以看出与set()方法类似首先会取出当前线程的ThreadLocalMap，然后将匹配的Entry取出，通过之前代码我们知道Looper的实例赋值给了value的，因此e.value的值即为当前线程的Looper。

总结

• 阅读Looper源码得知，我们可以通过Looper与Handler的组合让线程一直保持着待命的状态，当有消息时能及时处理消息。
• looper是存储在当前线程的ThreadLocalMap中的，ThreadLocal的get和set方法都是操作当前线程的ThreadLocalMap的，因此在不同的线程中调用ThreadLocal的get和set方法操作的对象是不同的。