handler有两个用途： 1：安排消息或者runnable在某个时间执行 2：将不同线程的消息排入消息队列
上面代码就是sendMessage方法，我们在子线程中发送消息，在主线程接收消息，更新ui。 这里产生了一个问题： 1：子线程的消息如何变成了主线程？ 这里先不做回答，后面统一解决这些问题。
当然除了post，我们还可以使用postAtTime(Runnable r, long uptimeMillis)定时发送等。
首先我们从handler的构造方法开始：
public Handler() { this(null, false); } public Handler(Callback callback) { this(callback, false); } public Handler(Looper looper) { this(looper, 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 " + Thread.currentThread() + " that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }可以看到当我们new handler(),也就是调用空构造时，执行的handler两个参数的构造方法，这个方法中主要初始化了looper，messageQueue，Callback。 接着我们看下是如何初始化的，首先Looper.myLooper()：
public static @Nullable Looper myLooper() { return sThreadLocal.get(); }这里我们看到looper是从sThreadLocal中获取的，而sThreadLocal是ThreadLocal。
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();那这个ThreadLocal是什么东西呢。 简单翻译下源码中对这个类的描述：
该类提供局部线程变量，每个线程通过get或者set方法获取自己的变量副本。接着我们看下get 和set方法
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(); }首先获取当前线程，通过getMap返回ThreadLocalMap ，
ThreadLocalMap getMap(Thread t) { return t.threadLocals; }可以看到ThreadLocalMap 是从线程的threadLocals变量中获取。 而ThreadLocalMap的key传入的是this，也就是变量threadLocals。
public class Thread implements Runnable { ThreadLocal.ThreadLocalMap threadLocals = null; }set很简单，就是传入value，根据map是否为空来做new map还是直接set的处理：
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); } void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }总结下：ThreadLocal的值是放入了当前线程的一个ThreadLocalMap实例中，所以只能在本线程中访问，其他线程无法访问。也就是说，在多线程处理同一个 ThreadLocal进行操作时，不影响其他线程，最终获取的还是当前线程的数据。 我们验证下这个结论：
public class MainActivity extends AppCompatActivity { ThreadLocal<Integer> threadLocal=new ThreadLocal<>(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); threadLocal.set(100); for (int i=0;i<10;i++){ LocalThread localThread1=new LocalThread (i); localThread1.start(); } try { Thread.sleep(3000); Log.e("xx","threadLocal-->"+threadLocal.get()); } catch (InterruptedException e) { e.printStackTrace(); } } class LocalThread extends Thread{ int pos; public LocalThread1(int size){ pos=size; } @Override public void run() { super.run(); threadLocal.set(pos); Log.e("xx","threadLocal-->"+threadLocal.get()+"--x="+(++s)); } } }看下打印结果：
07-25 16:50:36.287 3575-3598/? E/xx: threadLocal-->1--x=1 07-25 16:50:36.287 3575-3597/? E/xx: threadLocal-->0--x=2 07-25 16:50:36.287 3575-3599/? E/xx: threadLocal-->2--x=3 07-25 16:50:36.287 3575-3600/? E/xx: threadLocal-->3--x=4 07-25 16:50:36.287 3575-3601/? E/xx: threadLocal-->4--x=5 07-25 16:50:36.287 3575-3602/? E/xx: threadLocal-->5--x=6 07-25 16:50:36.287 3575-3603/? E/xx: threadLocal-->6--x=7 07-25 16:50:36.287 3575-3604/? E/xx: threadLocal-->7--x=8 07-25 16:50:36.287 3575-3605/? E/xx: threadLocal-->8--x=9 07-25 16:50:36.297 3575-3606/? E/xx: threadLocal-->9--x=10 07-25 16:50:39.297 3575-3575/com.zh.myview E/xx: threadLocal-->100可以看到虽然我们在不同线程操作threadLocal，但是通过get获取到的值是不会受到影响的。而且在经过set方法后，我们传入的value，也是和当前线程进行了绑定。 这里threadLocal就差不多分析完了，我们回到正题handler的初始化，从当前线程获取到looper后，我们看到源码中looper进行了判空操作：
if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread " + Thread.currentThread() + " that has not called Looper.prepare()"); }意思大概是在调用Looper.myLooper方法之前必须调用Looper.prepare方法，否则就会抛出异常。 我们看下prepare做了什么操作：
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)); }好吧，两步操作： 1：判断不为空就抛出异常，所以，这里我们可以得到一个结论，一个线程只有一个looper。 2：new looper。
这里会有人问了，上面handler的空构造中调用myLooper时，没有使用prepare方法啊，为什么没有报错呢。 好吧，这里activity已经帮我们做了这个操作了，准备来说是activityThread中调用了，我们看下activityThread的main方法。
public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); // Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line. // It will be in the format "seq=114" long startSeq = 0; if (args != null) { for (int i = args.length - 1; i >= 0; --i) { if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) { startSeq = Long.parseLong( args[i].substring(PROC_START_SEQ_IDENT.length())); } } } ActivityThread thread = new ActivityThread(); thread.attach(false, startSeq); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }瞅了半天发现没有prepare方法，但是有一个Looper.prepareMainLooper()：
public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } }这里looper.myLooper()结束。继续分析looper.mQueue;
final MessageQueue mQueue; private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }我们知道在prepare调用了set，从而初始化了Looper，所以，mQueue这里就直接获取了。
可以看到不管是sendMessage还是sendEmptyMessage最后调用的都是sendMessageDelayed，而sendMessageDelayed内部调用的是sendMessageAtTime，所以我们来看看sendMessageAtTime内部做了哪些操作：
public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); }两步操作：获取当前的消息队列，将消息当做参数，传入enqueueMessage：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }这里将this，也就是当前handler传给了message的target。 然后调用messagequeue的enqueueMessage方法：
boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); msg.when = when; Message p = mMessages; boolean needWake; if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don't have to wake // up the event queue unless there is a barrier at the head of the queue // and the message is the earliest asynchronous message in the queue. needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true; }enqueueMessage方法主要进行message插入messagequeue的操作。 那怎么从messagequeue取出message呢？ 答案是Looper.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; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); // Allow overriding a threshold with a system prop. e.g. // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start' final int thresholdOverride = SystemProperties.getInt("log.looper." + Process.myUid() + "." + Thread.currentThread().getName() + ".slow", 0); boolean slowDeliveryDetected = false; for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long traceTag = me.mTraceTag; long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs; if (thresholdOverride > 0) { slowDispatchThresholdMs = thresholdOverride; slowDeliveryThresholdMs = thresholdOverride; } final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0); final boolean logSlowDispatch = (slowDispatchThresholdMs > 0); final boolean needStartTime = logSlowDelivery || logSlowDispatch; final boolean needEndTime = logSlowDispatch; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0; final long dispatchEnd; try { msg.target.dispatchMessage(msg); dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0; } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (logSlowDelivery) { if (slowDeliveryDetected) { if ((dispatchStart - msg.when) <= 10) { Slog.w(TAG, "Drained"); slowDeliveryDetected = false; } } else { if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery", msg)) { // Once we write a slow delivery log, suppress until the queue drains. slowDeliveryDetected = true; } } } if (logSlowDispatch) { showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg); } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } }大致先获取当前线程的looper，从looper中获取messagequeue。 然后通过for循环（死循环）取出queue中的message，取出的方法是queue.next(). 其中处理的方法为:
msg.target.dispatchMessage(msg);我们知道在sendmessage时，通过handler的enqueueMessage方法将handler赋值给了msg.target。所以dispatchMessage还是handler的方法:
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }这里分为两种情况： 1：msg.callback ！=null，
private static void handleCallback(Message message) { message.callback.run(); }callback是message的变量Runnable ， 也就是说最后调用的是runnable的run方法。 2：当msg.callback==null时，判断mCallback 是不是为null；mCallback是在handler的构造方法中传入，如果不为null，就判断传入callback的handleMessage。如果是true，则直接返回； 也就是说，如果我们在handler的创建时传入callback，并且重写handleMessage，将返回值改成true，handler就会被拦截。 流程最后会调用 handleMessage(msg);
public void handleMessage(Message msg) { }handleMessage是空方法，需要我们自己处理，也就是在这里处理ui更新等操作。
这个问题在开头时，我们提出来，这里分析完源码我们应该可以总结出来了吧？
在handler发送message时，调用enquemessage与message绑定，并插入messagequeue，当执行looper.loop方法循环取出message时，通过msg.targer也就是handler的handlemessage处理，而我们无参创建的handler是主线程，
在handler的构造中可以传入looper参数，所以我们先创建一个handler：
private Handler mHandler; Looper myLooper; public void createHandler(){ handler=new Handler(myLooper){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); //ui操作 }; } }接着开启子线程，
new Thread(new Runnable() { @Override public void run() { //创建Looper和MessageQueue对象 Looper.prepare(); // 获取当前线程下的Looper对象 mLooper = Looper.myLooper(); createHandler(); // 开启Looper循环 Looper.loop(); } }).start();