在Android中，Handler非常重要，在主線程的main方法中就使用了Handler，並且由於UI只能在UI線程上更新，Handler的使用更廣泛了，當然，Handler的使用不止是更新界面，例如：在子線程做一些耗時操作，完成後可能需要更新UI，不更新UI做一些其他事情也是可以的。
一般在多線程裏使用Handler都是通過下面的方式。
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what){
case MESSAGE:
Log.d(TAG, "handleMessage: ");
break;
}
}
};
mHandler.sendEmptyMessage(MESSAGE);
Looper.loop();
}
}).start();但是由於我們在main線程經常用Handler，好多人可能會問，怎麼還有Looper？這是什麼鬼？
那麼接下來，我就從源碼的角度來解釋這個問題，順便梳理一下源碼。
源碼的理解
Handler mHandler = new Handler()；這一步，我們進去看一看：
Handler類：
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
......
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;
}上面構造方法中mLooper = Looper.myLooper();，接下來會有個判斷，那麼什麼時候mLooper == null？
進入Looper.myLooper()；的源碼一探究竟，
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}到這裏我們差不多是明白了些，原來Looper.myLooper()是得到該線程的Looper，而looper是存放在了ThreadLocal裏面，因此是從ThreadLocal裏面取出了Looper。但是ThreadLocal裏面會有looper嗎？我們把第一段代碼裏面的Looper.prepare();註釋掉運行一下試試看：
拋出的異常正是if語句塊裏的代碼，而加上Looper.prepare();這行代碼後就沒有該異常出現了，那麼Looper.prepare()做了什麼呢？進去看一下：
Looper類中：
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));
}原來是向ThreadLocal中設置looper，設置之後new Handler()的時候也不會拋出異常了。
那麼接下來的問題又來了，Handler是怎麼發送消息的呢，又是怎麼樣才能取到消息處理掉？還是看源碼！
Handler發送消息
Handler類中：
public final boolean sendEmptyMessage(int what){
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
上面的sendEmptyMessage、sendEmptyMessageDelayed和sendMessageDelayed都是我們經常用到的方法，最終都會走sendMessageAtTime方法。我們看一下這個方法，裏面有個MessageQueue，這又是什麼呢，這就是所謂的消息隊列，顧名思義，用來存放消息的。消息隊列的對象什麼時候創建的呢？
在Looper.prepare()中，會走sThreadLocal.set(new Looper(quitAllowed))這行代碼，我們看一看new Looper(quitAllowed)做了什麼？
Looper類：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}原來消息隊列的對象是再Looper的構造方法中創建的，瞬間就明白了Looper.prepare()的重要性，不光是創建了Looper對象並放入了ThreadLocal，還創建了MessageQueue對象。在enqueueMessage中，有msg.target = this，這句代碼是把當前的Handler對象賦值給msg.target。接着看queue.enqueueMessage。這個是MessageQueue類中的方法：
boolean enqueueMessage(Message msg, long when) {
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;//當前待處理的消息
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) {//將所有消息按照時間來排序
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;
}上述代碼是把消息插入到消息隊列當中，從上面代碼可以知道，消息隊列其實就是一個單鏈表。這個時候消息傳到了消息隊列，但是我們知道我們是再handler的handlerMessage中把消息處理掉了（不止這一種，還有通過post發送消息，在run方法中處理消息）。
在第一段代碼中，還有一行代碼沒有講到，我把源代碼貼出來看一下。
Looper類：
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();
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}通過代碼，我們看出首先得到當前線程的Looper對象，然後取出消息隊列（因爲消息隊列是在looper中創建的），接着進入了for語句塊，這是個無限循環，Message msg = queue.next()就很關鍵了，我們看一下
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
}
}從上面代碼中可以看出來，這是從單鏈表中移除一個節點的操作，而節點就是消息，而且是在一個死循環裏面，如果消息隊列中沒有消息，就會在這裏發生阻塞。但是一旦有消息到來，就會把消息從消息隊列中移除。
那我們再回到Looper.looper()中。當取到的消息爲空的時候，就會退出for循環。如果取到了消息，那麼接下來會走msg.target.dispatchMessage(msg)，而根據前面的講述msg.target就是當前的Handler對象，那麼dispatchMessage(msg)便是Handler中的方法，接着看源碼：
Handler類中：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}上面方法是對消息進行分發，首先判斷msg.callback是否爲空，msg.callback是Runnable對象，這又是何方神聖，我們有時候還喜歡用post來發送消息，例如下：
同樣是在一個子線程中：
Looper.prepare();
Handler mHandler = new Handler();
mHandler.post(new Runnable() {
@Override
public void run() {
}
});
Looper.loop();接着看post的源碼：
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}哦！原來msg.callback就是post的Runnable對象，如果不爲空，說明採用了post的方式發送消息，那麼接下來應該執行run方法了，接着看handleCallback方法：
private static void handleCallback(Message message) {
message.callback.run();
}果不其然！！在這裏執行了run方法。
接着檢查mCallBack是不是爲空，，如果不爲空的話，會執行mCallback.handleMessage(msg)。對應的例子代碼：
Looper.prepare();
Handler mHandler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
Looper.loop();因爲Callback是個接口，handleMessage是接口中的方法，該方法在上面的例子中實現，那麼會執行實現的方法。
如果msg.callback和mCallBack都爲空的話，那麼就會執行handleMessage(msg)，這是Handler中的方法：
Handler類：
public void handleMessage(Message msg) {
}空的！！！我們在new Handler()的時候覆蓋了該方法，那麼就會走我們覆蓋後的方法。
結尾
終於寫完了，有的人可以很詫異，爲什麼在UI線程中不需要寫Looper.prepare()和Looper.loop()？
我湊！UI線程那麼重要，當然已經被創建好了，在Looper.prepareMainLooper()方法中一看便知。Looper.loop()在ui線程也已經準備好了，這裏就不帶着去看了。
又有問題來了，Android中爲什麼主線程不會因爲Looper.loop()裏的死循環卡死？
這裏有個很好的答案，附上鍊接如下：
答案鏈接
