网上关于Handler源码分析的文章一大把，写的也不错，我写文章不是为了达到什么目的，只是再走一波流程，然后让自己的思维更加清晰，对着源码来一波，学习一下思想，熟悉一下系统源码，而不是去查看别人的文章然后死记硬背知识点，死记硬背的东西还是容易忘记的，就像英语单词一样，额，，，扯远了，开干吧。
Handler一般用于 用于子线程发送消息，主线程更新UI，废话不多说，直接看用法(2步，做你想做)，然后查看源码分析它
步骤1 主线程创建一个Handler 复写handleMessage(Message msg)方法，接收子线程发过来的消息，然后做你想做的操作即可。
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what){
case 100:
//do something
break;
}
}
};
步骤2 子线程创建发送的消息，携带数据
//创建Message 方式1 new 方式直接创建对象
Message message = new Message();
message.obj = "";
message.what = 100;
//方式2 通过Handler.obtainMessage方式创建Message,这种方式好一些，可以复用
//源码注释
/*Returns a new {@link android.os.Message Message}
from the global message pool. More efficient than
* creating and allocating new instances.*/
Message obtainMessage = mHandler.obtainMessage();
obtainMessage.obj = "";
obtainMessage.what = 101;
mHandler.sendMessage(obtainMessage);
源码分析走起（搞一波）
先从 mHandler.sendMessage(obtainMessage)方法开始看，看里面做了什么操作。
Handler
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
Handler
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
Handler
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;//消息队列，创建Handler的时候源码创 建赋值的，
并且是从Looper里面获取的赋值的，用来存放发送的消息的 是链表结构的（方便断开插入，插入快）
为什么用队列（先进先出），链表结构 （方便插队）
假设第一条消息发送执行的延时是1秒后执行，
第二条发送的是5秒后执行，
第三条发送的是3秒后执行，那么进入队列肯定需要根据时间排序放入队列的，方便Looper取出执行
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);
}
Handler
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//调用了MessageQueue里面里的enqueueMessage，继续，移步到MessageQueue
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue
//主要是把发送过来的消息按时间顺序加入到消息队列里面
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 (;;) {// 关键代码 死循环，链表方式存储消息，数据结构知识
//作用是消息入队列
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;
}
以上 Handler的发送消息部分的源码基本走完了
接下来是new Handler部分的源码
Handler
public Handler() {//new Handler的Handler构造
this(null, false);
}
Handler
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();//创建获取Looper 用来去MessageQueue里面死循环不断取消息用的
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;//获取消息队列
mCallback = callback;//Handler 的内部的callBack
mAsynchronous = async;
}
Looper
/**
* 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();//获取Looper
}
ThreadLoocal
public T get() { //map集合获取对应的Looper 后面会讲解在什么地方put进去的
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
到这里，new Handler部分的源码基本就完了
接下来看复写的Handler的handleMessage(Message msg) 方法源码
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
//空的，什么操作都没有，注释说必须继承实现 然后用来接收消息
//麻烦了，没办法往下看了，并没有看到是如何传递消息过来的
//别着急，看这个方法在哪里被调用了
//搜索一下，dispatchMessage这里被调用了
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {//分发消息
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//调用了我们复写的方法，传递了msg过去了
继续查看dispatchMessage 是哪里调用的，，然后，就发现，找不到
handleMessage(msg);
}
}
到这里new Handler复写handleMessage方法就分析完了，知道是调用dispatchMessage(Message msg) 的时候分发的消息 但是，找不到这个方法是在哪里被调用的，，，那么就要涉及到Activity的启动流程相关知识了。
Activity启动流程之ActivityThread
我们都知道一个程序只有一个入口，Activity也是，它的入口就是ActivityThread 的main方法，来看看源码分析吧
ActivityThread
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// 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();//Looper准备，程序开始运行，Activity通讯，消息基本上都是靠Looper去驱动的
ActivityThread thread = new ActivityThread();//创建一个主线程
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();//创建Activity的Handler
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();//Looper开始循环 取消息
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Looper
public static void prepareMainLooper() {
prepare(false);//准备创建Looper
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
Looper
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));//创建一个Looper 设置给当前线程
}
Looper
public void set(T value) {//用map存储Looper，对应前面的get方法去取Looper
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
Looper
private Looper(boolean quitAllowed) {
//创建消息队列
mQueue = new MessageQueue(quitAllowed);
//设置当前线程 所以一个消息队列对应一个线程
mThread = Thread.currentThread();
}
到这里，创建Looper基本上就完成了，Looper里面有了消息队列，可以通过Handler把消息存储到MessageQueue里面了。
到这里还是没看到调用 dispatchMessage 方法，别着急，马上就来。
ActivityThead
public static void main(String[] args) {
...
Looper.loop(); //开始循环，看看loop方法做了什么
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Looper
/**
* 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();//用get方法获取存储在集合中的Looper对象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//获取Looper的消息队列
// 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 (;;) {//关键代码 死循环 不断的去MessageQueue里面获取消息
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;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//取到消息就调用target的dispatchMessage 方法，而这个target正式Handler对象 下面接着分析源码
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();//移除消息 具体看源码，就是判断消息是否为空，然后对消息进行回收，置为空
}
}
Message
/*package*/ Handler target;//Message里面的target就是Handler
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
//这里也强调了，创建消息的方法首选obtain方式
public Message() {
}
//接着看target是在哪里赋值的
Handler
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//在这里给msg的target赋值的，赋的值是Handler
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
到这里，Handler整个执行的流程就基本分析完了，最好亲自看一下源码，跟着代码走一遍，才会记得更加清晰，理解起来才会容易一些。
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