前言：
之前的文章写的都是关于Bitmap和内存的优化技术，这一篇文章给大家谈谈Handler。
Handler是Android系统中比较重要的一个知识，在Android多线程面试经常会被问到，在实际项目中的确也经常用到。当然也比较复杂，知识比较多，牵扯到的类有Thread、Looper、Message、MessageQueue。
Android是支持多线程的，通常应用程序中与用户相关的UI事件都是运行在主线程中，比如点击屏幕、按钮等，为了保持主线程顺畅相应用户事件不被阻塞就需要把耗时的操作(主要是联网、操作大文件等)放到子线程中，这个时候你可能会想到Handler(当然还你可以用其他的比如：异步任务,，这个以后再讲)，但是Handler又是怎么和Thread联系起来的呢？这个咱们来看一下Android主线程是怎么创建的。
ActivityThread:
在ActivityThread.java中有一个main()函数，这个函数就是在一个应用启动的入口，调用关系是：ActivityManagerService.java中的startProcessLocked函数调用如下代码：
// Start the process. It will either succeed and return a result containing
// the PID of the new process, or else throw a RuntimeException.
Process.ProcessStartResult startResult = Process.start("android.app.ActivityThread",
app.processName, uid, uid, gids, debugFlags,
app.info.targetSdkVersion, null);
Process.start又做了如下的操作，只看方法注释就行，现在不需要知道具体做了什么：
/**
* Start a new process.
*
* <p>If processes are enabled, a new process is created and the
* static main() function of a <var>processClass</var> is executed there.
* The process will continue running after this function returns.
*
* <p>If processes are not enabled, a new thread in the caller‘s
* process is created and main() of <var>processClass</var> called there.
*
* <p>The niceName parameter, if not an empty string, is a custom name to
* give to the process instead of using processClass. This allows you to
* make easily identifyable processes even if you are using the same base
* <var>processClass</var> to start them.
*
* @param processClass The class to use as the process‘s main entry
* point.
* @param niceName A more readable name to use for the process.
* @param uid The user-id under which the process will run.
* @param gid The group-id under which the process will run.
* @param gids Additional group-ids associated with the process.
* @param debugFlags Additional flags.
* @param targetSdkVersion The target SDK version for the app.
* @param zygoteArgs Additional arguments to supply to the zygote process.
*
* @return An object that describes the result of the attempt to start the process.
* @throws RuntimeException on fatal start failure
*
* {@hide}
*/
public static final ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int debugFlags, int targetSdkVersion,
String[] zygoteArgs) {
try {
return startViaZygote(processClass, niceName, uid, gid, gids,
debugFlags, targetSdkVersion, zygoteArgs);
} catch (ZygoteStartFailedEx ex) {
Log.e(LOG_TAG,
"Starting VM process through Zygote failed");
throw new RuntimeException(
"Starting VM process through Zygote failed", ex);
}
}
通过注释也能看到上面的函数会找到ActivityThread的main函数并且执行。main函数中创建了Looper，Looper的作用就是利用线程创建一个消息处理队列，并且维护这个消息队列：
public static void main(String[] args) {
Looper.prepareMainLooper();//创建Looper
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}
ActivityThread thread = new ActivityThread();
thread.attach(false);//应用所有的逻辑都在这个方法中
Looper.loop();//开启一个消息循环，不断的读取MessageQueue中的Message。
}
Looper:
Looper.prepareMainLooper()的代码如下：
/**
* Initialize the current thread as a looper, marking it as an
* application‘s main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
myLooper().mQueue.mQuitAllowed = false;
}
上面的方法注释已经说的很明白，创建了主线程的Looper，这段代码是系统调用的。先看prepare方法做了什么操作。
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();//获取当前线程
}
创建主线程的Looper，每一个Looper对应一个Thread、一个MessageQueue，创建Looper的时候会创建一个MessageQueue。到目前位置创建了应用的主线程（Thread）、Looper、MessageQueue，调用Looper.loop()，开始不断的从MessageQueue中读取Message并处理，如果没有消息则等待。现在有了消息循环，有了管理消息循环的Looper就差发送消息和处理消息的Handler了。
Handler:
这个时候你在你的应用中创建一个Handler，一般都是下面的代码：
private static final Handler mHandler = new Handler() {
public void handleMessage(Message msg) {
..........
}
}
};
这个Handler是在主线程中创建的，Handler的构造函数如下：
/**
* Default constructor associates this handler with the queue for the
* current thread.
*
* If there isn‘t one, this handler won‘t be able to receive messages.
*/
public Handler() {
mLooper = Looper.myLooper();//获取上面在主线程创建的Looper
if (mLooper == null) {
throw new RuntimeException(
"Can‘t create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;//获取Looper的MessageQueue
mCallback = null;//默认为null在后面处理msg时会就行检查
}
创建完Handler你就可以用了，比如你发一个消息：
mHandler.sendEmptyMessage(MSG_WHAT);
在系统中会走最终走到Handler.java下面的方法：
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;//注意这行代码后面会用，把Handler赋值给Msg的target对象
sent = queue.enqueueMessage(msg, uptimeMillis);//把msg放到MsgQueue中
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
上面的方法第二个是延时毫秒数，queue.enqueueMessage把消息发送到MessageQueue后剩下的就是等待消息被处理，前面不是说了Looper.loop()方法开始轮询消息队列吗，你发送的消息就是在loop方法中读取到的，读取到后谁去处理呢？在loop()方法中有一句代码：
msg.target.dispatchMessage(msg);
msg就是你发送到MessageQueue的消息，被读取后调用target.dispatchMessage()，这个target就是上面Handler发送消息是赋值的，就是发送消息的Handler本身，然后Handler调用自己的下面方法就行消息处理：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);//在这会调用到上面重写的handleMessage方法。
}
}
因为在new Message的时候callback为空，并且Handler的mCallback = null，所以会调用到你上面new Handler时重写的handleMessage方法。
总结：
每一个线程中都对应一个Looper，每一个Looper都对应一个MessageQueue，这个Looper是用来管理消息队列的，主要是读取消息队列和把消息发送给Message的target去处理。到这你应该清除Thread、Handler、Message、MessageQueue和Looper他们之间的关系了吧。
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