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来源：伯乐在线专栏作者 - PleaseCallMeCoder
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Android中的消息处理机制概述
大家对于Android中的消息处理机制的用法一定都比较熟悉，至于工作原理估计不少人有研究。就像我们自己写的类我们用起来比较熟悉一样，如果我们熟悉了消息处理机制的具体实现，那么我们用起来肯定也会事半功倍。
博主之前只是稍有涉猎，对其中一些地方也还心存疑虑，比如既然Looper.loop()里是一个死循环，那它会不会很消耗CPU呢？死循环阻塞了线程，那我们其他的事务是如何被处理的呢？Android的UI线程是在哪里被初始化的呢？等等。索性今天就把他们放到一起，说道说道。
Android中线程的分类
有消息时就处理
没有消息时就睡眠
任务一旦执行完成便退出
带有消息队列线程概述
四要素
Message(消息)
MessageQueue(消息队列)
Looper(消息循环)
Handler(消息发送和处理)
四要素的交互过程
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具体工作过程
消息队列的创建
消息循环
消息的发送最基本的两个API
Handler.sendMessage
Handler.post
消息的处理
基于消息的异步任务接口
android.os.HandlerThread
android.os.AyncTask
带有消息队列线程的具体实现
ThreadLocal
ThreadLocal并不是一个Thread，而是Thread的局部变量。当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。
从线程的角度看，目标变量就象是线程的本地变量，这也是类名中“Local”所要表达的意思。
Looper
用于在指定线程中运行一个消息循环，一旦有新任务则执行，执行完继续等待下一个任务，即变成Looper线程。Looper类的注释里有这样一个例子：
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
//将当前线程初始化为Looper线程
Looper.prepare();
// ...其他处理，如实例化handler
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
// 开始循环处理消息队列
Looper.loop();
}
}
其实核心代码就两行，我们先来看下Looper.prepare()方法的具体实现
public final class Looper {
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocalLooper> sThreadLocal = new ThreadLocalLooper>();
private static Looper sMainLooper; // guarded by Looper.class
//Looper内的消息队列
final MessageQueue mQueue;
// 当前线程
final Thread mThread;
private Printer mLogging;
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
/** 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() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//试图在有Looper的线程中再次创建Looper将抛出异常
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
/**
* 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(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
//~省略部分无关代码~
}
从中我们可以看到以下几点：
prepare()其核心就是将looper对象定义为ThreadLocal
一个Thread只能有一个Looper对象
prepare()方法会调用Looper的构造方法，初始化一个消息队列，并且指定当前线程
在调用Looper.loop()方法之前，确保已经调用了prepare(boolean quitAllowed)方法，并且我们可以调用quite方法结束循环
说到初始化MessageQueue，我们来看下它是干什么的
/**
* Low-level class holding the list of messages to be dispatched by a
* {@link Looper}. Messages are not added directly to a MessageQueue,
* but rather through {@link Handler} objects associated with the Looper.
*
*
You can retrieve the MessageQueue for the current thread with
* {@link Looper#myQueue() Looper.myQueue()}.
*/
它是一个低等级的持有Messages集合的类，被Looper分发。Messages并不是直接加到MessageQueue的，而是通过Handler对象和Looper关联到一起。我们可以通过Looper.myQueue()方法来检索当前线程的MessageQueue。
接下来再看看Looper.loop()
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
//得到当前线程Looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//得到当前looper的MessageQueue
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();
//开始循环
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
//没有消息表示消息队列正在退出
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//将真正的处理工作交给message的target，即handler
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println(" + 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);
}
//回收消息资源
msg.recycleUnchecked();
}
}
通过这段代码可知，调用loop方法后，Looper线程就开始真正工作了，它不断从自己的MessageQueue中取出队头的消息(或者说是任务)执行。
除了prepare()和loop()方法，Looper类还有一些比较有用的方法，比如
Looper.myLooper()得到当前线程looper对象
getThread()得到looper对象所属线程
quit()方法结束looper循环这里需要注意的一点是，quit（）方法其实调用的是MessageWueue的quite（boolean safe）方法。
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
我们看到其实主线程是不能调用这个方法退出消息队列的。至于mQuitAllowed参数是在Looper初始化的时候初始化的，主线程初始化调用的是Looper.prepareMainLooper()方法，这个方法把参数设置为false。
Message
在整个消息处理机制中，message又叫task，封装了任务携带的信息和处理该任务的handler。我们看下这个类的注释
/**
*
* Defines a message containing a description and arbitrary data object that can be
* sent to a {@link Handler}. This object contains two extra int fields and an
* extra object field that allow you to not do allocations in many cases.
*
*
While the constructor of Message is public, the best way to get
* one of these is to call {@link #obtain Message.obtain()} or one of the
* {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
* them from a pool of recycled objects.
*/
这个类定义了一个包含描述和一个任意类型对象的对象，它可以被发送给Handler。
从注释里我们还可以了解到以下几点：
尽管Message有public的默认构造方法，但是你应该通过Message.obtain()来从消息池中获得空消息对象，以节省资源。
如果你的message只需要携带简单的int信息，请优先使用Message.arg1和Message.arg2来传递信息，这比用Bundle更省内存
用message.what来标识信息，以便用不同方式处理message。
Handler
从MessageQueue的注释中，我们知道添加消息到消息队列是通过Handler来操作的。我们通过源码来看下具体是怎么实现的
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread’s {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread’s message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it – from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
*
There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
*/
注释比较简单，这里就不过多翻译了，主要内容是：每一个Handler实例关联了一个单一的ghread和这个thread的messagequeue，当Handler的实例被创建的时候它就被绑定到了创建它的thread。它用来调度message和runnables在未来某个时间点的执行，还可以排列其他线程里执行的操作。
public class Handler {
//~省略部分无关代码~
final MessageQueue mQueue;
final Looper mLooper;
public Handler() {
this(null, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(boolean async) {
this(null, async);
}
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 that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
//~省略部分无关代码~
}
先看构造方法，其实里边的重点是初始化了两个变量，把关联looper的MessageQueue作为自己的MessageQueue，因此它的消息将发送到关联looper的MessageQueue上。
有了handler之后，我们就可以使用Handler提供的post和send系列方法向MessageQueue上发送消息了。其实post发出的Runnable对象最后都被封装成message对象
接下文
专栏作者简介（ 点击 → 加入专栏作者 ）
PleaseCallMeCoder：我是 PleaseCallMeCoder，一个小小的90后程序员。热衷于移动开发，喜欢研究新技术，奔跑在成为大神的路上。
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