android基础知识02——线程安全3：Message，MessageQueue，Handler，Looper

android的UI操作不是线程安全的，同时也只有主线程才能够操作UI，同时主线程对于UI操作有一定的时间限制（最长5秒）。为了能够做一些比较耗时的操作（比如下载、打开大文件等），android提供了一些列机制。《android基础知识02——线程安全》系列文章就是参考了网上许多网友的文章后，整理出来的一个系列，介绍了主要的方法。分别如下：

android基础知识02——线程安全1：定义及例子

android基础知识02——线程安全2：handler、message、runnable

android基础知识02——线程安全4：HandlerThread

android基础知识02——线程安全5： AsyncTask

在上文例子中，用到了消息队列，这里就不得不提与之相关的Message，MessageQueue，Handler，Looper。

线程，MessageQueue，Handler，Looper之间的关系可以通过一个图来展示：

1、Handler

--用来处理消息和线程

--每个Handler可以执行一个线程对象(Runnable实现)也可以发送和处理消息
>>post(Runnable)
>>postAtTime(Runnable,long)
>>postDelayed(Runnable,long)
>>sendEmptyMessage(int)
>>sendMessageAtTime(Message,long)
>>sendMessageDelayed(Message,long)
>>handleMessage(Message)
通过Handler你可以发布或者处理一个消息或者是一个Runnable的实例。每个Handler都会与唯一的一个线程以及该线程的消息队列管理。当你创建一个新的Handler时候，默认情况下，它将关联到创建它的这个线程和该线程的消息队列。也就是说，如果你通过Handler发布消息的话，消息将只会发送到与它关联的这个消息队列，当然也只能处理该消息队列中的消息。
主要的方法有：
1) public final boolean sendMessage(Message msg)
把消息放入该Handler所关联的消息队列，放置在所有当前时间前未被处理的消息后。
2) public void handleMessage(Message msg)
关联该消息队列的线程将通过调用Handler的handleMessage方法来接收和处理消息，通常需要子类化Handler来实现handleMessage。
removeMessages(0)来清除消息队列。

这个是我从网络上找到的介绍，如果你看到handler的源代码，则其介绍这样的：

* 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.

[java] view plain copy

public class Handler {
private static final boolean FIND_POTENTIAL_LEAKS = false;
private static final String TAG = "Handler";
public interface Callback {
public boolean handleMessage(Message msg);
}
final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
IMessenger mMessenger;
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
public Handler() {
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 = null;
}
/**
* 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);
}
}
public Handler() {
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 = null;
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}

从源码分析可以看出:
1）handler在无参数的构造方法中调用Looper.myLooper()方法,里面就是从当前线程里面获取一个Looper对象,如果没有则创建.这样对Looper就进行初始化,初始化Looper的同时一并初始化MessageQueue,并且从中得到looper的MessageQueue .可以看出Handler就是Looper和MessageQueue的管理者和调度者.

2）其中最重要的是:sendMessageAtTime(Message msg, long uptimeMillis)这个方法,当你往Handler中发送Message消息的时候,从代码看出他自己并不去处理Message ,而是交给了MessageQueue.由以下从这段代码来处理:

3）runnable对象在处理时，是创建一个新的Message并将其mCallback设置为该runnable对象，并将该Message插入到MessageQueue。

4）handler的功能包括两部分：处理消息（执行Message的内容）、转发消息，其中主要由handleMessage和dispatchMessage这两个函数处理；

5）handler中不管是send message还是post runnable都是将其加入到MessageQueue 。

queue.enqueueMessage(msg, uptimeMillis), 其具体实现要看下面的对。

2、Message

frameworks\base\core\java\android\Os\Message.java

--定义一个信息对象，该信息对象可以发送给Handler对象来处理。
--可以通过Message.obtain()或者Handler.obtainMessage()方法来实例化
Message是线程之间传递信息的载体，包含了对消息的描述和任意的数据对象。Message中包含了两个额外的 int字段和一个object字段，这样在大部分情况下，使用者就不需要再做内存分配工作了。虽然Message的构造函数是public的，但是最好是使用Message.obtain( )或Handler.obtainMessage( )函数来获取Message对象，因为Message的实现中包含了回收再利用的机制，可以提供效率。

* 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.

[java] view plain copy

public final class Message implements Parcelable {
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
long when;
Bundle data;
Handler target;
Runnable callback;
Message next;
private static Object mPoolSync = new Object();
private static Message mPool;
private static int mPoolSize = 0;
private static final int MAX_POOL_SIZE = 10;

When: 向Handler发送Message生成的时间
Data: 在Bundler 对象上绑定要线程中传递的数据
Next: 当前Message 对一下个Message 的引用
Handler: 处理当前Message 的Handler对象.
mPool: 通过字面理解可能叫他Message池,但是通过分析应该叫有下一个Message引用的Message链更加适合.
其中Message.obtain(),通过源码分析就是获取断掉Message链关系的第一个Message.

对于源码的解读，可以明确两点：

1）Message.obtain()是通过从全局Message pool中读取一个Message，回收的时候也是将该Message 放入到pool中。

2）Message中实现了Parcelable接口。

3、MessageQueue

--维护一个消息列表，该列表中的消息通过Looper对象来分发。
--调用Looper.myQueue()方法可以获得一个MessageQueue对象
Message Queue是一个消息队列，用来存放通过Handler发布的消息。消息队列通常附属于某一个创建它的线程，可以通过Looper.myQueue()得到当前线程的消息队列。Android在第一启动程序时会默认会为UI thread创建一个关联的消息队列，用来管理程序的一些上层组件，activities，broadcast receivers 等等。你可以在自己的子线程中创建Handler与UI thread通讯。

/**

* 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()}.
*/

[java] view plain copy

public class MessageQueue {
Message mMessages;
private final ArrayList mIdleHandlers = new ArrayList();
private boolean mQuiting = false;
boolean mQuitAllowed = true;
public static interface IdleHandler {
boolean queueIdle();
}
public final void addIdleHandler(IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
final boolean enqueueMessage(Message msg, long when) {
if (msg.when != 0) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
} else if (msg.target == null) {
mQuiting = true;
}
msg.when = when;
//Log.d("MessageQueue", "Enqueing: " + msg);
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
this.notify();
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
this.notify();
}
}
return true;
}

mMessages: 为当前序列的第一个Message, 通过源码分析 MessageQueue并不是对许多Message 之间的关系维护,这样也许可以省去很多事把,而Message 之间的关系
则统统丢给了Message自己去维护,这个可以从对Message源码分析可以理解.
mIdleHandler: 保存的是一系列的handler的集合.
其中final boolean enqueueMessage(Message msg, long when),
这个方法就是上面提到Handler 处理消息时调用到的方法,对她理解了就显的很重要了,其按延迟时间长短，将延迟时间短的放在前面，功能代码如下: [java] view plain copy

msg.when = when;
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
this.notify();
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
this.notify();
}

当向MessageQueue中添加消息的时候,判断当前的Message(mMessage)是否为空,
如果为空或者when=0或者when

如果不为空: 则循环把当前的Message(mMessage)的下一个Message(next)进行遍历,用prev记住当前的message,直到找到prev的下一个Message为空的时候就退出循环,最后将msg接到prev的屁股后面,即这段代码: prev.next = msg;

当然，需要注意到，在其中定义了一个IdleHandler接口，该接口只有一个方法申明：boolean queueIdle()。MessageQueue类中addIdleHandler(IdleHandler handler)函数用于添加队列为空时的IdleHandler。当队列为空时，需要一一通知这些IdleHandler。

4、Looper

--该类用来为一个线程维护一个消息队列
--默认的线程是没有和一个Looper实例关联的，需要在线程中使用Loop.prepare();关联一个线程，使用Looper.loop()处理消息直到结束。

Looper扮演着一个Handler和消息队列之间通讯桥梁的角色。程序组件首先通过Handler把消息传递给Looper，Looper把消息放入队列。Looper也把消息队列里的消息广播给所有的Handler，Handler接受到消息后调用handleMessage进行处理。
1) 可以通过Looper类的静态方法Looper.myLooper得到当前线程的Looper实例，如果当前线程未关联一个Looper实例，该方法将返回空。
2) 可以通过静态方法Looper. getMainLooper方法得到主线程的Looper实例。

[java] view plain copy

/**
* Class used to run a message loop for a thread. Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
*
*
Most interaction with a message loop is through the
* {@link Handler} class.
*
*
This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
*
```
 
```
* class LooperThread extends Thread {
* public Handler mHandler;
* public void run() {
* Looper.prepare();
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
* }
*/

[java] view plain copy

public class Looper {
private static final boolean DEBUG = false;
private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV;
// sThreadLocal.get() will return null unless you've called prepare().
private static final ThreadLocal sThreadLocal = new ThreadLocal();
final MessageQueue mQueue;
volatile boolean mRun;
Thread mThread;
private Printer mLogging = null;
private static Looper mMainLooper = null;
/** 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 final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
/** 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.
* {@link #prepare()}
*/
public static final void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
if (Process.supportsProcesses()) {
myLooper().mQueue.mQuitAllowed = false;
}
}
private synchronized static void setMainLooper(Looper looper) {
mMainLooper = looper;
}
/** Returns the application's main looper, which lives in the main thread of the application.
*/
public synchronized static final Looper getMainLooper() {
return mMainLooper;
}
public static final void loop() {
Looper me = myLooper();
MessageQueue queue = me.mQueue;
while (true) {
Message msg = queue.next(); // might block
if (msg != null) {
if (msg.target == null) {
return;
}
if (me.mLogging!= null) me.mLogging.println(
">>>>> Dispatching to " + msg.target + " "
+ msg.callback + ": " + msg.what
);
msg.target.dispatchMessage(msg);
if (me.mLogging!= null) me.mLogging.println(
"<<<<< Finished to " + msg.target + " "
+ msg.callback);
msg.recycle();
}
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static final Looper myLooper() {
return (Looper)sThreadLocal.get();
}

从源码可以看出Looper 封装的信息:
Looper实质上是对当前线程, ThreadLocal,MessageQueue的封装,也就是负责在多线程之间传递消息的一个循环器.

当你往Handler中添加消息的时候则,里面这个方法: public static final void loop()死循环的方法就会被系统调用,之后的功能代码是:
msg.target.dispatchMessage(msg),则从MessageQueue中得到一个Message(msg),之后调用Handler的dispatchMessage(msg),这个方法内部实际调用的就是 Handler.handleMessage(msg)方法,这个就是我们在activity要重写的方法,所以我们就能够得到其他子线程传递的Message了.

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