（一）Handler的常规使用方式
public class MainActivity extends AppCompatActivity {
public static final String TAG = MainActivity.class.getSimpleName();
private TextView texttitle = null;
/**
* 在主线程中定义Handler，并实现对应的handleMessage方法
*/
public static Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "接收到handler消息...");
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
texttitle = (TextView) findViewById(R.id.texttitle);
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
// 在子线程中发送异步消息
mHandler.sendEmptyMessage(101);
}
}.start();
}
});
}
}
可以看出，一般handler的使用方式都是在主线程中定义Handler，然后在子线程中调用mHandler.sendEmptyMessage();方法，然么这里有一个疑问了，我们可以在子线程中定义Handler么？
（二）如何在子线程中定义Handler？
我们在子线程中定义Handler，看看结果:
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。");
}
}
};
}
}.start();
}
});
点击按钮并运行这段代码：
这里写图片描述
可以看出来在子线程中定义Handler对象出错了，难道Handler对象的定义或者是初始化只能在主线程中？
其实不是这样的，错误信息中提示的已经很明显了，在初始化Handler对象之前需要调用Looper.prepare()方法，那么好了，我们添加这句代码再次执行一次：
texttitle.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
Looper.prepare();
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。");
}
}
};
}
}.start();
}
});
再次点击按钮执行该段代码之后，程序已经不会报错了，那么这说明初始化Handler对象的时候我们是需要调用Looper.prepare()的，那么主线程中为什么可以直接初始化Handler呢？
其实不是这样的，在App初始化的时候会执行ActivityThread的main方法：
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());
AndroidKeyStoreProvider.install();
// 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();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看到原来Looper.prepare()方法在这里调用了，所以在其他地方我们就可以直接初始化Handler了。
并且我们可以看到还调用了：Looper.loop()方法，通过参考阅读其他文章我们可以知道一个Handler的标准写法其实是这样的：
Looper.prepare();
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。");
}
}
};
Looper.loop();
（三）查看Handler源码
1）查看Looper.prepare()方法
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/** 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) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
可以看到Looper中有一个ThreadLocal成员变量，熟悉JDK的同学应该知道，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。具体参考：<a href="http://blog.csdn.net/lufeng20/article/details/24314381">彻底理解ThreadLocal</a>
由此可以看出在每个线程中Looper.prepare()能且只能调用一次，这里我们可以尝试一下调用两次的情况。
/**
* 这里Looper.prepare()方法调用了两次
*/
Looper.prepare();
Looper.prepare();
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
if (msg.what == 101) {
Log.i(TAG, "在子线程中定义Handler，并接收到消息。。。");
}
}
};
Looper.loop();
再次运行程序，点击按钮，执行该段代码：
这里写图片描述
可以看到程序出错，并提示prepare中的Excetion信息。
我们继续看Looper对象的构造方法，可以看到在其构造方法中初始化了一个MessageQueue对象：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
综上小结（1）：Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。
2）查看Handler对象的构造方法
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;
}
可以看出在Handler的构造方法中，主要初始化了一下变量，并判断Handler对象的初始化不应再内部类，静态类，匿名类中，并且保存了当前线程中的Looper对象。
综上小结（2）：Looper.prepare()方法初始话了一个Looper对象并关联在一个MessageQueue对象，并且一个线程中只有一个Looper对象，只有一个MessageQueue对象。而Handler的构造方法则在Handler内部维护了当前线程的Looper对象
3）查看handler.sendMessage(msg)方法
一般的，我们发送异步消息的时候会这样调用：
mHandler.sendMessage(new Message());
通过不断的跟进源代码，其最后会调用：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
原来msg.target就是Handler对象本身；而这里的queue对象就是我们的Handler内部维护的Looper对象关联的MessageQueue对象。查看messagequeue对象的enqueueMessage方法：
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 (;;) {
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;
}
可以看到这里MessageQueue并没有使用列表将所有的Message保存起来，而是使用Message.next保存下一个Message，从而按照时间将所有的Message排序；
4）查看Looper.Loop()方法
/**
* 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();
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
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();
}
}
可以看到方法的内容还是比较多的。可以看到Looper.loop()方法里起了一个死循环，不断的判断MessageQueue中的消息是否为空，如果为空则直接return掉，然后执行queue.next()方法：
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
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;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[I];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
可以看到其大概的实现逻辑就是Message的出栈操作，里面可能对线程，并发控制做了一些限制等。获取到栈顶的Message对象之后开始执行：
msg.target.dispatchMessage(msg);
那么msg.target是什么呢？通过追踪可以知道就是我们定义的Handler对象，然后我们查看一下Handler类的dispatchMessage方法：
/**
* 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);
}
}
可以看到，如果我们设置了callback（Runnable对象）的话，则会直接调用handleCallback方法：
private static void handleCallback(Message message) {
message.callback.run();
}
即，如果我们在初始化Handler的时候设置了callback（Runnable）对象，则直接调用run方法。比如我们经常写的runOnUiThread方法：
runOnUiThread(new Runnable() {
@Override
public void run() {
}
});
看其内部实现：
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
而如果msg.callback为空的话，会直接调用我们的mCallback.handleMessage(msg)，即handler的handlerMessage方法。由于Handler对象是在主线程中创建的，所以handler的handlerMessage方法的执行也会在主线程中。
综上可以知道：
1）主线程中定义Handler，直接执行：
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
而如果想要在子线程中定义Handler，则标准的写法为：
// 初始化该线程Looper，MessageQueue，执行且只能执行一次
Looper.prepare();
// 初始化Handler对象，内部关联Looper对象
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
// 启动消息队列出栈死循环
Looper.loop();
2）一个线程中只存在一个Looper对象，只存在一个MessageQueue对象，可以存在N个Handler对象，Handler对象内部关联了本线程中唯一的Looper对象，Looper对象内部关联着唯一的一个MessageQueue对象。
3）MessageQueue消息队列不是通过列表保存消息（Message）列表的，而是通过Message对象的next属性关联下一个Message从而实现列表的功能，同时所有的消息都是按时间排序的。
4）android中两个子线程相互交互同样可以通过Handler的异步消息机制实现，可以在线程a中定义Handler对象，而在线程b中获取handler的引用并调用sendMessage方法。
5）activity内部默认存在一个handler的成员变量，android中一些其他的异步消息机制的实现方法：
Handler的post方法：
mHandler.post(new Runnable() {
@Override
public void run() {
}
});
查看其内部实现：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
可以发现其内部调用就是sendMessage系列方法。。。
view的post方法：
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().post(action);
return true;
}
可以发现其调用的就是activity中默认保存的handler对象的post方法。
activity的runOnUiThread方法：
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
判断当前线程是否是UI线程，如果不是，则调用handler的post方法，否则直接执行run方法。
参考文章：
1. Android异步消息处理机制完全解析，带你从源码的角度彻底理解
2. Android异步消息处理机制详解及源码分析
