Handler是可以用来发送消息，或者一个Runable；消息／Runable可以在指定的线程（由Handler中的Looper决定）中接受／执行；
MessageQueue：消息队列，用来存放消息的
Looper：用来循环遍历消息队列中的消息，并负责消息的分发
我们看看使用Handler常用示例代码(如下
public class TestActivity extends Activity {
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case caseA:
doSomeThingA();
break;
case caseB;
doSomeThingB();
break;
default:
break;
}
}
};
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
sendMessage();
}
private void sendMessage() {
Message msg = Message.obtain();
msg.what = caseA;
msg.obj = data;
mHandler.sendMessage(msg);
}
}我们可以看到mHandler变量作为成员变量，覆写了handleMessage方法，且是通过无参构造器创建出来的；
在onCreate方法调用了sendMessage方法，这样消息最终被传送到handlerMessage中；下面我们结合Handler部分源码看看消息是如何一步一步到达Handler的handleMessage方法里的。
TestActivity类中的sendMessage方法中发送消息后，调用了mHandler.sendMessage(msg)，也就是如下代码，
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)
{
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}我们从上述源码可以看到，handler发送的消息最终是把消息放到消息队列中，之后就没做任何事情了，那么问题来了
那么消息队列是在什么时候创建？
其实每个Looper都包含一个消息队列成员，而消息队列也是在Looper构造器时被创建;
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }- 最终消息是如何分发到handlerMessage中？
我们看到Handler类中包含了Looper 和 消息队列成员变量，它们都是在构造器中被赋值（见下面Handler类部分源码）
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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());
}
}
//获取当前线程的Looper对象
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//从Looper对象中获取消息队列
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}因为示例代码中Handler对象是在UI线程中创建，而系统在初始化时会自动调用Looper.prepareMainLooper方法，该方法帮我们创建好了主线程的Looper对象(所以在UI线程中我们不需要主动调用Looper.prepareMainLooper去创建Looper对象);创建Looper对象后会启用loop方法，它会不断循环读取消息队列的消息。
我们看下Looper源码：
package android.os;
import android.annotation.NonNull;
import android.annotation.Nullable;
import android.util.Log;
import android.util.Printer;
/**
* 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.
*
* <p>Most interaction with a message loop is through the
* {@link Handler} class.
*
* <p>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.
*
* <pre>
* 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();
* }
* }</pre>
*/
public final class Looper {
/*
* API Implementation Note:
*
* This class contains the code required to set up and manage an event loop
* based on MessageQueue. APIs that affect the state of the queue should be
* defined on MessageQueue or Handler rather than on Looper itself. For example,
* idle handlers and sync barriers are defined on the queue whereas preparing the
* thread, looping, and quitting are defined on the looper.
*/
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
/** 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));
}
/**
* 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();
}
}
/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
/**
* 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);
}
//target其实就是Handler，此处就是将消息传递到Handler中dispatchMessage方法中
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();
}
}
/**
* 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();
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
}
我们可以看到上述代码中loop方法，其内部就是不断从消息队列中取消息，最终调用msg.target.dispatchMessage(msg);这里的target其实就是handler对象，也就是Handler的dispatchMessage方法；那么dispatchMessage内部是怎么实现？（看下面源码）
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//如果Message中的Runnable不空，直接调用Message的Runable.run方法
handleCallback(msg);
} else {
//如果Handler中构造器传入了CallBack不为空
if (mCallback != null) {
//如果CallBack不为空且handleMessage返回为true，直接返回（不执行handleMessage方法了）
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public interface Callback {
public boolean handleMessage(Message msg);
}我们的示例代码中发送的message的callback没有赋值所以为null，且我们的mHandler是无参构造的所以mCallback其实也是为空，所以最终执行到Handler的handleMessage方法；整个大体流程就是这样；
我们可以看到其实dispatchMessage方法运行在哪个线程其实是由Looper.loop决定的，至此整个流程逐渐清晰明朗起来了
梳理下流程：
通过Looper&Handler来达到线程间通讯
上面示例代码演示消息最终在UI线程被消费处理掉，如果要将一个消息／Runable传递给一个线程我们该如何做呢？我么可以这样做
class LooperThread extends Thread {
private Handler mHandler;
@Override
public void run() {
super.run();
//为当前线程创建一个Looper对象
Looper.prepare();
//handler会获取当前线程绑定的Looper对象
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
//循环分发消息队列中的消息
Looper.loop();
}
}首先定义一个LooperThread，我们覆写run方法，并对该线程绑定一个Looper对象和一个Handler对象，这样在外部我们可以通过Handler对象将消息／Runable交给Handler的handlerMessage处理了；这样就达到了其他线程消息交给此线程处理目的，这其实就实现了线程间通讯了。