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Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程。messagequeue意思是消息队列，它内部存储一组消息，有插入和删除的功能，其实内部是以单链表的形式来实现队列功能的。looper的意思是循环，它的主要功能是循环读取messagequeue里面的消息，然后加以处理，如果暂时还没有消息，则looper会一直等待。在android日常开发中，我们经常利用handler将工作线程切换到UI线程从而达到更新UI的目的。
在handler机制中，每个线程都会对应一个looper，但线程默认是没有looper的，需要自己创建，而我们在android的UI线程中可以直接使用handler是因为UI线程在创建的时候已经初始化了looper，然后looper对象是通过ThreadLocal保存在对应thread的threadlocalmap中的。ThreadLocal是什么？ThreadLocal不是一个线程，它的作用是可以为每个不同的线程存储数据，这些不同线程的数据互不干扰，像我们的looper就是通过它来保存在各自线程中的，并且可以通过threadlocal来获取每个线程的looper。下面我们来讲解它们各自的工作原理。
ThreadLocl的工作原理
先来看一个例子：
public class ThreadLocalDemo {
private java.lang.ThreadLocal<String> booleanThreadLocal = new ThreadLocal<>();
public void test() {
new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
booleanThreadLocal.set("主线程");
LogUtils.d(Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get());
new Thread("线程01") {
@Override
public void run() {
super.run();
booleanThreadLocal.set("线程01");
Log.d("tag", Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get());
}
}.start();
new Thread("线程02") {
@Override
public void run() {
super.run();
//booleanThreadLocal.set("线程02");
Log.d("tag", Thread.currentThread().getName() + "的值=" + booleanThreadLocal.get());
}
}.start();
}
}
执行结果：
通过上面的例子执行结果来看，threadlocal确实可以为不同的线程保存不同的结果，下面我们来看看它的源码是怎么实现的：
public void set(T value) {
//获取当前线程
Thread t = Thread.currentThread();
//获取当前线程的threadlocalmap
ThreadLocalMap map = getMap(t);
//如果threadlocalmap不为null，那么将value值保存，其中key值为当前threadlocal
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
//返回线程的threadlocalmap对象
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
//创建对应线程的threadlocalmap实例，并且保存对应的值
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}其实这里的关键是将value值保存到当前线程的threadlocalmap中去，而且对应的key是threadlocal本身，下面我们来看看threadlocal的get方法：
public T get() {
//获取对应的线程
Thread t = Thread.currentThread();
//获取对应线程的threadlocalmap
ThreadLocalMap map = getMap(t);
//如果不为null，将取出key为当前threadlocal对应的value
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
//threadlocalmap为null的时候，返回setInitialValue的值，其实就是null
return setInitialValue();
}
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
protected T initialValue() {
return null;
}
get方法其实就是返回我们保存的value值，当然如果我们之前没有对应set方法保存对应值的话，就返回null，这就是为什么上面的那个例子中，线程02会返回null的原因了。好了，其实threadlocal还是比较简单的。
MessageQueue的工作原理
messagequeue的插入和读取操作内部是由单链表结构来实现的，可能是因为单链表结构在插入和删除上效率比较高吧。插入用enqueueMessage方法，读取用next方法来实现，下面我们来看看他们的源码：
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;
}
上面是消息的插入操作，注意参数when代表这个消息何时取出来处理。下面我们看看，next方法源码：
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，next是一个无限循环的方法，当取到消息的时候返回msg，没有消息的时候，会一直阻塞直到有消息。注意上面的一个判断，如果now<msg.when，就先不取msg，其实就是说这个消息还没到执行时间，所以暂时不会取出来。还有当looper执行退出的时候，即上面的mQuitting为true的时候，此时next方法返回null。理解了messagequeue，下面我们来看看Looper的工作原理。
Looper的工作原理
looper在消息机制中扮演者消息循环的角色，就是说它会不停地从消息队列中查看有没有新消息，如果取出了新消息就会马上处理，否则就一直阻塞在那里。我们知道，开启looper的工作，需要在对应的线程中通过Looper.prepare()创建一个looper，然后通过loop（）方法来开启消息的循环，我们来看看这个两个方法的源码：
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
//一个线程只能创建一个looper
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//创建一个looper实例，通过threadlocal保存起来
sThreadLocal.set(new Looper(quitAllowed));
}来看看looper的构造方法：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}可以看到，构造方法中会创建一个消息队列和获取当前looper的线程。looper还提供了prepareMainLooper方法，这个方法是用来给主线程ActivityThread创建looper使用的，当然其本质也是通过prepare方法来实现的。由于主线程的looper比较特殊，所以其提供了getMainLooper方法，用来在任何地方都可以获取主线程的looper对象。looper提供了两个退出方法：quit和quitSafely，其中quit是立即退出，quitSafely是等消息队列中的消息处理完毕以后再安全退出。如果我们手动创建了looper，在不用的时候需要我们手动去退出，否则这个子线程会一直处于等待状态。下面我们来看看looper最关键的方法loop，只有调用了loop方法，消息循环系统才会真正起作用，如下：
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();
//无限循环
for (;;) {
//调用消息队列的next方法取消息
Message msg = queue.next(); // might block
//如果消息为null，结束方法
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
//调用对应handler对象的方法，将消息递交给handler处理
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}loop方法还是比较好理解的，首先这是一个死循环，唯一一个跳出循环的条件是取到的消息为null，就是当我们调用了looper的退出方法的时候，同时也会去调用messagequeue的退出方法，此时next方法就会返回一个null值。所以说我们不用的时候必须要手动退出looper，不然这个循环会一直持续下去。如果我们取出了新消息，后面就会调用msg.target.dispatchMessage(msg)，这里的target就是发送消息的handler对象，这样消息又交给了handler来处理了。值得注意的是，我们取出消息调用handler的dispatchMessage是在looper的loop方法中执行的，而looper对象是保存在创建时候的那个线程中的，所以此时就成功地实现了线程切换。讲解了消息队列和looper的工作原理，下面我们来看看Handler是怎么工作的。
Handler的工作原理
handler的工作主要包括消息的发送和接收，消息的发送主要用post和send系列方法，接收一般是通过重写对应的handleMessage方法。下面我们先来看看一个例子：
public class HandlerDemo {
//重写handler的handleMessage方法
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
LogUtils.d("handler接收到消息,what内容=" + msg.what);
}
};
//通过传递一个callback实现来处理消息
Handler handler1 = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});
public void sendMsg() {
//在一个子线程中通过主线程的handler发送消息给主线程处理
new Thread(new Runnable() {
@Override
public void run() {
try {
//延时两秒
TimeUnit.SECONDS.sleep(2);
//发送一个消息给handler处理
Message msg = new Message();
msg.what = 01;
handler.sendMessage(msg);
} catch (InterruptedException e) {
}
}
}).start();
}
public void post() {
handler1.post(new Runnable() {
@Override
public void run() {
LogUtils.d("handler1的post方法执行了run方法");
}
});
}
}执行结果：
可以看到，例子中创建了主线程的handler对象，演示了在子线程中通过sendMessage方法发送消息给主线程处理还有post方法的使用，下面我们来看看它们的源码是怎么实现的。
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);
}
//将消息保存到消息队列中
return queue.enqueueMessage(msg, uptimeMillis);
}其实上面的最终关键的就是把消息保存到对应的消息队列中，值得注意的时候，msg.target=this，这里给target赋值为handler本身。我们来看看这个消息队列是怎么来的
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());
}
}
//获取对应的looper实例
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//获取消息队列
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}可以看到，在handler的构造方法中，我们获得了线程对应的looper，如果是主线程当然就是获得主线程的looper了，然后通过looper获得消息队列mQueue。
在讲looper的时候，我们知道loop方法会一直去消息队列中取消息，一旦取出新消息，就会调用handler的dispatchMessage方法，下面我们来看看这个方法：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}其中上面的msg.callback就是post方法中的runnable对象，我们来看看：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}再来看看handleCallback(msg)
private static void handleCallback(Message message) {
message.callback.run();
}可以看到会调用runnable对象的run方法，这就是handler的post方法的原理。那mCallback 又是怎么来的
public Handler(Callback callback) {
this(callback, false);
}
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;
//给mCallback赋值
mCallback = callback;
mAsynchronous = async;
}
//Callback接口
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}可以看到，其实这个mCallback就是我们在例子中创建handler1的时候传递的参数。如果这个回调方法为null，我们就调用handleMessage(msg)方法处理，这是一个public方法里面为空实现，需要我们自己重写去处理，这个就是我们例子中创建handler的时候实现。
好了，关于android消息机制的工作原理，通过上面的讲解我们大概也清楚是怎么一回事了。