Handler机制是Andrdoid中很常用的线程任务控制，Handler类负责信息传送和分发执行，Looper是控制Message中心，MessageQueue保持和提供Message。
1、Message
顺便提一下Message.obtain()，Message会维护一个信息池，以单列表结构存储，使用它可以减少对象的创建，节省内存，代码如下
//池默认长度是MAX_POOL_SIZE=50
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
//执行完会进行回收
void recycleUnchecked() {
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
来看一下Message的结构
public final class Message implements Parcelable{
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
public int sendingUid = -1;
int flags;
long when;
Bundle data;
Handler target;//执行的Handler
Runnable callback;//post时传入的灰度，会优先回调
Message next;//链表结构
}
2、Handler
Handler不管的post方法还是sendMessage，信息都是封装成Message传递执行，最终会调到sendMessageDelayed方法
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
//先计算运行的时间点，启动时间+延迟时间
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
最后调用sendMessageAtTime插入到MessageQueue队列中
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);
}
Handler分发执行，优先级为：Runnable->Callback->handleMessage
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
3、MessageQueue
插入的代码会比较复杂，mMessage记录为信息队列的第一个
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) {
//判断线程的Looper是否停止
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
//空队列，或者执行时刻小于列表头的执行时刻，插入到列表头，
//
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 {
//判断是否要唤醒执行队列，一般不需要唤醒
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {//变量查找插入位置
prev = p;
p = p.next;
if (p == null || when < p.when) {//找到比自己执行时刻小的，将插入到它前面
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//插入操作
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
Looper会循环调用MessageQueue.next，获取Message执行
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();
}
//本地方法，原来阻塞队列并设置唤醒时间
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
//获取列表头信息
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) {
// 执行时刻未到，设置唤醒阻塞时间
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 获取到信息并返回
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
return msg;
}
} else {
// 没有信息
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
//以下代码是获取和运行空闲程序，暂时不去理解
// 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("MessageQueue", "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;
}
}
4、Looper
以下是我们经常在线程启动Looper的代码，
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();
}
}
先看一下prepare方法
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
关键在ThreadLocal，ThreadLocal是线程报错数据的一种方式，Thread.threadLocals会记录保持在ThreadLocalMap中的key，下次要再根据key来获取数据，所以线程不一样，可以也不一样，获取Looper对象也不是同一个，所以其他不同线程使用不用Looper的效果
//ThreadLocal.java
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);//获取当前线程的ThreadLocalMap
if (map != null)
map.set(this, value);//保存，
else
createMap(t, value);//创建
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
创建Handler，会调用myLooper获取当前线程的Looper
public static Looper myLooper() {
return sThreadLocal.get();
}
Looper.loop()是一个无限循环的方法，不断获取MessageQueue中的信息进行执行，代码比较简单
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(); // 从消息队列获取信息，可能会阻塞
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);
}
//通过Handrle处理信息，
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);
}
//回收Message
msg.recycleUnchecked();
}
}
## 附
MessageQueue 是在native层使用epoll进行阻塞和唤醒，有兴趣可以看下面的文章
[Android消息机制Native](http://gityuan.com/2015/12/27/handler-message-native/)
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