Android消息机制底层原理

1.概述

Android的消息机制主要是指Handler的运行机制，Handler的运行需要底层的MessageQueue和Looper的支撑。MessageQueue是消息队列。他的内存存储了一组消息，以队列的形式对外提供插入和删除的工作。他的内部存储结构并不是真正的队列，而是采用单链表的数据结构来存储消息列表。Looper为消息循环，由于MessageQueue只是一个消息的存储单元，它不能去处理消息，而Looper就填补了这个功能，Looper会以无限循环的形式去查找是否有新的消息，如果有的话就处理消息，否则就一直等待，Looper还有一个特殊的概念，那就是ThreadLocal，ThreadLocal并不是线程，它的作用可以在每个线程中存储数据。我们知道，Handler创建的时候会采用当前线程的Looper来构造消息循环系统，那么Handler内部如何获取到当前线程的Looper呢？这就要使用ThreadLocal了，ThreadLocal可以在不同的线程中互不干扰地存储并提供数据，通过ThreadLocal可以轻松获取每个线程的Looper。当然需要注意的是，线程是默认没有Looper的，如果需要使用Handler就必须为线程创建Looper。我们经常提到的主线程，也就是UI线程，它就是ActivityThread，ActivityThread被创建时会初始化Looper，这也是在主线程中默认可以使用Handler的原因。

2.ThreadLocal-线程局部变量

ThreadLocal是一个现场内部的数据存储类，通过它可以在指定的线程中存储数据，数据存储以后，只有在指定线程中可以获取到存储的数据。对于Handler来说，它需要获取当前线程的Looper，很显然Looper的作用域就是线程并且不同线程具有不同的Looper，这个时候通过ThreadLocal就可以轻松实现Looper在线程中的存储。ThreadLocal是一个泛型类。

1）存储机制

在localValues内部有一个数组；private Object[]table，ThreadLocal的值就存在这个table数组中，ThreadLocal的值在table数组中的存储位置总是为ThreadLocal的reference字段所标识的对象的下一个位置，比如ThreadLocal的reference对象在table数组中的索引为index，那么ThreadLocal的值在table数组中的索引就是index+1.最终ThreadLocal的值将会被存储在table数组中：table[index+1]=value

//value加入数据
void put(ThreadLocal<?> key, Object value) {
cleanUp();

// Keep track of first tombstone. That's where we want to go back
// and add an entry if necessary.
int firstTombstone = -1;

for (int index = key.hash & mask;; index = next(index)) {
Object k = table[index];

if (k == key.reference) {
// Replace existing entry.
table[index + 1] = value;
return;
}

if (k == null) {
if (firstTombstone == -1) {
// Fill in null slot.
table[index] = key.reference;
table[index + 1] = value;
size++;
return;
}

// Go back and replace first tombstone.
table[firstTombstone] = key.reference;
table[firstTombstone + 1] = value;
tombstones--;
size++;
return;
}

// Remember first tombstone.
if (firstTombstone == -1 && k == TOMBSTONE) {
firstTombstone = index;
}
}
}

//获取当前线程的数据
Values values(Thread current) {
return current.localValues;//当前线程存储的数组
}

//初始化当前线程的数据
Values initializeValues(Thread current) {
return current.localValues = new Values();
}

2）set

public void set(T value) {
Thread currentThread = Thread.currentThread();//获取当前的线程
Values values = values(currentThread);//
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}

3）get

public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}

return (T) values.getAfterMiss(this);
}

从ThreadLocal的set和get方法可以看出，他们所操作的对象都是当前线程localValues对象的table数组，因此在不同线程中访问同一个ThreadLocal的set和get方法，他们对ThreadLocal所做的读写操作仅限于各自线程的内部。

3.MessageQueue-消息队列

消息队列在Android中指的是MessageQueue，MessageQueue主要包含两个操作：插入和读取。读取操作本身会伴随着删除操作，插入和读取对应的方法分别为enqueueMessage和next，其中enqueueMessage的作用是往消息队列中 插入一条消息，而next的作用是从消息队列中取出一条消息并将其从消息队列中移除。MessageQueue内部是通过一个单链表的数据结构来维护消息列表，当链表在插入和删除上比较有优势。

1）enqueueMessage插入消息

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 &l
4000
t; 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;
}

2）next获取消息

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;
}
}

next方法是一个无限循环的方法，如果消息队列中没有消息，那么next方法会一直堵塞在这里。当有新消息到来时，next方法会返回这条消息并将其从链表中移除

4.Message- 消息实体

需要注意Message的一些成员变量
 Handler target;  //对应的Handler
 Runnable callback; //对应的回调
Message next;//单链表引用

5.Looper-消息循环

Looper在Android的消息机制中扮演着消息循环的角色，具体来说就是他会不停地从MessageQueue中查看是否有新消息，如果有新消息就会立刻处理，否则就一直阻塞在哪里。



Looper处理prepare方法外，还提供了prepareMainLooper方法，这个方法主要是给主线程也就是ActivityThread创建Looper使用的，其本质也是通过prepare方法来实现。由于主线程的Looper比较特殊，所以Looper提供一个getMainLooper方法，通过它可以在任何地方获取主线程的Looper。Looper也是可以退出的，Looper提供勒quit和quitSafely来退出一个Looper。quit会直接退出Looper，而quitSafely只是设定一个退出标记，然后把消息队列的已有消息处理完毕后才安全退出。Looper退出后，通过Handler发送的消息会失败，这个时候Handler的send方法会返回false。在子线程，如果手动为其创建了Looper，那么所有的事情完成以后应该调用quit方法来终止消息循环，否则这个子线程就会一直处理等待的状态。
Looper最重要的一个方法是Loop方法：

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();
}
}

loop方法是一个死循环，唯一跳出循环的方式是MessageQueue的next方法返回了null。当Looper的quit方法被调用时，Looper就会调用MessageQueue的quit或者quitSafely方法来通知消息队列退出，当消息队列被标记为退出状态时，他的next方法会返回null。loop方法会调用MessageQueue的next方法来获取新消息，而next是一个阻塞操作，当没有消息时，next方法会一直阻塞在哪里，这也导致loop方法一直阻塞在哪里。若有新消息，Looper会调用msg。target。dispatchMessage（msg），这里的msg.target是发送这条消息的Handler对象，这样Handler发送的消息最终又交给它的dispatchMessage方法来处理了。但是这里不同的是，Handler的dispatchMessage方法是在创建Handler时所使用的Looper中执行，这样就成功将代码逻辑切换到指定的线程中去执行了。

6.Handle-消息处理

Handler的工作主要包含消息的发送和接收过程。消息的发送可以通过post的一系列方法以及send的一系列方法来实现，post的一系列方法最终是通过send的一系列方法来实现的。

1）创建

使用Handler必须要有Looper，不然会报异常

public Handler(Callback callback) {
this(callback, false);
}

/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, 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;
mCallback = callback;
mAsynchronous = async;
}

2   发送

Handler发送消息的过程仅仅是向消息队列中插入了一条消息

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);
}

3）接收

 当消息队列插入消息后，MessageQueue的next方法就会返回这条消息给Looper，Looper收到消息后就开始处理了，最终消息由Looper交由Handler处理

public interface Callback {
public boolean handleMessage(Message msg);
}

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

/**
* 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);
}
}

7.主线程的消息循环

Android的主线程就是ActivityThread，主线程的入口方法为main，在main方法中系统会通过Looper.prepareMainLooper()来创建主线程的Looper以及MessageQueue，并通过Looper。loop（）来开启主线程的消息循环

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");
}

主线程的消息循环开始以后，ActivityThread还需要一个Handler来和消息队列进行交互，这个Handler就是ActivityThread.H，他的内部定义了一组消息类型，主要管理Activity的生命周期及四大组件的启动和停止过程等

private class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
public static final int PAUSE_ACTIVITY_FINISHING= 102;
public static final int STOP_ACTIVITY_SHOW = 103;
public static final int STOP_ACTIVITY_HIDE = 104;
public static final int SHOW_WINDOW = 105;
public static final int HIDE_WINDOW = 106;
public static final int RESUME_ACTIVITY = 107;
public static final int SEND_RESULT = 108;
public static final int DESTROY_ACTIVITY = 109;
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int NEW_INTENT = 112;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;

public static final int CONFIGURATION_CHANGED = 118;
public static final int CLEAN_UP_CONTEXT = 119;
public static final int GC_WHEN_IDLE = 120;
public static final int BIND_SERVICE = 121;
public static final int UNBIND_SERVICE = 122;
public static final int DUMP_SERVICE = 123;
public static final int LOW_MEMORY = 124;
public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
public static final int RELAUNCH_ACTIVITY = 126;
public static final int PROFILER_CONTROL = 127;
public static final int CREATE_BACKUP_AGENT = 128;
public static final int DESTROY_BACKUP_AGENT = 129;
public static final int SUICIDE = 130;
public static fin
b5af
al int REMOVE_PROVIDER = 131;
public static final int ENABLE_JIT = 132;
public static final int DISPATCH_PACKAGE_BROADCAST = 133;
public static final int SCHEDULE_CRASH = 134;
public static final int DUMP_HEAP = 135;
public static final int DUMP_ACTIVITY = 136;
public static final int SLEEPING = 137;
public static final int SET_CORE_SETTINGS = 138;
public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
public static final int TRIM_MEMORY = 140;
public static final int DUMP_PROVIDER = 141;
public static final int UNSTABLE_PROVIDER_DIED = 142;
public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
public static final int INSTALL_PROVIDER = 145;
public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
public static final int CANCEL_VISIBLE_BEHIND = 147;
public static final int BACKGROUND_VISIBLE_BEHIND_CHANGED = 148;
public static final int ENTER_ANIMATION_COMPLETE = 149;
}

另外经常使用的runOnUIThread(Runable action),通过源码分析也是使用了mHandler，而mHandler的Looper也是使用的UI线程的mainLooper。

public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}