Android的消息机制————读书笔记

Android的消息机制

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。

Android的消息机制分析

ThreadLocal的工作原理

ThreadLocal是一个线程内部的数据存储类，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，对于其他线程来说则无法获取到数据。当某些数据是以线程为作用域并且不同线程具有不同的数据副本的时候，就可以考虑采用ThreadLocal。另一个使用场景是复杂逻辑下的对象传递。

ThreadLocal#set

public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}

分析，首先会通过values方法来获取当前线程中的ThreadLocal数据。Thread类的内部有一个成员专门用于存储线程的ThreadLocal的数据：ThreadLocal.Values localValues。

在localValues内部有一个数组private Object[] table，ThreadLocal的值就存储在table数组里。如下代码所示：

/**
* Sets entry for given ThreadLocal to given value, creating an
* entry if necessary.
*/
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;
}
}
}

ThreadLocal#set

/**
* Returns the value of this variable for the current thread. If an entry
* doesn't yet exist for this variable on this thread, this method will
* create an entry, populating the value with the result of
* {@link #initialValue()}.
*
* @return the current value of the variable for the calling thread.
*/
@SuppressWarnings("unchecked")
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);
}

逻辑：取出当前线程的localValues对象，若对象为空返回初始值，不为空就取出它的table数组并找出ThreadLocal的reference对象在数组中的位置，然后table数组中的下一个位置所储存的数据就是ThreadLocal的值。

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

消息队列的工作原理

MessageQueue主要包含两个操作：插入和读取（伴随着删除操作）。分别对应的方法为：enqueueMessage和next。MessageQueue内部实现是通过一个单链表的数据结构来维护消息列表，插入和删除上有优势。

MessageQueue#enqueueMessage

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;
}

主要是对单链表的插入操作。

MessageQueue#next

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MessageQueue没有消息，next方法阻塞在那，有新消息next方法返回此消息并将其从单链表中移除。

Looper的工作原理

Looper的工作原理：

a. prepare方法，为当前没有Looper的线程创建Looper。

b.prepareMainLooper和getMainLooper方法用于创建和获取ActivityThread的Looper。

c.quit和quitSafely方法，前者立即退出，后者只是设定一个标记，当消息队列中的所有消息处理完毕后会才安全退出。子线程中创建的Looper建议不需要的时候都要手动终止。

d.loop方法，死循环，阻塞获取msg并丢给msg.target.dispatchMessage方法去处理，这里的target就是handler。

Looper#loop

/**
* 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();
}
}

Handler的工作原理

Handler的工作主要包含消息的发送和接受过程。

a. 无论sendMessage还是post最终都是调用的sendMessageAtTime方法。

b. 发送消息其实就是把一条消息通过MessageQueue的enqueueMessage方法加入消息队列，Looper收到消息就会调用handler的dispatchMessage方法。它的处理过程参考书page388的流程图，一看就懂

/**
* 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);
}
}

首先，检查Message的callback是否为null，不为null就通过handleCallback来处理消息。Message的callback是一个Runnable对象，实际上就是Handler的post方法所传递的Runnable参数。

private static void handleCallback(Message message) {
message.callback.run();
}

其次检查mCallback是否为null，不为null就通过mCallback的handleMessage方法处理消息。

/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}

c. 这里我补充一个东西，当我们直接Handler h = new Handler()时，本质调用的是Handler(Callback callback, Boolean async)构造方法，这个方法里会调用Looper.myLooper()方法，这个方法其实就是返回的ThreadLocal里保存的当前线程的Looper，这也就解释了为什么我们在主线程中这样new没有问题，子线程中如果不先Looper.prepare会抛出异常的原因，前面多次说了，因为ActivityThread会在初始化的时候创建自己的Looper。

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。

ActivityThread通过ApplicationThread和AMS进行进程间通信，AMS以进程间通信的方式完成ActivityThread的请求后会回调ApplicationThread中的Binder方法，然后ApplicationThread会向H发送消息，H收到消息后会将ApplicationThread中的逻辑切换到ActivityThread中去执行，即切换到主线程中去执行。这就是主线程的消息循环模型。