Andorid异步处理之Handler消息机制分析
在学习Handler需要明白一下几个问题：
（1）为什么要使用Handler？
（2）为什么不能在线程中更新UI？
（3）Handler如何实现从子线程到主线程的切换（这里其实是涉及到java内存模型中多线程共享堆内存）
（4）Looper中的死循环为什么不会引起ANR，会消耗大量资源吗
（5）主线程的消息循环机制是什么（死循环如何处理其它事务）？
（6）ActivityThread 的动力是什么？（ActivityThread执行Looper的线程是什么）
（7）如何处理Handler 使用不当导致的内存泄露？
（8）子线程有哪些更新UI的方法。
（9）ThreadLocal的作用是什么？
一、Handler基础
通常我们的耗时操作会在子线程中执行，比如访问网络或者文件操作或者数据库操作，而在耗时任务执行完毕后，如果我们需要跟新UI，则需要在主线程中更新UI，此时需要Handler来处理，Handler可以实现从子线程到主线程的切换。
Handler的消息机制由五部分组成，Message，MessageQueue，Looper，Handler，和ThreadLocal。如果理解了这五个部分，那么Handler消息机制也就明白了。接下来详细讲一下。
1、Message
Message翻译过来就是消息，而它就是在线程之间传递到消息。可以携带少量的数据，what，arg1,arg2,obj。这里我们分析一下message的源码;
public final class Message implements Parcelable {
/**
* User-defined message code so that the recipient can identify
* what this message is about. Each {@link Handler} has its own name-space
* for message codes, so you do not need to worry about yours conflicting
* with other handlers.
*/
public int what;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg1;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg2;
/**
* An arbitrary object to send to the recipient. When using
* {@link Messenger} to send the message across processes this can only
* be non-null if it contains a Parcelable of a framework class (not one
* implemented by the application). For other data transfer use
* {@link #setData}.
*
* <p>Note that Parcelable objects here are not supported prior to
* the {@link android.os.Build.VERSION_CODES#FROYO} release.
*/
public Object obj;
/**
* Optional Messenger where replies to this message can be sent. The
* semantics of exactly how this is used are up to the sender and
* receiver.
*/
public Messenger replyTo;
/**
* Optional field indicating the uid that sent the message. This is
* only valid for messages posted by a {@link Messenger}; otherwise,
* it will be -1.
*/
public int sendingUid = -1;
/** If set message is in use.
* This flag is set when the message is enqueued and remains set while it
* is delivered and afterwards when it is recycled. The flag is only cleared
* when a new message is created or obtained since that is the only time that
* applications are allowed to modify the contents of the message.
*
* It is an error to attempt to enqueue or recycle a message that is already in use.
*/
/*package*/ static final int FLAG_IN_USE = 1 << 0;
/** If set message is asynchronous */
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
/** Flags to clear in the copyFrom method */
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;
这里Message具体的方法我们就忽略了，主要分析几个很重要的参数，对于理解后面的消息队列有很大的帮助。
（1）携带数据
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;用于Messenger跨进程通信
（2） Message next;
这里会有一个Message next的参数，其实属于链表的话就会很敏感的发现，就是指向它的下一个元素
（3）Handler target;
这里的target就是指发送该Message的Handler
2、Handler
Handler的作用就是用来发送和接收数据。
通过sendXXX来发送数据。在接收消息则是在handleMessage（）方法中进行处理。
首先我们通过Handler mainHandler = new Handler（）创建一个Handler实例，那么我们找到Handler的构造方法
public Handler() {
this(null, false);
}
可以看到调用的是Handler（Callback callback，booolean async）
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;
}
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
这里也就说明了为什么在子线程里使用Handler需要先创建Looper。继续往下看mQueue = mLooper.mQueue;这里通过实例化的Looper获取到了消息队列，可以看到拿到的是Looper的mQueue。
接下来我们发送消息，比如sendEmptyMessage（）
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
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);
}
可以看到最后实际上调用了sendMessageAtTime（），在这个方法里，我们最终调用了MessageQueue的enqueueMessage(queue, msg, uptimeMillis);方法，把Message插入到消息队列中。
3、Message Queue
消息队列。用于存放所有通过Handler发送的消息，也就是存放的是Message。MessageQueue以队列的形式对外提供了插入和删除 方法（先进先出），虽然说是以队列的形式，但是其实内部是单链表。为什么说是单链表呢？
其实前面我们分析了Message的参数，其中一个参数就是Message next，也就是指定它指向的下一个元素。
插入：enqueueMessage(Message msg, long when)
将消息插入队列中。
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;
}
这里可以看到在发送消息时,最后调用的是MessageQueue的enqueueMessage（）方法，那么具体是怎么插入的呢，看这段代码
prev = p;
p = p.next;
msg.next = p; // invariant: p == prev.next
prev.next = msg;
删除：next()
就是将消息读取，并从队列中删除。这里的next（）执行的是一个无限循环的方法，如果队列中没有消息，就会处于阻塞状态。有新消息时，next（）方法就会立即返回这条消息。并从单链表中移除。具体代码如下：
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;
}
}
通过源码可以看到，在next（）方法中有一个 for (;;)来实现死循环，并返回新的Message，同时从链表中删除这个消息代码如下：
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
此外，一个线程中只会有一个MessageQueue。这一点从插入和读取消息的方法中synchronized (this)就可以看出。
4、Looper
轮询器/消息泵/循环，Looper有一个looper（）方法，在该方法中是一个死循环，会不断轮询MessageQueue， 当发现MessageQueue中有新消息时，就会传递到Handler的handleMessage（）方法中，如果没有新消息，则处于阻塞状态。
（1）Looper的构造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper在创建时，就会创建一个消息队列，同时会保存Looper对象所在的线程。而Handler在创建对象时，所在的线程里必须已经实例化了Looper，否则会报错Can't create handler inside thread that has not called Looper.prepare()")。
（2）UI线程的Looper
线程默认是没有Looper，因此在线程中实例化Handler，则需要先创建Looper，调用Looper.prepare()，实例化Handler再调用Looper.loop()，开始轮询。但是UI线程我们并没有看到创建Looper。其实是因为在UI线程创建时就已经创建了Looper，因此不需要我们手动创建。
（3）Looper中的ThreadLocal
ThreadLocal的作用其实就是帮助Handler获取当前线程的Looper
（4）looper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
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 {
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();
}
}
这里可以看到，也是利用for(;;)来执行死循环，在MessageQueue的next（）方法得到新消息后，则loop（）方法则会调用 msg.target.dispatchMessage(msg);其中msg.target就是Handler对象，而
dispatchMessage（）方法接下来会调用Handler的handleMessage（）方法。
5、ThreadLocal
TheadLocal是JAVA提供的用于在线程内部获取数据的存储类。具体如下：
public class ThreadLocal<T> {
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
static class ThreadLocalMap {
}
}
可以看出，在TheadLocal有一个静态内部类是ThreadLocalMap，就是一个用来保存数据的类。同时在ThreadLocal中提供了一个set（）方法，用来存储数据，而get（）方法则用来获取保存的数据。那么接下来回到Looper。
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
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));
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
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
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 {
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();
}
}
分析Looper源码，可以看到初始化变量时有一个static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();也就是说明了在ThreadLocal保存的就是当前线程的Looper
接下来在prepare方法中会创建当前线程的looper，并通过ThreadLocal中的set方法sThreadLocal.set(new Looper(quitAllowed));
将创建的looper对象保存起来。
而在调用Looper的loop（）方法进行轮询时，我们分析loop（）方法，可以看到 final Looper me = myLooper();而myLooper（）方法中return sThreadLocal.get();返回了我们之前保存的looper对象，因此保证了轮询的就是我们当前的线程的looper对象。之后在死循环中，通过looper对象拿到MessageQueue中的消息。再通过 msg.target.dispatchMessage(msg);将消息分发给msg.target，也就是发送消息的Handler，再dispatchMessage会调用handlerMessage（）从而就可以处理消息了。
二、重要问题
在从源码角度分析了Handler机制后，现在我们来回到之前提到的问题。
（1）为什么要使用Handler？为什么不能在线程中更新UI？
在线程中更新UI的问题，就要从UI的设计的设计角度来讲，android中设计的UI是单线程模型的，因为如果改为多线程的话线程同步和线程安全问题将会很麻烦，因此就设计为单线程模型。但是是不是可以考虑加锁来解决多线程的问题呢？加锁会有以下两个问题：
首先加上锁机制会让UI访问的逻辑变得复杂
锁机制会降低UI访问的效率，因为锁机制会阻塞某些线程的执行。
也正因为在非UI线程中无法更新UI，因此才会设计一个Handler
（2）为什么Handler可以实现线程的切换
首先假设在A线程创建Handler，同时调用Looper.prepare()创建Looper对象和MessageQueue对象。之后开启一个线程B发送消息，那么调用handler.sendMessage()其实就是调用handler对应的线程中的MessageQueue插入一个消息，之后轮询器looper会轮询这个消息，并调用msg.target.dispatchMessage()，也就是handler的dispatchMessage（），此时就是在B线程中调用A中handler对象方法。在这个方法中会回调handlerMessage（）方法。
当然最根本的原因是我们创建的Handler保存在堆中，是可以线程共享的，然后才有上面分析。
（3）Looper中的死循环为什么不会引起ANR，会消耗大量资源吗
不会，因为从上面loop方法可以看出，有消息时会处理，没有消息时会阻塞