Binder/Socket用于进程间通信，而Handler消息机制用于同进程的线程间通信，Handler消息机制是由一组MessageQueue、Message、Looper、Handler共同组成的，为了方便且称之为Handler消息机制。
有人可能会疑惑，为何Binder/Socket用于进程间通信，能否用于线程间通信呢？答案是肯定，对于两个具有独立地址空间的进程通信都可以，当然也能用于共享内存空间的两个线程间通信，这就好比杀鸡用牛刀。接着可能还有人会疑惑，那handler消息机制能否用于进程间通信？答案是不能，Handler只能用于共享内存地址空间的两个线程间通信，即同进程的两个线程间通信。很多时候，Handler是工作线程向UI主线程发送消息，即App应用中只有主线程能更新UI，其他工作线程往往是完成相应工作后，通过Handler告知主线程需要做出相应地UI更新操作，Handler分发相应的消息给UI主线程去完成，如下图：
由于工作线程与主线程共享地址空间，即Handler实例对象mHandler位于线程间共享的内存堆上，工作线程与主线程都能直接使用该对象，只需要注意多线程的同步问题。工作线程通过mHandler向其成员变量MessageQueue中添加新Message，主线程一直处于loop()方法内，当收到新的Message时按照一定规则分发给相应的handleMessage()方法来处理。所以说，Handler消息机制用于同进程的线程间通信，其核心是线程间共享内存空间，而不同进程拥有不同的地址空间，也就不能用handler来实现进程间通信。
消息机制主要包含:
- Message：消息分为硬件产生的消息(如按钮、触摸)和软件生成的消息；Message中有一个用于处理消息的Handler；
- MessageQueue：消息队列的主要功能向消息池投递消息(MessageQueue.enqueueMessage)和取走消息池的消息(MessageQueue.next)；MessageQueue有一组待处理的Message；
- Handler：消息辅助类，主要功能向消息池发送各种消息事件(Handler.sendMessage)和处理相应消息事件(Handler.handleMessage)；Handler中有Looper和MessageQueue的成员变量。
- Looper：不断循环执行(Looper.loop)，按分发机制将消息分发给目标处理者。Looper有一个MessageQueue消息队列；
首先想一想平时我们是怎么使用的:
private Handler mHandler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(@NonNull Message msg) {
// xxxx.
return false;
}
});
mHandler.sendMessage(msg);所以这里分析还是要从Handler开始，主要看它的构造函数和sendMessage()方法:
final Looper mLooper;
final MessageQueue mQueue;
@UnsupportedAppUsage
final Callback mCallback;
final boolean mAsynchronous;
@UnsupportedAppUsage
IMessenger mMessenger;
public Handler() {
this(null, false);
}
public Handler(@Nullable Callback callback) {
this(callback, false);
}
// 可以设置Looper进来
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
public Handler(@Nullable Callback callback, boolean async) {
// 匿名类、内部类和本地类都必须申请为static，否则会警告可能出现内存泄露
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());
}
}
// 1. 如果不设置Looper对象进来，Looper会通过Looper.myLooper()获取Looper对象
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
// 获取Looper中的MessageQueue赋值给当前的MessageQueue变量
mQueue = mLooper.mQueue;
// 回调接口
mCallback = callback;
// 设置消息是否为异步处理方式，默认都是同步的
// If true, the handler calls {@link Message#setAsynchronous(boolean)} for
// each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
// 如果是异步的会调用Message的setAsynchronous方法，然后在MessageQueue中会判断是不是异步的消息
mAsynchronous = async;
}// sThreadLocal.get() will return null unless you've called prepare().
@UnsupportedAppUsage
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}上面注释写的很明白：除非你调用了prepare()方法，不然sThreadLocal.get()会返回null。那sThreadLocal是啥？ prepare()方法又是干啥的？这里还是分两步:
ThreadLocal:线程本地存储区（Thread Local Storage，简称为TLS），每个线程都有自己的私有的本地存储区域，不同线程之间彼此不能访问对方的TLS区域。TLS常用的操作方法:
-
ThreadLocal.set(T value)：将value存储到当前线程的TLS区域:
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); }
-
ThreadLocal.get():获取当前线程TLS区域的数据:
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(); }
而sThreadLocal就是线程本地存储变量，它的意义就是在本线程内的任何对象内保持一致。
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");
}
// 这个参数是是否允许退出
sThreadLocal.set(new Looper(quitAllowed));
}
final MessageQueue mQueue;
final Thread mThread;
private Looper(boolean quitAllowed) {
// 创建MessageQueue，并将quitAllowed传入MessageQueue的构造函数
mQueue = new MessageQueue(quitAllowed);
// 保存当前的线程
mThread = Thread.currentThread();
}从代码上可以看到prepare()方法的作用是创建一个Looper对象，然后将该Looper对象保存到sThreadLocal中。也就是说，prepare函数通过ThreadLocal机制，巧妙地把Looper和调用prepare的线程（也就是最终的处理线程）绑定在一起了。当事件源向这个Looper发送消息的时候，其实是把消息加到这个Looper的消息队列里了。那么，该消息就将由和Looper绑定的处理线程来处理。
这里有点麻烦了，我们上面使用的代码中并没有调用Looper.prepare()方法啊，理论上这里应该是null，Handler是无法使用的，为什么我们还能正常使用Handler？Looper.prepare()究竟是什么时候调用的？那我们需要看一下prepare()和prepare(boolean quitAllowed)方法都有哪些地方调用了:
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}发现在Looper类中有个prepareMainLooper()的方法，注释上面写的也比较清楚，说这是Android运行环境创建的应用程序的主looper，你不能自己调用这个方法，那我们看一下这个方法是从哪里调用的，ActivityThread类中的main函数，看到这个main函数，就知道这个类不简单啊：
/**
* This manages the execution of the main thread in an
* application process, scheduling and executing activities,
* broadcasts, and other operations on it as the activity
* manager requests.
*
* {@hide}
*/
public final class ActivityThread extends ClientTransactionHandler {
// ....
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
// Install selective syscall interception
AndroidOs.install();
// 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();
// 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>");
// 调用Looper.prepareMainLooper()方法
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
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()启动
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
} 从上面ActivityThread类的注释中可以看到说ActivityThread是应用程序的主线程。ActivityThread类是Android APP进程的初始类，它的main函数是这个APP进程的入口。
ActivityThread类的详细介绍可以参考这篇文章，这里就不细说ActivityThread类了，我们只需要知道Android运行环境会启动ActivityThread类，他是主线程，而ActivityThread类中的main函数会调用Looper.prepareMainLooper()和Looper.loop()方法。
到这里梳理一下上面的TODO：
- Looper类构造函数创建MessageQueue
- ActivityThread调用了Looper.prepareMainLooper()后又调用了Looper.loop()方法，要分析Looper.loop()方法的实现。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
public final class MessageQueue {
private final boolean mQuitAllowed;
private long mPtr; // used by native code
// Message中的next指向了下一个Message
Message mMessages;
private boolean mBlocked;
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
@UnsupportedAppUsage
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
} MessageQueue中有一个native的方法;
- nativeInit()
- 创建了NativeMessageQueue对象，增加其引用计数，并将NativeMessageQueue指针mPtr保存在Java层的MessageQueue
- 创建了Native Looper对象
- 调用epoll的epoll_create()/epoll_ctl()来完成对mWakeEventFd和mRequests的可读事件监听
- nativeDestroy()方法
- 调用RefBase::decStrong()来减少对象的引用计数
- 当引用计数为0时，则删除NativeMessageQueue对象
- nativePollOnce
- 调用Looper::pollOnce()来完成，空闲时停留在epoll_wait()方法，用于等待事件发生或者超时
- nativeWake
- 调用Looper::wake()来完成，向管道mWakeEventfd写入字符；
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
// 从sThreadLocal获取当前的Looper
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取当前Looper的MessageQueue
final MessageQueue queue = me.mQueue;
// 确保在权限检查时是基于本地进程，而不是调用进程
// 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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
// 不断去循环执行
for (;;) {
// 去MessageQueue中的下一个Message，可能会堵塞
Message msg = queue.next(); // might block
// 没有消息就退出该循环
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);
}
try {
// 分发Message，Message.target是Message中保存的当前handler对象，这里相对于调用Handler的dispatchMessage(msg)方法
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
// 恢复调用者信息
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
// 将Message放入消息池
msg.recycleUnchecked();
}
}
// Looper的quit方法是调用MessageQueue.quit()方法
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}loop()进入循环模式，不断重复下面的操作，直到没有消息时退出循环
- 读取MessageQueue的下一条Message；
- 把Message分发给相应的target；
- 再把分发后的Message回收到消息池，以便重复利用。
这里我们需要看一下Message类中的target及recycleUncheked()方法:
public final class Message implements Parcelable {
// 消息类别
public int what;
// 参数1
public int arg1;
// 参数2
public int arg2;
// 消息内容
public Object obj;
// 消息响应方Handler
/*package*/ Handler target;
// 维护下一个消息
/*package*/ Message next;
// 消息的回调方法
/*package*/ Runnable callback;
public static final Object sPoolSync = new Object();
// sPool也是Message对象，
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
public static Message obtain(Handler h) {
Message m = obtain();
// 将handler赋值给target保存
m.target = h;
return m;
}
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = UID_NONE;
workSourceUid = UID_NONE;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
// 如果当前消息池小于最大的数量限制，就把消息放到消息池中
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
// 把新放入的Message加到链表的表头
sPool = this;
sPoolSize++;
}
}
}
} public void handleMessage(@NonNull Message msg) {
}
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
// 如果Message存在回调方法，就回调Message.callback.run()方法
handleCallback(msg);
} else {
if (mCallback != null) {
// 如果Message没有回调方法，而Handler对象中设置了Callback接口，这里就回调Callback.handleMessage(msg)方法
if (mCallback.handleMessage(msg)) {
return;
}
}
// handler自己的handleMessage方法，默认空实现，子类可重写该方法
handleMessage(msg);
}
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
public final void removeCallbacksAndMessages(@Nullable Object token) {
mQueue.removeCallbacksAndMessages(this, token);
}通过调用sendMessageAtFrontOfQueue计入一个when为0的message到队列，即插入到队列的头部。可以尽量保证快速执行。
MessageQueue非常重要，因为quit、next等都最终调用的是MessageQueue类:
public final class MessageQueue {
private final boolean mQuitAllowed;
private long mPtr; // used by native code
// 消息在MessageQueue中使用Message表示，而Message包含一个next变量，该变量指向下一个消息
// 所以队列中的消息以链表的结构进行保存
Message mMessages;
// 获取MessageQueue的下一个Message
Message next() {
// 如果MessageQueue已退出就直接返回
// 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();
}
// 该方法的作用是从消息队列中取出一个消息。MessageQueue中没有保存消息队列，真正的消息队列在JNI的C代码中
// 也就是在C环境中创建了一个NativeMessageQueue数据对象。该方法的第一个参数是int型变量，在C环境中该变量
// 会被强制转换为一个NativeMessageQueue对象。如果消息队列中没消息，当前线程会被挂起。
// 堵塞操作，当等待nextPollTimeoutMillis时长或消息队列别唤醒，都会返回
// nextPollTimeoutMillis代表下一个消息到来前，还需要等待的时长；当nextPollTimeoutMillis = -1时，表示消息队列中无消息，会一直等待下去。
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) {
// 当当前的消息的Handler为空时，就查询异步消息
// 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) {
// 仅仅是为了判断消息所指定的执行时间是否到了，如果到了就返回该消息
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);
// 设置消息的使用状态，即FLOAG_IN_USE的flag
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);
}
// 空闲回调函数
// 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;
}
}
// 添加一条消息到消息队列
boolean enqueueMessage(Message msg, long when) {
// 每条消息必须有一个target
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;
// 如果是堵塞的，这里就需要唤醒
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) {
// 内部会将mMessages消息添加到C环境中的消息队列中，并且如果消息线程正处于挂起状态，则唤醒该线程
nativeWake(mPtr);
}
}
return true;
}
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
// 清除所有的message
private void removeAllMessagesLocked() {
Message p = mMessages;
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
mMessages = null;
}
void removeCallbacksAndMessages(Handler h, Object object) {
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// 从消息队列的头开始，移除所有符合条件的消息
// Remove all messages at front.
while (p != null && p.target == h
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycleUnchecked();
p = n;
}
// 移除剩余符合条件的消息
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && (object == null || n.obj == object)) {
Message nn = n.next;
n.recycleUnchecked();
p.next = nn;
continue;
}
}
p = n;
}
}
}根据前面的分析可知，Handler中的消息队列实际就是某个Looper的消息队列，那么，Handler如此安排的目的何在？ 在回答这个问题之前，我先来问一个问题：怎么往Looper的消息队列插入消息？如果不知道Handler，这里有一个很原始的方法可解决上面这个问题：
- 调用Looper的myQueue，它将返回消息队列对象MessageQueue。
- 构造一个Message，填充它的成员，尤其是target变量。
- 调用MessageQueue的enqueueMessage，将消息插入消息队列。 这种原始方法的确很麻烦，且极容易出错。但有了Handler后，我们的工作就变得异常简单了。Handler更像一个辅助类，帮助我们简化编程的工作。
有关内存泄露请猛戳内存泄露
在上面分析Handler类源码时，其构造函数中第一部分的代码就是 匿名类、内部类和本地类都必须申请为static，否则会警告可能出现内存泄露。那为什么会导致内存泄露呢？
Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
// do something.
}
}当我们这样创建Handler的时候Android Lint会提示我们这样一个warning： In Android, Handler classes should be static or leaks might occur.。
一直以来没有仔细的去分析泄露的原因，先把主要原因列一下：
Android程序第一次创建的时候，默认会创建一个Looper对象，Looper去处理Message Queue中的每个Message,主线程的Looper存在整个应用程序的生命周期.Hanlder在主线程创建时会关联到Looper的Message Queue,Message添加到消息队列中的时候Message(排队的Message)会持有当前Handler引用， 当Looper处理到当前消息的时候，会调用Handler#handleMessage(Message).就是说在Looper处理这个Message之前， 会有一条链MessageQueue -> Message -> Handler -> Activity，由于它的引用导致你的Activity被持有引用而无法被回收- 在java中，no-static的内部类会隐式的持有当前类的一个引用。static的内部类则没有。
public class SampleActivity extends Activity {
private final Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
// do something
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// 发送一个10分钟后执行的一个消息
mHandler.postDelayed(new Runnable() {
@Override
public void run() { }
}, 600000);
// 结束当前的Activity
finish();
}
}在finish()的时候，该Message还没有被处理，Message持有Handler,Handler持有Activity,这样会导致该Activity不会被回收，就发生了内存泄露.
-
通过程序逻辑来进行保护。
- 如果
Handler中执行的是耗时的操作，在关闭Activity的时候停掉你的后台线程。线程停掉了，就相当于切断了Handler和外部连接的线，Activity自然会在合适的时候被回收。 - 如果
Handler是被delay的Message持有了引用，那么在Activity的onDestroy()方法要调用Handler的remove*方法，把消息对象从消息队列移除就行了。- 关于
Handler.remove*方法removeCallbacks(Runnable r)——清除r匹配上的Message。removeCallbacks(Runnable r, Object token)——清除r匹配且匹配token（Message.obj）的Message，token为空时，只匹配r。removeCallbacksAndMessages(Object token)——清除所有callback以及token匹配上的Message，如果token是null就会清楚所有callback和message。我们更多需要的是清除以该Handler为target的所有Message(Callback)就调用如下方法即可handler.removeCallbacksAndMessages(null);removeMessages(int what)——按what来匹配removeMessages(int what, Object object)——按what来匹配
- 关于
- 如果
-
将
Handler声明为静态类。 静态类不持有外部类的对象，所以你的Activity可以随意被回收。但是不持有Activity的引用，如何去操作Activity中的一些对象？ 这里要用到弱引用
public class MyActivity extends Activity {
private MyHandler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mHandler = new MyHandler(this);
}
@Override
protected void onDestroy() {
// Remove all Runnable and Message.
mHandler.removeCallbacksAndMessages(null);
super.onDestroy();
}
static class MyHandler extends Handler {
// WeakReference to the outer class's instance.
private WeakReference<MyActivity> mOuter;
public MyHandler(MyActivity activity) {
mOuter = new WeakReference<MyActivity>(activity);
}
@Override
public void handleMessage(Message msg) {
MyActivity outer = mOuter.get();
if (outer != null) {
// Do something with outer as your wish.
}
}
}
}- 邮箱 ：charon.chui@gmail.com
- Good Luck!