Handler作为Android消息通信的基础它的使用是每一个开发者都必须掌握的。开发者从一开始就被告知必须在主线程中进行UI操作。但Handler是如何实现线程间通信的呢本文将从源码中分析Handler的消息通信机制。
0x00 Handler使用
首先看看我们平时是如何使用的Handler的。先看看以下代码
//定义HandlerHandler mHandler = new Handler(){ public void handleMessage(Message msg){ switch(msg.what){ case UPDATE_UI:
updateUI(msg); break;
}
}
};class MyThread extends Thread{ public void run(){ //do same work!
... //send message
Message msg = mHandler.obtainMessage(UPDATE_UI);
mHandler.sendMessage(msg);
}
}private void updateUI(Message msg){ //update UI}
在子线程中sendMessage(Message)发送消息然后在Handler的handleMessage(Message)接收消息执行更新UI操作。那么Handler是如何把消息从MyThread传递到MainThread中来呢我们从sendMessage()开始慢慢揭开它的面纱。
0x01 sendMessage(Message)
public final boolean sendMessage(Message msg){ return sendMessageDelayed(msg, 0);
}
...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);
}
...private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this; if (mAsynchronous) {
msg.setAsynchronous(true);
} return queue.enqueueMessage(msg, uptimeMillis);
}
我们发现调用sendMessage()方法最后都走到enqueueMessage()这个方法一开始就把当前Handler实例赋给了Message.target的属性里面后面可以知道这个target是用来执行处理函数回调的。enqueueMessage方法是把Message信息放入到一个MessageQueue的队列中。顾名思义MessageQueue就是消息队列。从sendMessageAtTime()方法知道这个MessageQueue是Handler中的一个成员。它是在Handler的构造函数中通过Loopger对象来初始化的。
0x02 Handler构造函数
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;
}
这时候我们脑海知道创建Handler的时候同时也创建了Looper实例和MessageQueue引用MessageQueue对象其实是在Looper中构造的。Looper是何物呢简单地说就是消息循环这个我们稍后会分析。
0x03 enqueueMessage(MessageQueue)
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; //这里把消息插入到队列中
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;
}
在MessageQueue中可以看到这个入列方法中有一个for循环就是把当前的需要处理Message放到队列的合适位置。因为需要处理的Message对象都有一个开始处理的时间when这个队列是按照when排序的。至此Handler调用sendMessage()方法后就把Message消息通过enqueueMessage()插入MessageQueue队列中。而这个MessageQueue是在Looper中维护的。
0x04 prepare()创建Looper
在0x02中我们知道创建Handler时就使用静态方法Looper.myLooper()得到当前线程的Looper对象。
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/public static @Nullable Looper myLooper() { return sThreadLocal.get();
}
sThreadLocal是一个ThreadLocal类型的静态变量。什么时候会把Looper对象放在sThreadLocal中呢通过prepare()方法。
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));
}
继续翻阅源码知道Looper在构造函数中创建MessageQueue对象
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
调用prepare()方法将一个Looper对象放在了静态的ThreadLocal对象中。这个是一个与线程绑定的对象且在内存中仅保存了一份引用。使用ThreadLocal对象这一点非常巧妙也非常重要这是线程间通信的基础。即在线程中调用prepare()时就在该线程中绑定了Looper对象而Looper对象中拥有MessageQueue引用。所以每个线程都有一个消息队列。这样Handler、Looper、MessageQueue这几个类关系大概就可以画出来了。
0x05 启动循环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(); //这里执行消息队列循环
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 traceTag = me.mTraceTag; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
} //执行处理消息的回调
try {
msg.target.dispatchMessage(msg);
} finally { if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
} 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()方法中有一个无限循环不停地读取调用MessageQueue的next()方法。当next()没有返回时就阻塞在这里。当获取到MessageQueue中的消息时就执行了处理消息的回调函数msg.target.dispatchMessage(msg)。前面0x01分析我们知道msg.target是在Handler中的enqueueMessage()进行赋值即它指向当前的Handler实例。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this; if (mAsynchronous) {
msg.setAsynchronous(true);
} return queue.enqueueMessage(msg, uptimeMillis);
}
执行msg.target.dispatchMessage(msg)后便走到了以下流程
/**
* 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);
}
}
这里就是回调handleMessage(msg)函数处理消息的地方。Handler负责将Message入列Looper则负责循环从MessageQueue中取出需要处理的Message并交由Handler来处理。
0x06 启动主线程的消息循环
我们知道通过静态方法Looper.prepare()创建了绑定当前线程的Looper对象而通过loop()启动一个循环不停地读取队列中Message。但是Android系统是什么时候启动了主线程的消息循环呢要理解这一点就必须进入Android应用程序的入口ActivityThread的main方法。
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited");
}
可以看出main方法中先后执行了Looper.prepareMainLooper()方法和Looper.loop()方法。正常情况下main方法不会退出只有loop()方法发生异常后将会抛出RuntimeException。
0x07 Looper.prepareMainLooper()
/**
* 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();
}
}
prepareMainLooper()方法其实是调用了prepare()方法。当我们启动应用时系统就调用了prepareMainLooper()并在主线程中绑定了一个Looper对象。这时候我们回过来看看一开始的Handler使用方式。在主线程中我们创建了Handler对象在Handler构造函数中初始化了Looper即获取到了绑定在主线程中的Looper对象。当在子线程MyThread中通过mHandler.sendMessage(msg)方法发送一个消息时就把Message放在与主线程绑定的MessageQueue中。这样在子线程中使用Handler就实现了消息的通信。
可以简单的使用以下类图表示每个线程都由一个Handler每个Handler都是与当前所在线程的Looper绑定。
0x08 主线程是否会阻塞
在0x06中知道在ActivityThead的main方法中启动了一个死循环。那主线程是不是就一直阻塞在这里呢其实不然。可以看到ActivityThread类里面有一个自定义的Handler对象mH在这里对象中handleMessage()回调中定义了Activity的各种交互如管理Activity生命周期启动service显示window等都是通过Handler进行处理的。同时可以看出只有当应用退出EXIT_APPLICATION之后才回调用Looper.quit()停止消息循环。
public void handleMessage(Message msg) {
... switch (msg.what) { case LAUNCH_ACTIVITY: {
...
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
... case PAUSE_ACTIVITY: {
...
handlePauseActivity((IBinder) args.arg1, false,
(args.argi1 & USER_LEAVING) != 0, args.argi2,
(args.argi1 & DONT_REPORT) != 0, args.argi3);
...
} break;
... case SHOW_WINDOW:
...
handleWindowVisibility((IBinder)msg.obj, true);
... break;
... case EXIT_APPLICATION: if (mInitialApplication != null) {
mInitialApplication.onTerminate();
}
Looper.myLooper().quit(); break;
...
}
...
}
0x09 总结
当创建Handler时将通过ThreadLocal在当前线程绑定一个Looper对象而Looper持有MessageQueue对象。执行Handler.sendMessage(Message)方法将一个待处理的Message插入到MessageQueue中这时候通过Looper.loop()方法获取到队列中Message然后再交由Handler.handleMessage(Message)来处理。
转载于:https://blog.51cto.com/kiujyhgt/1915864
