android线程消息机制主要由Handler,Looper,Message和MessageQuene四个部分组成。平常在开发中,我们常用来在子线程中通知主线程来更新,其实整个安卓生命周期的驱动都是通过Handler(ActivityThread.H)来实现的。
首先我们先介绍这四个类的作用:
Handler:消息的发送者。负责将Message消息发送到MessageQueue中。以及通过Runnable,Callback或者handleMessage()来实现消息的回调处理
Looper:是消息的循环处理器,它负责从MessageQueue中取出Message对象进行处理。(Looper含有MessageQueue的引用)
Message:是消息载体,通过target来指向handler的引用。通过object来包含业务逻辑数据。其中MessagePool为消息池,用于回收空闲的Message对象的。
MessageQueue:消息队列,负责维护待处理的消息对象。
通过上面的图,我们可以比较清楚地知道他们的作用以及关系。接下来,我们从源码角度来分析这种关系是如何建立的。
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
hander的其它构造方法可以自己去查看,通过这个构造方法,我们知道,handler持有MessageQueue的引用。所以可以方便地将Message加入到队列中去。
通过源码我们发现,sendMessage- sendMessageDelayed- sendMessageAtTime- enqueueMessage
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);
}
都是通过enqueueMessage将message将加入到MessageQueue中。
接下来,我们看Message是如何构造的。通过Message的构造方法。
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
我们看到,Message是通过obtain的静态方法从消息池sPool中拿到的。这样可以做到消息的复用。
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
其中有一个重载方法中m.target = h;这段代码非常重要,便于后面找到消息的目标handler进行处理。
接下来,我们来看Looper。我们知道Looper通过过Looper.loop来进入循环的,而循环是通过线程的run方法的驱动的。
首先我们知道,我们在创建Handler的时候,都没有去创建Looper,那么Looper哪里来的呢?
public Handler(Callback callback, boolean async) {
...
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;
}
再看看Looper.myLooper()
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
ThreadLocal是线程创建线程局部变量的类。表示此变量只属于当前线程。
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();
}
我们看到了sThreadLocal.get()的方法实际是取当前线程中的Looper对象。
那么我们主线程的Looper到底在哪里创建的呢? 而我们清楚地知道,如果在子线程中创建handler调用,则需要使用Looper.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));
}
我们看到此方法中,如果此线程中没有Looper对象,则创建一个Looper对象。接下来我们在源码中看到一个熟悉的方法。
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
此方法单独的创建了一个sMainLooper用于主线程的Looper。这个prepareMainLooper到底在哪里调用呢?
高过引用指向发现,我们在ActivityThread.main()方法中发现
public static void main(String[] args) {
...
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");
}
而ActivityThread.main()是程序的入口方法。这样我们就非常清楚了,主线程的Looper在程序的启动过程中就已经创建并循环。
那么如果在子线程中创建Looper该如何正确调用呢?
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
接下来,我们需要看下Looper.loop()的执行方法
public static void loop() {
final Looper me = myLooper();//拿到当前线程的looper
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//拿到当前looper的消息队列
// 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 (;;) {//死循环遍历消息体。如果为null,则休眠。
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);//此处是真正的分发消息。此处的target即是handler对象
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();
}
}
最后我们看下dispatchMessage的处理方法。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
我们看到,dispatchMessage是优化处理msg.callback,然后就是实现的Callback接口,最后才是handleMessage方法。
重点说明:
1、handler在实例化的时候,持有Looper的引用。是通过ThreadLocal与Handler进行关联的。
2、Message在实例化的过程中,通过target 持有Handler的引用。
3、通常一个线程对应一个Looper.一个Looper可以属于多个Handler。
以上就是本文的全部内容,希望对大家的学习有所帮助。