了解Handler,Looper,MessageQueue，Message的工作流程
Handler的作用
异步通信，消息传递Handler的基本用法
Handler的用法，示例1、（子线程向主线程发送消息）
public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Toast.makeText(HandlerActivity.this, "接收到了消息", Toast.LENGTH_SHORT).show();
}
};
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
}
public void onClickView(View v) {
switch (v.getId()) {
case R.id.button: {
Thread thread = new Thread() {
@Override
public void run() {
Message msg = Message.obtain();
handler.sendMessage(msg);
}
};
thread.start();
break;
}
}
}
}这个界面只有一个Button, 点击事件是：启动一个线程，在这个线程里使用handler，发送一个消息；这个消息不带任何数据。
Message: 这个可以使用new来生成，但是最好还是使用Message.obtain()来获取一个实例。这样便于消息池的管理。
handler初始化的时候，我们复写了handleMessage方法，这个方法用来接收，子线程中发过来的消息。
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示例2、主线程向子线程发送消息
public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
Log.d("child", "----" + Thread.currentThread().getName());
}
};
Handler childHandler = null;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler);
Thread child = new Thread("child thread") {
@Override
public void run() {
Looper.prepare();
childHandler = new Handler(Looper.myLooper()) {
@Override
public void handleMessage(Message msg) {
Log.d("child", "----" + Thread.currentThread().getName() + ", " + msg.obj);
}
};
Looper.loop();
}
};
child.start();
}
public void onClickView(View v) {
switch (v.getId()) {
case R.id.button: {
Thread thread = new Thread() {
@Override
public void run() {
Message msg = Message.obtain();
handler.sendMessage(msg);
}
};
thread.start();
break;
}
case R.id.button1: {
if (childHandler != null) {
Message msg = Message.obtain(childHandler);
msg.obj = "123---" + SystemClock.uptimeMillis();
childHandler.sendMessage(msg);
} else {
Log.d("-----", "onClickView: child Handler is null");
}
break;
}
}
}
}主线程向子线程发送消息时，我们需要使用的是handler,但是这个handler是需要在子线程中实例化，否则子线程无法接收到消息。
在child thread中准备的工作：
-
- Looper.prepare();
-
- 实例化childHandler的时候出入了一个Looper实例。
-
Looper.loop()调用
假如我们直接在子线程中直接实例化一个handler，而不传入Looper实例。程序会直接抛出异常：java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
这个异常是在hanlder源码：public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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;
}
出现的，从这里可知，在线程中实例化一个handler需要一个Looper实例。如果其他线程的实例出来的Looper会怎么样？
首先看Looper源码中是怎么写的。Looper的构造方法是私有的，这样我们只能通过Looper的静态方法来实例化一个Looper.在Looper类中还有一个ThreadLocal
sThreadLocal 变量，而在上面的代码中使用了Looper.myLooper()方法来给handler一个Looper实例。 * Looper.myLooper()
public static @Nullable Looper myLooper() { return sThreadLocal.get(); }这个方法返回了一个存放在ThreadLocal
中的Looper实例。 为什么在调用Looper.myLooper()方法之前需要先调用 Looper.prepare()呢？
Looper.prepare源码：
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");
}
sThreadLocal.set(new Looper(quitAllowed));
}
方法很简单，就是检查sThreadLocal里面是否有有值，然后将Looper的实例存放到ThreadLocal，如果不为空，直接就是 RuntimeException 异常。这也保证了一个线程中只能有一个Looper实例。因此，Looper.prepare方法只能调用一次。那么ThreadLocal的作用是什么呢？
在这个程序中，可以大概理解成线程间数据的隔离。意思就是我存放在ThreadLocal中的数据，只能在我本线程中可以获得到值，在其他线程中获取不到（其他线程中获取的是null）。
这样就能得出，其他线程中的Looper，在本线程中通过Looper.myLooper()获取不到数据。那么为什么在主线程中不需要传入Looper实例呢？
通过查找Android源码，可以知道在ActivityThread中main方法里面，UI线程已经初始化了Looper.prepareMainLooper();这样就在UI线程中有Looper实例了。当然在main方法的下面，也调用了Looper.loop()方法。
Looper.loop()方法是做什么的
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(); **}**}
这个方法里面，我们需要注意到：
- Looper me这个不允许为空
- for (;;)循环
- Message msg = queue.next()
msg.target.dispatchMessage(msg);
- 在looper.loop方法里面，检查了本线程中的Looper实例，也对应了在调用looper.loop方法之前必须先调用looper.prepare方法。
for循环是一个死循环，这个需要一直取出MessageQueue队列中的数据，也就是刚才所列的第三个 queue.next方法。这个方法会阻塞，直到有消息从消息队列中取出来。
next方法源码：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; }}
- queue.next方法，返回一个Message，而这个message会被传入到msg.target的dispatchMessage方法中。msg.target是一个handler，就是发送消息的实例。
dispatchMessage(Message)源码：public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Handler初始化的时候，我们并没有传入msg.callback和mCallback这两个回调。所以这个方法最终执行的是handleMessage方法。
上面我们分析了ThreadLocal,Looper,Message,MessageQueue,下面开始分析handler发送消息的方式。Handler发送消息
Handler 通过sendMessage方法发送消息。这个方法最终调用的是
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
参数：MessageQueue queue则是Handler中mQueue变量，这个变量在Handler(Callback callback, boolean async)初始化完成。它是Looper中一个final MessageQueue的变量，在初始化Looper的时候，就开始初始化MessageQueue。这也是一个线程中只有一个MessageQueue的原因.
Message msg 是封装的消息。
long uptimeMillis 延迟发送时间。
之前分析looper.loop方法的时候，说了msg.target.dispatchMessage, 这个target就是在这个方法里面赋值的。
从这个方法里面，可以知道handler的sendMessage，只是把消息(Message实例)添加到了queue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;
}Handler, Looper, Message, MessageQueue之间的关系
通过handler发送Message，将Message压入MessageQueue队列中；而Looper.loop方法又在不停的循环这个消息队列，取出压入MessageQueue的Message， 然后dispatchMessage分发，最终会调用handler.handleMessage方法来处理发送过来的Message.
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