Handler
我们都知道Handler由Message、MessageQueue、Handler和Looper组成，接下来我们带着问题，从源码中寻找 Handler 的具体流程与实现。
问题
- 消息是如何发送出去的？
- 消息时如何被得到的？
- 轮询器的启动在什么时候？
- 轮询器与消息队列的绑定是如何建立的？
- 如何确保总是能够在对应的线程中获取到正确的轮询器实例？
- 对消息队列的操作，消费者和生产者，主线程与子线程如何进行通信？
- Handler只能用于主线程UI更新？
解析
首先从轮询器的启动开始，所有的java程序都有一个main方法作为程序的入口，而Android中这个main方法在ActivityThread中
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
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();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
}
在代码中我们可以看到 Looper.prepareMainLooper()和Looper.loop() 两个方法
先从Looper.prepareMainLooper()进行分析
private static Looper sMainLooper; // guarded by Looper.class 保存Looper类
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
在其中prepareMainLooper，Looper将全局静态变量 sMainLooper 赋值和调用了 prepare() 方法
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
//myLooper就是调用了sThreadLocal的get方法
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在 prepare 中调用了全局静态变量 sThreadLocal 的 set 方法
那么ThreadLocal是什么呢？
ThreadLocal本质是一个Map，不过其中的 key 值是线程 Thread ，它通过线程来存储和读取数据。正如其名，用来存储线程本地数据【避免其他线程读取或修改】。
可就是说在此处，sThreadLocal 中存储了与主线程对应的 Looper 实例，只要是主线程中调用sThreadLocal 的get方法就能获取这个轮询器，若是其他子线程就获取不到这个轮询器
然后就到了讲解 Looper.loop() ，也就是在这里开始死循环，使得主线程一直存活
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;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
/*
* 经过上述步骤消息都未被处理，于是将其回收
*/
msg.recycleUnchecked();
}
}
在 loop 中我们主要看到 MessageQueue 、 msg.target.dispatchMessage(msg) 这三处地方
我们先看 msg.target.dispatchMessage(msg) 之后，回来了解 MessageQueue
public final class Message implements Parcelable {
...
Handler target;
Runnable callback;
...
}
从 Message 中了解 target 就是 Handler，而 callback 是用户设置的回调任务，只要不设置这个 callback 就会进入 handleMessage
public class Handler {
...
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//这里调用了用户自定义的handleMessage去处理业务逻辑
handleMessage(msg);
}
}
...
}
这里我们看到了消息时如何被得回应的，那么我们只需要知道 Message 是怎么发送的和在什么时候给 target 赋值确保Handler对象不出错。
Message 是如何发送的？这个问题想必都知道答案
public class Handler {
final Looper mLooper;
final MessageQueue mQueue;
...
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);
}
//最终进入该方法,此处就出现了 MessageQueue
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;//在此处Handler给Message的target赋值
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
...
}
经过一层一层的调用，最终Handler调用了 enqueueMessage 方法，将 Message 放入它的全局变量MessageQueue中，且将Message的target赋值为this【发送消息的Handler本身】
那么我们知道了发送的消息最终去向【MessageQueue】，那么Handler中的MessageQueue又是什么时候初始化的？
//我们平常使用的Handler无参构造函数最终都会到这里
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());
}
}
//此处就是为什么子线程不能创建Handler的原因
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
mLooper = Looper.myLooper() 之前我们分析过 myLooper 就是调用了sThreadLocal的get方法，此处只有主线程才有对应的 Looper 实例存在，这也就是为什么子线程中不能用无参构造方法实例化Handler，如果创建会报下列错误提示
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
当然也不是没有解决方案，不是没有 Looper 报错吗，那就给呗！！！解决方案
继续向下，我们看到 mQueue = mLooper.mQueue ，也就是说 MessageQueue 是从Looper中获取的
public final class Looper {
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
...
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
...
}
嗯，MessageQueue 对象在私有构造函数中就已经实例化了，那么还记得什么时候调用了Looper的构造函数吗？
绕了一圈终于到了讲解 MessageQueue 了
public final class MessageQueue {
Message mMessages;//
...
boolean enqueueMessage(Message msg, long when) {
...
synchronized (this) {
if (mQuitting) {//是否退出，只有调用了quit方法后是true
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();
//SystemClock.uptimeMillis() + delayMillis
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;
}
}
此时你可能有些疑惑，不是队列吗，那么为什么没有数组或者List呢？因为 MessageQueue 采用链表的方式实现队列。
public final class Message implements Parcelable {
...
// sometimes we store linked lists of these things
/*package*/ Message next;
//最大Message池为50个
private static final int MAX_POOL_SIZE = 50;
...
}
我们将 if 语句分开看
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;
}
首先Message p被赋值为全局变量 mMessages【我称其为“即将发送Message”】如果
- “即将发送Message”是null
- msg 的 when 是0 【when = SystemClock.uptimeMillis() + delayMillis】
- msg 的 when 小于“即将发送Message”的 when 【 msg 发送事件的等待时间 小于 mMessages】
其中一个成立，将 msg 的 next 指向 p ，在将 mMessages 赋值为 msg【实际上就是将msg放在队列最前面】
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;
}
到else，基本就是 mMessages有值且 msg 发送事件的等待时间 大于 mMessages，于是就把 msg 放入链表中，通过循环将 msg 放入链表的合适位置，确保队列中的元素等待时间是递增的
既然已经讲了存放，那么就该到读取了
Message next() {
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) {
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;
}
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.
//基本不会进入这个if
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;
}
}
此处我们看到了MessageQueue调用了 native 方法【就是java调用了c方法】，其具体的实现在 /frameworks/base/core/jni/android_os_MessageQueue.cpp 点击此处查看
//可以理解为阻塞，ptr相当于Message指针，timeoutMillis阻塞时间
private native void nativePollOnce(long ptr, int timeoutMillis);
//唤醒，之前在enqueueMessage中有调用该方法唤醒
private native static void nativeWake(long ptr);
之后我们主要看 next 对 return 有关的部分
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
//msg 对应的 Handler 被销毁，于是取出队列中的下一个 Message
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//还没有到 msg 的发送时间，需要阻塞等待
if (now < msg.when) {
// 下一条消息未准备好。设置一个超时时间，当它准备好时唤醒。
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 到 msg 的发送时间
mBlocked = false;
//将 mMessages 变为 msg 的 next
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 {
// nextPollTimeoutMillis = -1 代表nativePollOnce将一直阻塞直到被唤醒
nextPollTimeoutMillis = -1;
}
到了这里Handler的源码分析就结束了，可以再回去看看Handler的流程图是不是会有新的感悟
