在Android系统中，Handler机制应用较广，尤其是在App层面，基本每个App都会用到。使用的场景主要是向主线程发送事件，更新UI。但大家真的了解Handler机制吗？看一下面的几个问题是否可以回答：
a.Handler是如何实现多个线程之前事件传递的？
b.Handler、Message、MessageQueue、Looper相互之间的数量比是多少，都是1:1吗？
c.每个变量运行的线程是那个？
d.ThreadLocal是怎么回事，在Handler机制中起什么作用？
能准确回答上述问题，说明对Handler机制的理解已相当到位了。
一、下面论述一下Handler机制各类的实现原理，从而揭露内部的工作流程。（源码环境为Android8.0）
1. 先来看Handler类：
1.1 构造方法：
Handler的构造方法总共有如下几个，最终实现为下面两个方法：
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;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
最终都调至Handler(Callback callback, boolean async)和Handler(Looper looper, Callback callback, boolean async)，看一下源码可知，主要为初始化几个变量：mLooper、mQueue、mCallback，mAsynchronous，前三个变量非常重要，有以下几个结论：
a.一个Handler中含有一个Looper
b.Handler中持有的MessageQueue源于Looper中，与Looper中该变量行为一致
c.一个Handler中含有一个Callback
Handler中还有如下几个重要的方法，提供了Handler对外的能力：
1.2 obtainMessage方法，对外提供一个获取Message缓存池中Message的方法，避免应用产生过多的Message对象。根据参数的不同，重载有多个该方法，实现原理都一样，以其中之一分析一下：
//obtainMessage@Handler.java
public final Message obtainMessage()
{
return Message.obtain(this);
}
//obtain@Message.java
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
// obtain@Message.java
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();
}
sPool提供复用功能的实现，其存储结构为链表，通过sPoolSize的大小及SPool头部指针位置作到这一点，感兴趣的同学可以研究下
1.3 post方法、sendMessage方法，Handler提供了多个post方法，按参数不同，提供了诸如指定时间、延时的功能，基本功能是一样的，以其中某一个方法说一下其实现：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
从getPostMessage方法可以看出，传入的Runnable被Message所持有。
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);
}
由上面可知如下结论：
a.post方法的实现是与sendMessage一致的，最终都调至sendMessage中
b.干了个什么事呢：得到一个message，将它的callback设为传入的callback（如果有的话），将它的target设为自身，然后调用MessageQueue的enqueueMessage处理该Message，实现原理讲到MessageQueue再行讨论。
1.4 dispatchMessage方法，该方法从Handler发起流程中难以看到在什么地方调用了，是一个消息处理时的回调，先讲一下该方法的逻辑，后面再说调用时机
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
从上面的逻辑中看到如下结论：
消费该Hanlder事件总共有三种可能的途径，分别为Message中的callback、Handler所持有的Callback对象、Handler的自身的handleMessage方法（一般写法是构造匿名子类，实现该方法），三种途径的优先级从高到低。
Handler在该机制中的作用为：
a.对外提供可复用的Message
b.对外提供发消息的接口
c.对外提供回调的实现，三种方式
2.Looper类
在主线程中使用Handler，看不到Looper，因为系统已经帮我们自动生成了Looper，在工作线程中使用Handler则必须先调用Looper的prepare生成Looper，否则会报错，原因如下：
测试代码：
Thread t = new Thread(new Runnable() {
@Override
public void run() {
Handler handler = new Handler();
}
});
t.start();
运行后报错：
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
Handler最终会调至（1.1中第一个构造方法）
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
因此正确的写法为：
Thread t = new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler handler = new Handler();
// ignore
Looper.loop();
}
});
2.1 prepare方法：
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));
}
sThreadLocal变量定义如下：
static final ThreadLocal sThreadLocal = new ThreadLocal();
使用ThreadLocal类对Looper进行了一层封装。
像c++的内联函数一样，此处我们直接内联一下ThreadLocal这个类，看一下该类的作用，先从Looper中跳出去一下
--------------------------------此处插入ThreadLocal类----------------------------------
3. ThreadLocal类
ThreadLocal达到的目的是：每一个ThreadLocal的变量在多个线程中都有一份copy，相互间独立存储，互不影响，但变量名只有一个。也就是说对于该变量的使用方来讲，看起来像是只定义了一个变量，但实际上在多线程环境中，有互不影响的、每个线程都有一份的对象存在。如何作到这一点呢，是通过其对外提供的get方法和set方法做到的。
3.1 先来看set方法：
public void set(T value) {
Thread t = Thread.currentThread();
// 第一步，拿到每个线程所对应的ThreadLocalMap
ThreadLocalMap map = getMap(t);
// 第二步，将该值存入或更新map的对应位置
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
// ThreadLocalMap的构造方法
ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
// set@ThreadLocalMap
private void set(ThreadLocal key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
上述逻辑第一步拿到每个线程的ThreadLocalMap对象
// threadlocals#Thread
ThreadLocal.ThreadLocalMap threadLocals = null;
也就是说每个线程维护一个ThreadLocalMap来存储该线程所用到的所有ThreadLocal变量，ThreadLocalMap的实现原理与HashMap基本一致，之前讨论过HashMap的实现原理，感兴趣的同学对比一下源码就能明白。
第二步中将传入的值存入ThreadLocalMap的对应位置，该位置的key由ThreadLocal本身的hash值去运算得到的。从上面的代码中可以看出。每一个线程持有一个ThreadLocalMap，set的value将和ThreadLocal自身组成key-value结构存在于该Map中。
3.2 get方法
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();
}
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
理解了set方法之后，get方法就较为简单了，反向拿到存入的值即可，ThreadLocal对象本身在多个线程中只有一份，但通过其set和get，可以得到其包装对象在每个线程中只有一份。
--------------------------ThreadLocal分析完毕，回至Looper----------------------------------
Looper的prepare为一个静态方法，目的是可以生成每个线程都有一份的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));
}
2.2 Looper构造方法：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
持有一个MessageQueue对象，持有当前的线程对象
2.3 loop方法
public static void loop() {
// 第一步：myLooper的实现看下方，得到当前线程的Looper
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();
// 第二步：定义一个无限循环体，遍历MessageQueue，取其中的有效Message，进行处理，处理语名为：msg.target.dispatchMessage(msg);该方法请看上面1.4
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);
}
// 此处实现Message的循环利用
msg.recycleUnchecked();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在loop的处理中，有一点需重点关注，loop在线程中是一个无限循环，跳出该循环的条件为MessageQueue的next返回值为空，看一下4.2的next方法实现，返回空依赖于mQuitting变量，也就是说在线程中不需要一直等待事件时，要把MessageQueue该变量置为true，设置入口为quit@Looper
2.4 quit方法
public void quit() {
mQueue.quit(false);
}
//quit@MessageQueue
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);
}
}
总结一下：Looper在Handler机制中起的作用是：
a.提供每个线程唯一的实例
b.触发MessageQueue的循环遍历
4. MessageQueue类
顾名思义，该类需提供三个能力：
a.Message队列存储
b.Message入队
c.Message出队
Message队列以链表形式存储，其头部指针存放于mMessages
4.1 enqueueMessage方法，该方法用于Message入队
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;
// 以下为入队的关键代码，找到链表的队尾，放入该Message
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;
// 注意，入队的时候，已经按时间次序将Message插入队列中的合适位置
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;
}
需要注意，在上面的入队处理中，要按Handler传入的执行时间插入
enqueueMessage调用的入口请查看1.3
4.2 next方法，该方法用于MessageQueue出队
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;
// 第一步：拿到链表的头部
Message msg = mMessages;
// 第二步：取到最早入队的有效的Message
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;
}
}
next方法的调用入口位于2.3中
小结一下，通过上面的代码讲解，可以得到以下几个结论：
1. 首先Handler对象本身位于构造Handler的当前线程，他的事件要发往那个线程，取决于构造方法传 的Looper变量
2. MessageQueue与Looper位于Handler所要发往的工作线程中，Looper持有MessageQueue对象，MessageQueue中看不到Looper
3. Looper与线程为1对1的关系，也就是说一个线程只有一个Looper，一个Looper只服务于一个线程，达到这个目的是通过ThreadLocal包装Looper实现的
4. 从线程的角度讲，Handler对象自身与线程没有任何关系，它是通过Looper持有的MessageQueue实现向Looper所服务的线程事件队列（MessageQueue）插入事件（Message）的
看一下文章开头所说的几个问题是不是已经有了答案：
a.Handler是如何实现多个线程之前事件传递的？
请看上面总结的第4点
b.Handler、Message、MessageQueue、Looper相互之间的数量比是多少，都是1:1吗？
Handler n--1 Looper（以主线程为例，主线程的Looper可以定义非常多Handler）
Looper持有一个MessageQueue（不可逆，看上面总结的第2点）
MessageQueue 1--n Message
c.每个变量运行的线程是那个？
Handler位于定义线程、Looper、MessageQueue位于工作线程，线程的轮询依赖于Looper.loop
d.ThreadLocal是怎么回事，在Handler机制中起什么作用？
可查看代码分析部分的第3部分
