在Android系統中，Handler機制應用較廣，尤其是在App層面，基本每個App都會用到。使用的場景主要是向主線程發送事件，更新UI。但大家真的瞭解Handler機制嗎？看一下面的幾個問題是否可以回答：
a.Handler是如何實現多個線程之前事件傳遞的？
b.Handler、Message、MessageQueue、Looper相互之間的數量比是多少，都是1:1嗎？
c.每個變量運行的線程是那個？
d.ThreadLocal是怎麼回事，在Handler機制中起什麼作用？
能準確回答上述問題，說明對Handler機制的理解已相當到位了。
下面論述一下Handler機制各類的實現原理，從而揭露內部的工作流程。（源碼環境爲Android8.0）
1. 先來看Handler類：
1.1 構造方法：
Handler的構造方法總共有如下幾個，最終實現爲下面兩個方法：
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;
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
最終都調至Handler(Callback callback, boolean async)和Handler(Looper looper, Callback callback, boolean async)，看一下源碼可知，主要爲初始化幾個變量：mLooper、mQueue、mCallback，mAsynchronous，前三個變量非常重要，有以下幾個結論：
a.一個Handler中含有一個Looper
b.Handler中持有的MessageQueue源於Looper中，與Looper中該變量行爲一致
c.一個Handler中含有一個Callback
Handler中還有如下幾個重要的方法，提供了Handler對外的能力：
1.2 obtainMessage方法，對外提供一個獲取Message緩存池中Message的方法，避免應用產生過多的Message對象。根據參數的不同，重載有多個該方法，實現原理都一樣，以其中之一分析一下：
// obtainMessage@Handler.java
public final Message obtainMessage()
{
return Message.obtain(this);
}
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提供複用功能的實現，其存儲結構爲鏈表，通過sPoolSize的大小及SPool頭部指針位置作到這一點，感興趣的同學可以研究下
1.3 post方法、sendMessage方法，Handler提供了多個post方法，按參數不同，提供了諸如指定時間、延時的功能，基本功能是一樣的，以其中某一個方法說一下其實現：
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;
}
從getPostMessage方法可以看出，傳入的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);
}
由上面可知如下結論：
a.post方法的實現是與sendMessage一致的，最終都調至sendMessage中
b.幹了個什麼事呢：得到一個message，將它的callback設爲傳入的callback（如果有的話），將它的target設爲自身，然後調用MessageQueue的enqueueMessage處理該Message，實現原理講到MessageQueue再行討論。
1.4 dispatchMessage方法，該方法從Handler發起流程中難以看到在什麼地方調用了，是一個消息處理時的回調，先講一下該方法的邏輯，後面再說調用時機
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();
}
從上面的邏輯中看到如下結論：
消費該Hanlder事件總共有三種可能的途徑，分別爲Message中的callback、Handler所持有的Callback對象、Handler的自身的handleMessage方法（一般寫法是構造匿名子類，實現該方法），三種途徑的優先級從高到低。
Handler在該機制中的作用爲：
a.對外提供可複用的Message
b.對外提供發消息的接口
c.對外提供回調的實現，三種方式
2.Looper類
在主線程中使用Handler，看不到Looper，因爲系統已經幫我們自動生成了Looper，在工作線程中使用Handler則必須先調用Looper的prepare生成Looper，否則會報錯，原因如下：
測試代碼：
Thread t = new Thread(new Runnable() {
@Override
public void run() {
Handler handler = new Handler();
}
});
t.start();
運行後報錯：
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
Handler最終會調至（1.1中第一個構造方法）
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
因此正確的寫法爲：
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變量定義如下：
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
使用ThreadLocal類對Looper進行了一層封裝。
像c++的內聯函數一樣，此處我們直接內聯一下ThreadLocal這個類，看一下該類的作用，先從Looper中跳出去一下
--------------------------------此處插入ThreadLocal類----------------------------------
3. ThreadLocal類
ThreadLocal達到的目的是：每一個ThreadLocal的變量在多個線程中都有一份copy，相互間獨立存儲，互不影響，但變量名只有一個。也就是說對於該變量的使用方來講，看起來像是隻定義了一個變量，但實際上在多線程環境中，有互不影響的、每個線程都有一份的對象存在。如何作到這一點呢，是通過其對外提供的get方法和set方法做到的。
3.1 先來看set方法：
public void set(T value) {
Thread t = Thread.currentThread();
// 第一步，拿到每個線程所對應的ThreadLocalMap
ThreadLocalMap map = getMap(t);
// 第二步，將該值存入或更新map的對應位置
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的構造方法
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();
}
上述邏輯第一步拿到每個線程的ThreadLocalMap對象
// threadlocals#Thread
ThreadLocal.ThreadLocalMap threadLocals = null;
也就是說每個線程維護一個ThreadLocalMap來存儲該線程所用到的所有ThreadLocal變量，ThreadLocalMap的實現原理與HashMap基本一致，之前討論過HashMap的實現原理，感興趣的同學對比一下源碼就能明白。
第二步中將傳入的值存入ThreadLocalMap的對應位置，該位置的key由ThreadLocal本身的hash值去運算得到的。從上面的代碼中可以看出。每一個線程持有一個ThreadLocalMap，set的value將和ThreadLocal自身組成key-value結構存在於該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方法之後，get方法就較爲簡單了，反向拿到存入的值即可，ThreadLocal對象本身在多個線程中只有一份，但通過其set和get，可以得到其包裝對象在每個線程中只有一份。
--------------------------ThreadLocal分析完畢，回至Looper----------------------------------
Looper的prepare爲一個靜態方法，目的是可以生成每個線程都有一份的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構造方法：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
持有一個MessageQueue對象，持有當前的線程對象
2.3 loop方法
public static void loop() {
// 第一步：myLooper的實現看下方，得到當前線程的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：定義一個無限循環體，遍歷MessageQueue，取其中的有效Message，進行處理，處理語名爲：msg.target.dispatchMessage(msg);該方法請看上面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);
}
// 此處實現Message的循環利用
msg.recycleUnchecked();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在loop的處理中，有一點需重點關注，loop在線程中是一個無限循環，跳出該循環的條件爲MessageQueue的next返回值爲空，看一下4.2的next方法實現，返回空依賴於mQuitting變量，也就是說在線程中不需要一直等待事件時，要把MessageQueue該變量置爲true，設置入口爲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);
}
}
總結一下：Looper在Handler機制中起的作用是：
a.提供每個線程唯一的實例
b.觸發MessageQueue的循環遍歷
4. MessageQueue類
顧名思義，該類需提供三個能力：
a.Message隊列存儲
b.Message入隊
c.Message出隊
Message隊列以鏈表形式存儲，其頭部指針存放於mMessages
4.1 enqueueMessage方法，該方法用於Message入隊
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;
// 以下爲入隊的關鍵代碼，找到鏈表的隊尾，放入該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，入隊的時候，已經按時間次序將Message插入隊列中的合適位置
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，在上面的入隊處理中，要按Handler傳入的執行時間插入
enqueueMessage調用的入口請查看1.3
4.2 next方法，該方法用於MessageQueue出隊
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：拿到鏈表的頭部
Message msg = mMessages;
// 第二步：取到最早入隊的有效的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方法的調用入口位於2.3中
小結一下，通過上面的代碼講解，可以得到以下幾個結論：
1. 首先Handler對象本身位於構造Handler的當前線程，他的事件要發往那個線程，取決於構造方法傳 的Looper變量
2. MessageQueue與Looper位於Handler所要發往的工作線程中，Looper持有MessageQueue對象，MessageQueue中看不到Looper
3. Looper與線程爲1對1的關係，也就是說一個線程只有一個Looper，一個Looper只服務於一個線程，達到這個目的是通過ThreadLocal包裝Looper實現的
4. 從線程的角度講，Handler對象自身與線程沒有任何關係，它是通過Looper持有的MessageQueue實現向Looper所服務的線程事件隊列（MessageQueue）插入事件（Message）的
看一下文章開頭所說的幾個問題是不是已經有了答案：
a.Handler是如何實現多個線程之前事件傳遞的？
請看上面總結的第4點
b.Handler、Message、MessageQueue、Looper相互之間的數量比是多少，都是1:1嗎？
Handler n--1 Looper（以主線程爲例，主線程的Looper可以定義非常多Handler）
Looper持有一個MessageQueue（不可逆，看上面總結的第2點）
MessageQueue 1--n Message
c.每個變量運行的線程是那個？
Handler位於定義線程、Looper、MessageQueue位於工作線程，線程的輪詢依賴於Looper.loop
d.ThreadLocal是怎麼回事，在Handler機制中起什麼作用？
可查看代碼分析部分的第3部分