Handler
我們都知道Handler由Message善玫、MessageQueue禁悠、Handler和Looper組成粉私，接下來我們帶著問題绷旗，從源碼中尋找 Handler 的具體流程與實現(xiàn)。
問題
- 消息是如何發(fā)送出去的际乘？
- 消息時如何被得到的坡倔？
- 輪詢器的啟動在什么時候？
- 輪詢器與消息隊列的綁定是如何建立的脖含？
- 如何確弊锼總是能夠在對應的線程中獲取到正確的輪詢器實例？
- 對消息隊列的操作养葵，消費者和生產(chǎn)者征堪，主線程與子線程如何進行通信？
- Handler只能用于主線程UI更新关拒？
解析
首先從輪詢器的啟動開始佃蚜，所有的java程序都有一個main方法作為程序的入口庸娱，而Android中這個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");
}
}
在代碼中我們可以看到 Looper.prepareMainLooper()和Looper.loop() 兩個方法
先從Looper.prepareMainLooper()進行分析
private static Looper sMainLooper; // guarded by Looper.class 保存Looper類
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將全局靜態(tài)變量 sMainLooper 賦值和調(diào)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就是調(diào)用了sThreadLocal的get方法
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
在 prepare 中調(diào)用了全局靜態(tài)變量 sThreadLocal 的 set 方法
那么ThreadLocal是什么呢谐算？
ThreadLocal本質(zhì)是一個Map熟尉，不過其中的 key 值是線程 Thread ，它通過線程來存儲和讀取數(shù)據(jù)洲脂。正如其名斤儿，用來存儲線程本地數(shù)據(jù)【避免其他線程讀取或修改】。
可就是說在此處恐锦，sThreadLocal 中存儲了與主線程對應的 Looper 實例往果，只要是主線程中調(diào)用sThreadLocal 的get方法就能獲取這個輪詢器，若是其他子線程就獲取不到這個輪詢器
然后就到了講解 Looper.loop() 一铅，也就是在這里開始死循環(huán)陕贮，使得主線程一直存活
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);
}
}
...
/*
* 經(jīng)過上述步驟消息都未被處理，于是將其回收
*/
msg.recycleUnchecked();
}
}
在 loop 中我們主要看到 MessageQueue 潘飘、 msg.target.dispatchMessage(msg) 這三處地方
我們先看 msg.target.dispatchMessage(msg) 之后飘蚯，回來了解 MessageQueue
public final class Message implements Parcelable {
...
Handler target;
Runnable callback;
...
}
從 Message 中了解 target 就是 Handler，而 callback 是用戶設置的回調(diào)任務福也，只要不設置這個 callback 就會進入 handleMessage
public class Handler {
...
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//這里調(diào)用了用戶自定義的handleMessage去處理業(yè)務邏輯
handleMessage(msg);
}
}
...
}
這里我們看到了消息時如何被得回應的，那么我們只需要知道 Message 是怎么發(fā)送的和在什么時候給 target 賦值確保Handler對象不出錯攀圈。
Message 是如何發(fā)送的暴凑？這個問題想必都知道答案
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);
}
//最終進入該方法,此處就出現(xiàn)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;//在此處Handler給Message的target賦值
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
...
}
經(jīng)過一層一層的調(diào)用，最終Handler調(diào)用了 enqueueMessage 方法赘来，將 Message 放入它的全局變量MessageQueue中现喳，且將Message的target賦值為this【發(fā)送消息的Handler本身】
那么我們知道了發(fā)送的消息最終去向【MessageQueue】，那么Handler中的MessageQueue又是什么時候初始化的犬辰？
//我們平常使用的Handler無參構(gòu)造函數(shù)最終都會到這里
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());
}
}
//此處就是為什么子線程不能創(chuàng)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() 之前我們分析過 myLooper 就是調(diào)用了sThreadLocal的get方法嗦篱，此處只有主線程才有對應的 Looper 實例存在，這也就是為什么子線程中不能用無參構(gòu)造方法實例化Handler幌缝，如果創(chuàng)建會報下列錯誤提示
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
當然也不是沒有解決方案灸促，不是沒有 Looper 報錯嗎，那就給唄：选Ｔ≡浴！解決方案
繼續(xù)向下轿偎，我們看到 mQueue = mLooper.mQueue 典鸡，也就是說 MessageQueue 是從Looper中獲取的
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 對象在私有構(gòu)造函數(shù)中就已經(jīng)實例化了坏晦，那么還記得什么時候調(diào)用了Looper的構(gòu)造函數(shù)嗎萝玷？
繞了一圈終于到了講解 MessageQueue 了
public final class MessageQueue {
Message mMessages;//
...
boolean enqueueMessage(Message msg, long when) {
...
synchronized (this) {
if (mQuitting) {//是否退出嫁乘，只有調(diào)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;
}
}
此時你可能有些疑惑，不是隊列嗎球碉，那么為什么沒有數(shù)組或者List呢蜓斧？因為 MessageQueue 采用鏈表的方式實現(xiàn)隊列。
public final class Message implements Parcelable {
...
// sometimes we store linked lists of these things
/*package*/ Message next;
//最大Message池為50個
private static final int MAX_POOL_SIZE = 50;
...
}
我們將 if 語句分開看
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被賦值為全局變量 mMessages【我稱其為“即將發(fā)送Message”】如果
- “即將發(fā)送Message”是null
- msg 的 when 是0 【when = SystemClock.uptimeMillis() + delayMillis】
- msg 的 when 小于“即將發(fā)送Message”的 when 【 msg 發(fā)送事件的等待時間 小于 mMessages】
其中一個成立汁尺，將 msg 的 next 指向 p 法精，在將 mMessages 賦值為 msg【實際上就是將msg放在隊列最前面】
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 發(fā)送事件的等待時間 大于 mMessages痴突，于是就把 msg 放入鏈表中搂蜓，通過循環(huán)將 msg 放入鏈表的合適位置，確保隊列中的元素等待時間是遞增的
既然已經(jīng)講了存放辽装，那么就該到讀取了
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.
//基本不會進入這個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;
}
}
此處我們看到了MessageQueue調(diào)用了 native 方法【就是java調(diào)用了c方法】帮碰，其具體的實現(xiàn)在 /frameworks/base/core/jni/android_os_MessageQueue.cpp 點擊此處查看
//可以理解為阻塞，ptr相當于Message指針拾积，timeoutMillis阻塞時間
private native void nativePollOnce(long ptr, int timeoutMillis);
//喚醒殉挽，之前在enqueueMessage中有調(diào)用該方法喚醒
private native static void nativeWake(long ptr);
之后我們主要看 next 對 return 有關的部分
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
//msg 對應的 Handler 被銷毀，于是取出隊列中的下一個 Message
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//還沒有到 msg 的發(fā)送時間拓巧，需要阻塞等待
if (now < msg.when) {
// 下一條消息未準備好斯碌。設置一個超時時間，當它準備好時喚醒肛度。
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 到 msg 的發(fā)送時間
mBlocked = false;
//將 mMessages 變?yōu)?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將一直阻塞直到被喚醒
nextPollTimeoutMillis = -1;
}
到了這里Handler的源碼分析就結(jié)束了傻唾，可以再回去看看Handler的流程圖是不是會有新的感悟
