Handler的運行機(jī)制
Handler的作用：
當(dāng)我們需要在子線程處理耗時的操作（例如訪問網(wǎng)絡(luò)，數(shù)據(jù)庫的操作），而當(dāng)耗時的操作完成后，需要更新UI，這就需要使用Handler來處理，因為子線程不能做更新UI的操作。Handler能幫我們很容易的把任務(wù)（在子線程處理）切換回它所在的線程。簡單理解，Handler就是解決線程和線程之間的通信的。
Handler的使用
使用的handler的兩種形式：
1.在主線程使用handler；
2.在子線程使用handler。
在主線程使用handler的示例：
public class TestHandlerActivity extends AppCompatActivity {
private static final String TAG = "TestHandlerActivity";
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//獲得剛才發(fā)送的Message對象，然后在這里進(jìn)行UI操作
Log.e(TAG,"------------> msg.what = " + msg.what);
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler_test);
initData();
}
private void initData() {
//開啟一個線程模擬處理耗時的操作
new Thread(new Runnable() {
@Override
public void run() {
SystemClock.sleep(2000);
//通過Handler發(fā)送一個消息切換回主線程（mHandler所在的線程）
mHandler.sendEmptyMessage(0);
}
}).start();
}
在主線程使用handler很簡單，只需在主線程創(chuàng)建一個handler對象，在子線程通過在主線程創(chuàng)建的handler對象發(fā)送Message，在handleMessage（）方法中接受這個Message對象進(jìn)行處理。通過handler很容易的從子線程切換回主線程了。
那么來看看在子線程中使用是否也是如此。
public class TestHandlerActivity extends AppCompatActivity {
private static final String TAG = "TestHandlerActivity";
//主線程中的handler
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//獲得剛才發(fā)送的Message對象，然后在這里進(jìn)行UI操作
Log.e(TAG,"------------> msg.what = " + msg.what);
}
};
//子線程中的handler
private Handler mHandlerThread = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler_test);
initData();
}
private void initData() {
//開啟一個線程模擬處理耗時的操作
new Thread(new Runnable() {
@Override
public void run() {
SystemClock.sleep(2000);
//通過Handler發(fā)送一個消息切換回主線程（mHandler所在的線程）
mHandler.sendEmptyMessage(0);
//在子線程中創(chuàng)建Handler
mHandlerThread = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.e("sub thread","---------> msg.what = " + msg.what);
}
};
mHandlerThread.sendEmptyMessage(1);
}
}).start();
}
程序崩潰了。報的錯誤是沒有在子線程調(diào)用Looper.prepare()的方法。而為什么在主線程中使用不會報錯？通過源碼的分析可以解析這個問題。
在子線程中正確的使用Handler應(yīng)該是這樣的。
public class TestHandlerActivity extends AppCompatActivity {
private static final String TAG = "TestHandlerActivity";
//主線程的Handler
private Handler mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//獲得剛才發(fā)送的Message對象，然后在這里進(jìn)行UI操作
Log.e(TAG,"------------> msg.what = " + msg.what);
}
};
//子線程中的Handler
private Handler mHandlerThread = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handler_test);
initData();
}
private void initData() {
//開啟一個線程模擬處理耗時的操作
new Thread(new Runnable() {
@Override
public void run() {
SystemClock.sleep(2000);
//通過Handler發(fā)送一個消息切換回主線程（mHandler所在的線程）
mHandler.sendEmptyMessage(0);
//調(diào)用Looper.prepare（）方法
Looper.prepare();
mHandlerThread = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.e("sub thread","---------> msg.what = " + msg.what);
}
};
mHandlerThread.sendEmptyMessage(1);
//調(diào)用Looper.loop（）方法
Looper.loop();
}
}).start();
}
可以看到，通過調(diào)用Looper.prepare()運行正常，handleMessage方法中就可以接收到發(fā)送的Message。
至于為什么要調(diào)用這個方法呢？去看看源碼。
Handler的源碼分析
Handler的消息處理主要有五個部分組成，Message，Handler，Message Queue，Looper和ThreadLocal。首先簡要的了解這些對象的概念
Message：Message是在線程之間傳遞的消息，它可以在內(nèi)部攜帶少量的數(shù)據(jù)，用于線程之間交換數(shù)據(jù)。Message有四個常用的字段，what字段，arg1字段，arg2字段，obj字段。what，arg1，arg2可以攜帶整型數(shù)據(jù)，obj可以攜帶object對象。
Handler：它主要用于發(fā)送和處理消息的發(fā)送消息一般使用sendMessage（）方法,還有其他的一系列sendXXX的方法，但最終都是調(diào)用了sendMessageAtTime方法，除了sendMessageAtFrontOfQueue（）這個方法
而發(fā)出的消息經(jīng)過一系列的輾轉(zhuǎn)處理后，最終會傳遞到Handler的handleMessage方法中。
Message Queue：MessageQueue是消息隊列的意思，它主要用于存放所有通過Handler發(fā)送的消息，這部分的消息會一直存在于消息隊列中，等待被處理。每個線程中只會有一個MessageQueue對象。
Looper：每個線程通過Handler發(fā)送的消息都保存在，MessageQueue中，Looper通過調(diào)用loop（）的方法，就會進(jìn)入到一個無限循環(huán)當(dāng)中，然后每當(dāng)發(fā)現(xiàn)Message Queue中存在一條消息，就會將它取出，并傳遞到Handler的handleMessage（）方法中。每個線程中只會有一個Looper對象。
ThreadLocal：MessageQueue對象，和Looper對象在每個線程中都只會有一個對象，怎么能保證它只有一個對象，就通過ThreadLocal來保存。Thread Local是一個線程內(nèi)部的數(shù)據(jù)存儲類，通過它可以在指定線程中存儲數(shù)據(jù)，數(shù)據(jù)存儲以后，只有在指定線程中可以獲取到存儲到數(shù)據(jù)，對于其他線程來說則無法獲取到數(shù)據(jù)。
了解了這些基本概念后，我們深入源碼來了解Handler的工作機(jī)制。
MessageQueue的工作原理
MessageQueue消息隊列是通過一個單鏈表的數(shù)據(jù)結(jié)構(gòu)來維護(hù)消息列表的。下面主要看enqueueMessage方法和next（）方法。如下：
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;
}
可以看出，在這個方法里主要是根據(jù)時間的順序向單鏈表中插入一條消息。
next（）方法。如下
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;
}
}
在next方法是一個無限循環(huán)的方法，如果有消息返回這條消息并從鏈表中移除，而沒有消息則一直阻塞在這里。
Looper的工作原理
每個程序都有一個入口，而Android程序是基于java的，java的程序入口是靜態(tài)的main函數(shù)，因此Android程序的入口也應(yīng)該為靜態(tài)的main函數(shù)，在android程序中這個靜態(tài)的main在ActivityThread類中。我們來看一下這個main方法，如下：
public static void main(String[] args) {
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Security.addProvider(new AndroidKeyStoreProvider());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
//######
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在main方法中系統(tǒng)調(diào)用了 Looper.prepareMainLooper();來創(chuàng)建主線程的Looper以及MessageQueue，并通過Looper.loop（）來開啟主線程的消息循環(huán)。來看看Looper.prepareMainLooper()是怎么創(chuàng)建出這兩個對象的。如下：
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
可以看到，在這個方法中調(diào)用了 prepare(false);方法和 myLooper();方法，我在進(jìn)入這個兩個方法中，如下：
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));
}
在這里可以看出，sThreadLocal對象保存了一個Looper對象，首先判斷是否已經(jīng)存在Looper對象了，以防止被調(diào)用兩次。sThreadLocal對象是ThreadLocal類型，因此保證了每個線程中只有一個Looper對象。Looper對象是什么創(chuàng)建的，我們進(jìn)入看看，如下：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看出，這里在Looper構(gòu)造函數(shù)中創(chuàng)建出了一個MessageQueue對象和保存了當(dāng)前線程。從上面可以看出一個線程中只有一個Looper對象，而Message Queue對象是在Looper構(gòu)造函數(shù)創(chuàng)建出來的，因此每一個線程也只會有一個MessageQueue對象。
對prepare方法還有一個重載的方法：如下
public static void prepare() {
prepare(true);
}
prepare()僅僅是對prepare(boolean quitAllowed) 的封裝而已，在這里就很好解釋了在主線程為什么不用調(diào)用Looper.prepare（）方法了。因為在主線程啟動的時候系統(tǒng)已經(jīng)幫我們自動調(diào)用了Looper.prepare()方法。
在Looper.prepareMainLooper（）方法中還調(diào)用了一個方法myLooper()，我們進(jìn)去看看，如下：
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
在調(diào)用prepare（）方法中在當(dāng)前線程保存一個Looper對象sThreadLocal.set(new Looper(quitAllowed));my Looper（）方法就是取出當(dāng)前線程的Looper對象，保存在sMainLooper引用中。
在main（）方法中還調(diào)用了Looper.loop()方法，如下：
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
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.recycle();
}
}
在這個方法里，進(jìn)入一個無限循環(huán)，不斷的從MessageQueue的next方法獲取消息，而next方法是一個阻塞操作，當(dāng)沒有消息的時候一直在阻塞，當(dāng)有消息通過 msg.target.dispatchMessage(msg);這里的msg.target其實就是發(fā)送給這條消息的Handler對象。
Handler的運行機(jī)制
看看Handler的構(gòu)造方法。如下：
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
我們?nèi)タ纯礇]有Looper 對象的構(gòu)造方法：
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;
}
可以看到，到looper對象為null，拋出 "Can't create handler inside thread that has not called Looper.prepare()"異常由這里可以知道，當(dāng)我們在子線程使用Handler的時候要手動調(diào)用Looper.prepare()創(chuàng)建一個Looper對象，之所以主線程不用，是系統(tǒng)啟動的時候幫我們自動調(diào)用了Looper.prepare()方法。
handler的工作主要包含發(fā)送和接收過程。消息的發(fā)送主要通過post和send的一系列方法，而post的一系列方法是最終是通過send的一系列方法來實現(xiàn)的。而send的一系列方法最終是通過sendMessageAtTime方法來實現(xiàn)的，除了sendMessageAtFrontOfQueue（）這個方法。去看看這些一系列send的方法，如下：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
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);
}
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以看出，handler發(fā)送一條消息其實就是在消息隊列插入一條消息。在Looper的loop方法中，從Message Queue中取出消息調(diào)msg.target.dispatchMessage(msg);這里其實就是調(diào)用了Handler的dispatchMessage(msg)方法，進(jìn)去看看，如下：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
判斷msg.callback是否為空，不為空調(diào)用 handleCallback(msg);來處理消息。其實callback是一個Runnable對象，就是Handler發(fā)送post消息傳過來的對象。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
public final boolean postAtTime(Runnable r, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
進(jìn)去handleCallback方法看看怎么處理消息的，如下：
private static void handleCallback(Message message) {
message.callback.run();
}
可以看出，其實就是回調(diào)Runnable對象的run方法。Activity的runOnUiThread，View的postDelayed方法也是同樣的原理，我們先看看runOnUiThread方法，如下：
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
View的postDelayed方法。如下：
public boolean postDelayed(Runnable action, long delayMillis) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.postDelayed(action, delayMillis);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);
return true;
}
實質(zhì)上都是在UI線程中執(zhí)行了Runnable的run方法。
如果msg.callback是否為null，判斷mCallback是否為null？mCallback是一個接口，如下：
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
CallBack其實提供了另一種使用Handler的方式，可以派生子類重寫handleMessage（）方法，也可以通過設(shè)置CallBack來實現(xiàn)。
我們梳理一下我們在主線程使用Handler的過程。
首先在主線程創(chuàng)建一個Handler對象 ，并重寫handleMessage（）方法。然后當(dāng)在子線程中需要進(jìn)行更新UI的操作，我們就創(chuàng)建一個Message對象，并通過handler發(fā)送這條消息出去。之后這條消息被加入到MessageQueue隊列中等待被處理，通過Looper對象會一直嘗試從Message Queue中取出待處理的消息，最后分發(fā)會Handler的handler Message（）方法中。
END.
