參考文章:
http://blog.csdn.net/bboyfeiyu/article/details/38555547
http://www.jianshu.com/p/6f25729ef62a
在Android應用啟動時,會預設有一個主線程(UI線程),在這個線程中會關聯一個消息隊列,所有的操作都會被封裝成消息然後交給主線程來處理。為了保證主線程不會主動退出,會将取消息的操作放在一個死循環中,這樣程式就相當于一直在執行死循環,是以不會退出。
示例圖如下 :
Android應用程式的入口為ActivityThread.main方法:
public static void main(String[] args) {
SamplingProfilerIntegration.start();
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();// 1、建立消息循環Looper
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler(); // UI線程的Handler
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop(); // 2、執行消息循環
throw new RuntimeException("Main thread loop unexpectedly exited");
}
執行ActivityThread.main方法後,應用程式就啟動了,并且會一直從消息隊列中取消息,然後處理消息。那麼系統是如何将消息投遞到消息隊列中的?又是如何從消息隊列中擷取消息并且處理消息的呢? 答案就是Handler。
每個Handler都會關聯一個消息隊列,消息隊列被封裝在Lopper中,而每個Looper又會關聯一個線程(ThreadLocal),也就是每個消息隊列會關聯一個線程。Handler就是一個消息處理器,将消息投遞給消息隊列,然後再由對應的線程從消息隊列中挨個取出消息,并且執行。預設情況下,消息隊列隻有一個,即主線程的消息隊列,這個消息隊列是在ActivityThread.main方法中建立的,通過Lopper.prepareMainLooper()來建立,然後最後執行Looper.loop()來啟動消息循環。那麼Handler是如何關聯消息隊列以及線程的呢?我們看看如下源碼 :
public Handler() {
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(); // 擷取Looper
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; // 擷取消息隊列
mCallback = null;
}
從Handler預設的構造函數中我們可以看到,Handler會在内部通過Looper.getLooper()來擷取Looper對象,并且與之關聯,最重要的就是消息隊列。那麼Looper.getLooper()又是如何工作的呢?我們繼續往下看.
/**
* 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();
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
myLooper().mQueue.mQuitAllowed = false;
}
private synchronized static void setMainLooper(Looper looper) {
mMainLooper = looper;
}
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
Looper與MessageQueue
建立了Looper後,如何執行消息循環呢?通過Handler來post消息給消息隊列( 連結清單 ),那麼消息是如何被處理的呢?答案就是在消息循環中,消息循環的建立就是通過Looper.loop()方法。源碼如下 :
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
MessageQueue queue = me.mQueue;// 1、擷取消息隊列
// 代碼省略
while (true) { // 2、死循環,即消息循環
Message msg = queue.next(); // 3、擷取消息 (might block )
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}
long wallStart = 0;
long threadStart = 0;
// 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);
wallStart = SystemClock.currentTimeMicro();
threadStart = SystemClock.currentThreadTimeMicro();
}
msg.target.dispatchMessage(msg);// 4、處理消息
// 代碼省略
msg.recycle();
}
}
}
可以看到,loop方法中實質上就是建立一個死循環,然後通過從消息隊列中挨個取出消息,最後處理消息的過程。對于Looper我們總結一下 : 通過Looper.prepare()來建立Looper對象(消息隊列封裝在Looper對象中),并且儲存在sThreadLoal中,然後通過Looper.loop()來執行消息循環,這兩步通常是成對出現的!! 最後我們看看消息處理機制,我們看到代碼中第4步通過msg.target.dispatchMessage(msg)來處理消息。其中msg是Message類型,我們看源碼 :
public final class Message implements Parcelable {
public int what;
public int arg1;
public int arg2;
public Object obj;
int flags;
long when;
Bundle data;
Handler target; // target處理
Runnable callback; // Runnable類型的callback
// sometimes we store linked lists of these things
Message next; // 下一條消息,消息隊列是鍊式存儲的
// 代碼省略 ....
}
從源碼中可以看到,target是Handler類型。實際上就是轉了一圈,通過Handler将消息投遞給消息隊列,消息隊列又将消息分發給Handler來處理。我們繼續看:
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
private final void handleCallback(Message message) {
message.callback.run();
}
/**
* 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);
}
}
可以看到,dispatchMessage隻是一個分發的方法,如果Runnable類型的callback為空則執行handlerMessage來處理消息,該方法為空,我們會将更新UI的代碼寫在該函數中;如果callback不為空,則執行handleCallback來處理,該方法會調用callback的run方法。其實這是Handler分發的兩種類型,比如我們post(Runnable callback)則callback就不為空,當我們使用Handler來sendMessage時通常不會設定callback,是以也就執行handlerMessage這個分支。我們看看兩種實作:
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private final Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
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)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this; // 設定消息的target為目前Handler對象
sent = queue.enqueueMessage(msg, uptimeMillis); // 将消息插入到消息隊列
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
可以看到,在post(Runnable r)時,會将Runnable包裝成Message對象,并且将Runnable對象設定給Message對象的callback字段,最後會将該Message對象插入消息隊列。sendMessage也是類似實作 :
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
子線程中建立Handler為何會抛出異常 ?
“Can’t create handler inside thread that has not called Looper.prepare()”異常
解決:
new Thread(){
Handler handler = null;
public void run() {
Looper.prepare(); // 1、建立Looper,并且會綁定到ThreadLocal中
handler = new Handler();
Looper.loop(); // 2、啟動消息循環
};
}.start();
在代碼中我們加了2處,第一是通過Looper.prepare()來建立Looper,第二是通過Looper.loop()來啟動消息循環。這樣該線程就有了自己的Looper,也就是有了自己的消息隊列。如果之建立Looper,而不啟動消息循環,雖然不會抛出異常,但是你通過handler來post或者sendMessage也不會有效,因為雖然消息被追加到消息隊列了,但是并沒有啟動消息循環,也就不會從消息隊列中擷取消息并且執行了!
總結
在應用啟動時,會開啟一個主線程(UI線程),并且啟動消息循環,應用不停地從該消息隊列中取出、處理消息達到程式運作的效果。Looper對象封裝了消息隊列,Looper對象是ThreadLocal的,不同線程之間的Looper對象不能共享與通路。而Handler通過與Looper對象綁定來實作與執行線程的綁定,handler會把Runnable(包裝成Message)或者Message對象追加到與線程關聯的消息隊列中,然後在消息循環中挨個取出消息,并且處理消息。當Handler綁定的Looper是主線程的Looper,則該Handler可以在handleMessage中更新UI,否則更新UI則會抛出異常!其實我們可以把Handler、Looper、Thread想象成一個生産線,勞工(搬運工)相當于Handler,負責将貨物搬到傳輸帶上(Handler将消息傳遞給消息隊列);傳送帶扮演消息隊列的角色,負責傳遞貨物,貨物會被挨取出,并且輸送到目的地 ( target來處理 );而貨物到達某個工廠中的房間後再被勞工處理,工廠中的房間就扮演了Thread這個角色,每個工廠中的房間有自己獨立的傳送帶,工廠中的房間A的貨物不能被工廠中的房間B的拿到,即相當于ThreadLocal( 工廠中的房間獨有 )。