关于这几个之间的关系以及源码分析的文章应该挺多的了，不过既然学习了，还是觉得整理下，印象更深刻点，嗯，如果有错误的地方欢迎反馈。
转载请注明出处：http://www.cnblogs.com/John-Chen/p/4396268.html
对应关系：
1、Handler
不带Looper的构造器
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
* * Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
* * Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
* * @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*/ 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(); //myLooper() return sThreadLocal.get(), mLooper每个线程独有，腾讯有次面试问到了 if (mLooper == null) {
//Handler创建时必须有Looper对象
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
} mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
“In Android, Handler classes should be static or leaks might occur.”：
public class MainActivity extends ActionBarActivity {
//warn:In Android, Handler classes should be static or leaks might occur.
private final Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
}
原因：
no-static的内部类handler会隐式的持有当前类MainActivity的一个引用，Handler 的 post/sendMessage 系列方法最后都是通过转调 MessageQueue 的 enqueueMessage 来实现的,每个Message都持有一个Handler的引用（Message.target）,所以最终会有一条MessageQueue -> Message -> Handler -> Activity的链，导致你的Activity被引用而无法被回收。
解决办法：
1、在关闭Activity的时候停掉与Handler有关的后台线程；
2、如果你的Handler是被delay的Message持有了引用，那么使用相应的Handler的removeCallbacks()方法，把消息对象从消息队列移除就行了；
3、将Handler声明为静态类：
static class MyHandler extends Handler {
WeakReference<Activity> mActivityReference;
MyHandler(Activity activity) {
mActivityReference= new WeakReference<Activity>(activity);
}
@Override
public void handleMessage(Message msg) {
final Activity activity = mActivityReference.get();
if (activity != null) {
//......
}
}
};
2、Looper
一个典型的Loop Thread实现：
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
变量：
//sThreadLocal变量保证looper对象每个线程独享，prepare()中set值 // sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); //通过Loop.class管理
private static Looper sMainLooper; // guarded by Looper.class //对应1个消息队列
final MessageQueue mQueue; //对应1个线程
final Thread mThread;
Looper构造器：
//Main thread not allowed to quit
private Looper(boolean quitAllowed) {
//创建一个消息队列
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper方法：
/** 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(){
prepare(true);
} private static void prepare(boolean quitAllowed){
if (sThreadLocal.get() != null) {
//每个线程只能有1个Looper对象
throw new RuntimeException("Only one Looper may be created per thread");
} //线程内创建一个Looper对象
sThreadLocal.set(new Looper(quitAllowed));
} /**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/ public static void loop() {
final Looper me = myLooper(); if (me == null) {
//必须先调用Looper.prepare()
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 (;;) {
//如果没有消息会阻塞
Message msg = queue.next(); // might block if (msg == null) {
// No message indicates that the message queue is quitting.
//没有消息表示消息队列已终止，即已调用mQueue.quit(true)，消息循环终止
return;
} ...... msg.target.dispatchMessage(msg); //通过handler分发消息 ...... msg.recycleUnchecked(); //Recycles a Message that may be in-use
}
}
MainLooper：
/**
* 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(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
} /**
Returns the application's main looper, which lives in the main thread of the application.
*/ public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
ActivityThread.main:
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>");
//主线程中，系统已经帮我们自动调用了Looper.prepare()方法，所以在主线程中可以直接创建Handler对象
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
3、消息队列MessageQueue
底层实现：
android/frameworks/base/core/jni/android_os_MessageQueue.h
android/frameworks/base/core/jni/android_os_MessageQueue.cpp
epoll模型找到几篇不错的文章：
http://blog.chinaunix.net/uid-7944836-id-2938541.html
http://blog.****.net/ashqal/article/details/31772697
http://book.2cto.com/201208/1946.html
Message是链表结构：
public final class Message implements Parcelable {
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
public int sendingUid = -1;
/*package*/ static final int FLAG_IN_USE = 1 << 0;
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
/*package*/ Message next;
......
}
MessageQueue变量：
// True if the message queue can be quit.
//与 Looper.prepare(boolean quitAllowed) 中参数含义一致，是否允许中止，主线程的消息队列是不允许中止的
private final boolean mQuitAllowed; //MessageQueue 是通过调用 C++ native MessageQueue 实现的，mPtr是指向native MessageQueue的指针
private long mPtr; // used by native code //消息链表
Message mMessages; //表示当前队列是否处于正在退出状态
private boolean mQuitting; // Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
//表示next()调用是否被block在timeout不为0的pollOnce上
private boolean mBlocked;
MessageQueue构造函数：
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
native函数：
//创建 NativeMessageQueue 对象，并将这个对象的指针复制给 Android MessageQueue 的 mPtr
private native static long nativeInit(); //通过等待被激活，然后从消息队列中获取消息
private native static void nativePollOnce(long ptr, int timeoutMillis); //激活处于等待状态的消息队列，通知它有消息到达了
private native static void nativeWake(long ptr); //消息队列是否是空置状态
private native static boolean nativeIsIdling(long ptr); //销毁消息队列
private native static void nativeDestroy(long ptr);
//添加消息
boolean enqueueMessage(Message msg, long when) {
//检测消息的合法性，必须有Handler对象以及未处理
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) {
//退出状态，状态异常
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
//按照时间从小到大，when == 0插入到头部
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
//消息添加到链表的头部
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.
//当前消息队列已经处于 Blocked 状态，且队首是一个消息屏障(和内存屏障的理念一样，
//这里是通过 p.target == null 来判断队首是否是消息屏障)，并且要插入的消息是所有异步消息中最早要处理的
//才会 needwake 激活消息队列去获取下一个消息
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//根据时间插入到链表的合适位置
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;
}
Handler 的 post/sendMessage 系列方法最后都是通过转调 MessageQueue 的 enqueueMessage 来实现的:
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);
}
Handler 中与Message 相关的静态方法也都是通过 MessageQueue 的对应的静态方法实现的，比如 removeMessages, hasMessages, hasCallbacks 等等。
//取消息
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) {
//已经dispose()
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}
}
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);
}
}
IdleHandler：
IdleHandler接口表示当MessageQueue发现当前没有更多消息可以处理的时候则顺便干点别的事情的callback函数（即如果发现idle了，
那就找点别的事干）。
callback函数有个boolean的返回值，表示是否keep。如果返回false，则它会在调用完毕之后从mIdleHandlers
中移除。
ActivityThread.java里的一个内部类，代码如下：
final class GcIdler implements MessageQueue.IdleHandler {
@Override
public final boolean queueIdle() {
doGcIfNeeded();
return false;
}
}
这是一个gc相关的IdleHandler，即如果没有更多的消息可以处理就会抽空doGcIfNeeded()，最后返回false表示不保留在mIdleHandlers
中，即用一次就扔了，只执行一遍。
参考文章：