在android中Handler通信机制大家早就耳濡目染，而Looper在Handler中扮演着重要的角色。
先大致看下Looper的源码：（为什么要先大致看下源码？先看源码做到心中有数，对于这个类有哪些东西、使用了哪些东西，心里要有个大概，然后再结合常用的API功能去分析源码，这样会有一个深刻的认知）
public final class Looper {
private static final String TAG = "Looper";
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
private long mTraceTag;
private long mSlowDispatchThresholdMs;
private long mSlowDeliveryThresholdMs;
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));
}
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
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;
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
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 traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
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);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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.recycleUnchecked();
}
}
private static boolean showSlowLog(long threshold, long measureStart, long measureEnd,
String what, Message msg) {
final long actualTime = measureEnd - measureStart;
if (actualTime < threshold) {
return false;
}
// For slow delivery, the current message isn't really important, but log it anyway.
Slog.w(TAG, "Slow " + what + " took " + actualTime + "ms "
+ Thread.currentThread().getName() + " h="
+ msg.target.getClass().getName() + " c=" + msg.callback + " m=" + msg.what);
return true;
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
public void setMessageLogging(@Nullable Printer printer) {
mLogging = printer;
}
public void setTraceTag(long traceTag) {
mTraceTag = traceTag;
}
public void setSlowLogThresholdMs(long slowDispatchThresholdMs, long slowDeliveryThresholdMs) {
mSlowDispatchThresholdMs = slowDispatchThresholdMs;
mSlowDeliveryThresholdMs = slowDeliveryThresholdMs;
}
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
public @NonNull Thread getThread() {
return mThread;
}
public @NonNull MessageQueue getQueue() {
return mQueue;
}
public void dump(@NonNull Printer pw, @NonNull String prefix) {
pw.println(prefix + toString());
mQueue.dump(pw, prefix + " ", null);
}
public void dump(@NonNull Printer pw, @NonNull String prefix, Handler handler) {
pw.println(prefix + toString());
mQueue.dump(pw, prefix + " ", handler);
}
/** @hide */
public void writeToProto(ProtoOutputStream proto, long fieldId) {
final long looperToken = proto.start(fieldId);
proto.write(LooperProto.THREAD_NAME, mThread.getName());
proto.write(LooperProto.THREAD_ID, mThread.getId());
mQueue.writeToProto(proto, LooperProto.QUEUE);
proto.end(looperToken);
}
@Override
public String toString() {
return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
+ ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
}
}
可以看到Looper中使用了静态的对象sThreadLocal来存储Looper，了解ThreadLocal用法的都知道ThreadLocal是用来在线程中隔离存储数据的（对ThreadLocal不了解的人可以看我的这篇文章），那么在这里保存Looper对象的作用是为了隔离保存各线程中的Looper对象（对于同一线程的Looper，这里用ThreadLocal保存就相当于是单例：也就是同一个线程通过Looper.myLooper和Looper.getMainLooper方法拿到的Looper都是同一个Looper），使得不同线程间的Looper对象相互独立。
Looper是一个final类，不允许继承。
下边进入源码分析：
前边说了threadLocal用来存储Looper，那么就从threadLocla存储Looper切入：
// 对外开放的Api，
//也就是在子线程中使用handler跟其他线程通信的时候，需要给该线程实例化Looper的方法
public static void prepare() {
//这里的参数传递的true，也就是说子线程中实例化Looper的时候参数传递的是true
prepare(true);
}
// 私有的API
private static void prepare(boolean quitAllowed) {
//判断sThreadLocal中是否存在Looper，存在就抛出异常
//说明一个线程中只能实例化一个Looper对象
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//如果sThreadLocal中没有实例化当前线程的Looper，
//那么就实例化Looper并赋值给sThreadLocal缓存
sThreadLocal.set(new Looper(quitAllowed));
}
上边分析说到了子线程实例化Looper，下边看下主线程实例化Looper
// 开放API，主线程实例化Looper
public static void prepareMainLooper() {
//这里也调用了私有的prepare方法，但是主线程这传递的参数是false，
//而子线程传递的参数是true，从参数名称的意思上看表示主线程创建的Looper是不会退出的，
//子线程创建的Looper会退出（这样说不是特别准确，暂时可以这样理解，在Looper构造的时候会明白）
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//给sMainLooper变量赋值
sMainLooper = myLooper();
}
}
// 从sThreadLocal中获取Looper对象
public static @Nullable Looper myLooper() {
//再次强调ThreadLocal具有线程隔离的作用
return sThreadLocal.get();
}
// 获取主线程的Looper
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
看看Looper的构造方法
//获取Looper绑定的MessageQueue对象
public static @NonNull MessageQueue myQueue() {
//先拿到当前线程的Looper，再拿到Looper绑定的MessageQueue对象
return myLooper().mQueue;
}
//Looper的构造方法
private Looper(boolean quitAllowed) {
//实例化消息队列的对象，并赋值给Looper中的mQueue变量（可以理解为将MessageQueue对象绑定到当前Looper）
//参数quitAllowed表示实例化的消息队列是否可以退出
mQueue = new MessageQueue(quitAllowed);
//将当前Looper与当前的线程绑定
mThread = Thread.currentThread();
}
//这个方法的作用是：判断调用当前方法的线程与Looper绑定的线程是否是同一个线程
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
重点说一下Looper构造方法中的MessageQueue的实例化，参数为true表示消息队列可以退出，参数为false表示消息队列不能退出，这里立马能够想到Android主线程的Handler消息机制是一直存活的，这个参数就是一个体现，说明主线程的消息队列中的looper一直处于循环的存活状态。主线程的Looper实例化是在应用的入口函数ActivityThread，这里不做展开（可以查看我的文章主线程Handler消息机制：）
Looper作为消息队列的循环器，那么功能的核心肯定是在loop循环这个方法
// 循环从消息队列获取消息的方法
public static void loop() {
// 获取当前线程的Looper对象
final Looper me = myLooper();
if (me == null) {
// 如果当前线程没有实例化Looper抛出异常
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 取出当前线程的Looper中绑定的Queue对象
final MessageQueue queue = me.mQueue;
// 清空远程进程的PID和UIP，确保此线程的标识是本地进程的标识
//（涉及到Binder进程间通信机制，这里不做扩展，其他文章会讲解）
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
// 开启死循环不断从消息队列中试图取出消息
for (;;) {
// 从消息队列中取出消息，这个方法可能会阻塞（其他文章会讲解MessageQueue）
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
// 没有消息的时候退出消息队列循环，什么时候会走到这里，这个目前我还真不清楚
return;
}
//下边是线程调度跟踪的一些消息（不是关心的重点）
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
try {
//拿到msg绑定的target（handler）对象下发消息
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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.recycleUnchecked();
}
}
总结：
looper方法中开启一个死循环，从MessageQueue中不断的尝试取出消息msg，后然通过msg绑定的handler对象下发消息。消息队列中没有消息的时候，处于阻塞状态
这里会出现一个面试中经常会问到的一个假命题：主线程的MessageQueue发生阻塞的时候为什么不会阻塞主线程？
答案：Android是基于事件驱动的，事件就需要通过消息机制来传递。Android进程间的消息机制大多是基于Binder，而Android线程间的消息传递是基于Handler通信。Android应用本质是一段可执行代码，对于一个应用来说，肯定不允许执行完一段代码后就死亡，那么最简单的方式就是开启一段死循环保证程序不会退出。
真正会发生卡死主线程的操作是在回调方法onCreate()/onStart()/onResume()等操作时间过长，会导致掉帧，甚至发生ANR，looper.loop()方法本身是不回导致应用卡死的。
还有一个点：Looper.loop()方法一直运行那不是很耗费资源吗？
其实不然，这里就涉及到Linux pipe/epoll机制，简单说就是在主线程的MessageQueue没有消息时，便阻塞在loop的queue.next()中的nativePollOnce()方法里，详情见Android消息机制1-Handler(Java层)，此时主线程会释放CPU资源进入休眠状态，直到下个消息到达或者有事务发生，通过往pipe管道写端写入数据来唤醒主线程工作。这里采用的epoll机制，是一种IO多路复用机制，可以同时监控多个描述符，当某个描述符就绪(读或写就绪)，则立刻通知相应程序进行读或写操作，本质同步I/O，即读写是阻塞的。 所以说，主线程大多数时候都是处于休眠状态，并不会消耗大量CPU资源。
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