概述
- Handler是Android比较基础，也是非常重要的一个组成部分；整个App运行就是基于消息驱动运行起来的，也可以理解为消费者-生产者模式；
- 原理及实现是相对比较简单的，主要涉及四个类：Message，Handler，MessageQueue，Looper；Message是消息的载体；Handler是消息的发送者及处理者；MessageQueue是消息队列，核心操作是入列和出列；Looper从MessageQueue获取Message并进行分发；
- 消息的执行是异步的；Message分为同步Message和异步Message，可以向MessageQueue中发送同步栅栏，阻止同步Message执行，但是异步Message不受影响；
- Handler可以用于异步/延迟/定时执行，也可用于线程间通信；
- 源码基于Android-SDK-29；
源码
Message
- Message是消息的载体，生命周期从发送（获取Message对象）到处理（Handler处理结束）；
- Message是单向链式结构；
- Message
public final class Message implements Parcelable {
public int what;
Handler target;
public long when;
Runnable callback;
public int arg1;
public int arg2;
public Object obj;
Bundle data;
}
- what：用来标识什么消息，作用域为对应的Handler，只要Handler能区分所有的what即可；
- target：Message的发送者和处理者；
- when：Message执行的时间，自Android系统启动以来的时间（不包括系统睡眠时间）为起点；Message不一定正好在when时间执行（可能前面积压了很多Message），但是肯定不会早于when时间；
- callback：callback也是Message的处理者；
- arg1/arg2/obj：可携带的数据；是data的另一种替换方案，不涉及内存分配；
- data：复杂数据或者跨进程携带数据；
- 对象池
@UnsupportedAppUsage
/*package*/ Message next; //链式结构
/** @hide */
public static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
- Message是生命周期很短（从发送到处理完毕）的对象，并且很多地方都会用到；大量创建生命周期短的对象这种场景下，创建对象本身需要比较耗时，也可能触发GC，导致系统卡顿不稳定；所以需要对象池；
- Message是单向链式结构，对象池只要持有一个Message对象即可；
- 获取Message对象尽量通过Message.obtain/Handler.obtain方法；源码中会对sPool加锁，防止并发调用；
- 跨进程
/**
* Optional Messenger where replies to this message can be sent. The
* semantics of exactly how this is used are up to the sender and
* receiver.
*/
public Messenger replyTo;
/**
* Indicates that the uid is not set;
*
* @hide Only for use within the system server.
*/
public static final int UID_NONE = -1;
/**
* Optional field indicating the uid that sent the message. This is
* only valid for messages posted by a {@link Messenger}; otherwise,
* it will be -1.
*/
public int sendingUid = UID_NONE;
/**
* Optional field indicating the uid that caused this message to be enqueued.
*
* @hide Only for use within the system server.
*/
public int workSourceUid = UID_NONE;
Handler
- Handler是Message的发送者和处理者；
- 每个Handler对象都绑定到Looper对象，Handler发送Message最终发送到绑定的Looper对象的MessageQueue中；如果创建Handler时未指定了Looper对象，那么就绑定到创建Handler对象时所处的线程的Looper对象；
- 创建Handler对象
public Handler(@Nullable Callback callback, boolean async) {
//提醒可能内存泄漏
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());
}
}
//获取当前线程的Looper
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;
}
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
- 如果构造函数没有指定Looper，那么获取当前线程的Looper对象；构造函数就是为了生成Looper，MessageQueue，Callback（Handler的Callback，不是Message的Callback，类似于Handler.handleMessage方法），Asynchronous（为true时，该Handler发送的都是异步Message）；
- 发送消息
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
- 发送Message和Runnable，最终都会调用sendMessageAtTime；Runnable会被封装为Message对象；sendMessage，sendMessageDelayed通过时间转化最终都会转化为sendMessageAtTime；
- Message入列是直接调用MessageQueue.enqueueMessage方法，如果Handler.mAsynchronous为true，则该Handler所有的Message都是异步Message；
public final boolean runWithScissors(@NonNull Runnable r, long timeout) {
if (Looper.myLooper() == mLooper) {
调用线程和执行线程是同一个线程时，直接run
r.run();
return true;
}
BlockingRunnable br = new BlockingRunnable(r);
return br.postAndWait(this, timeout);
}
- 阻塞发送消息：如果Handler绑定的Looper和调用者所在的线程Looper是同一个，直接调用Runnable.run；否则调用者所在的线程阻塞，直到执行结束；
- 发送立即处理消息：postAtFrontOfQueue/sendMessageAtFrontOfQueue方法，设置Message.when为0，Message会排在MessageQueue的最前面，Looper取出的下一个Message就是该Message，所以是立即处理消息；
- 删除消息
- Handler的删除消息都会调用MessageQueue对应的方法；
- 处理消息
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg); // Runnable转化的Message
} else {
if (mCallback != null) {
//Handler.Callback优先处理Message，如果返回true，表示处理结束
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public void handleMessage(@NonNull Message msg) {
}
- 如果Message的callback不为null（Runnable转化的Message），直接调用callback处理；否则，优先Handler.Callback处理，如果返回true，表示处理结束；否则，handleMessage处理；
Looper
- Looper从MessageQueue获取Message并进行分发；主要功能就是消息循环以及监控；
- Looper是保存在ThreadLocal中，MessageQueue是Looper的实例变量，所以MessageQueue对象也是线程内唯一；
- 线程本地存储
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
final MessageQueue mQueue;
final Thread mThread;
- Looper保存在ThreadLocal中，确保线程内单例；MessageQueue是Looper的成员变量，外部不可创建，所以MessageQueue也是线程内单例；
- 初始化
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
- Looper构造函数中，初始化了MessageQueue，并将Looper保存在ThreadLocal中；
- 初始化必须在对应的线程中调用；
- 默认是可退出的（Looper/MessageQueue）；
- 循环
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
// 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;
//死循环
for (;;) {
Message msg = queue.next(); // 可阻塞
if (msg == null) { //MessageQueue已退出
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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
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;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
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);
}
msg.recycleUnchecked(); //Message回收
}
}
- 主要代码就是从MessageQueue中取出要执行的下一个Message（可能阻塞），然后调用Handler.dispatchMessage进行分发；最后回收Message对象；
- 监控
- 在loop方法中，分发Message前后，根根据设置的阈值/Printer/全局Observer，执行对应的Log以及回调；
- 主要用于系统监控，业务层只能设置Printer，但是返回的只有一个拼接好的String，用法有限；
MessageQueue
- MessageQueue是消息队列，负责Message入列，出列，空闲时间IdleHandler的处理，以及线程的挂起；
- 初始化
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
private native static long nativeInit();
- 初始化Looper对象时，也会初始化MessageQueue，进而在Native层也初始化一个NativeMessageQueue，并在Java层保存起来（mPtr）；
- Native层也有消息队列，和Java层类似，MessageQueue是Native和Java层消息队列的纽带；
- 入列
boolean enqueueMessage(Message msg, long when) {
synchronized (this) { //加锁
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) {
// 插入到头部
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.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//根据when插入到合适位置
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) { //唤醒
nativeWake(mPtr);
}
}
return true;
}
- 可能并发，所以在MessageQueue对象上加锁；
- 根据Message.when插入到合适位置；
- 如果需要唤醒，则从Native唤醒；
- 出列
Message next() {
final long ptr = mPtr;
if (ptr == 0) { //消息队列已经退出
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) { //加锁
// 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) {
//同步栅栏，获取下一个异步Message
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
//头部Message还未到执行时间
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
//成功获取到要分发的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 (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;
}
}
- 整体是个死循环，循环获取Message（Java层）；
- 调用nativePollOnce，每个迭代优先处理Native层，Native消息处理完，判断nextPollTimeoutMillis，如果等于0直接返回，如果等于-1进入休眠，如果大于0进入超时休眠；
- 处理Java层，如果头部Message执行时间已到（包括同步栅栏逻辑）则直接返回Message，否则处理IdleHandler，进入下个迭代；
- 删除
- IdleHandler
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
public void removeIdleHandler(@NonNull IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
public boolean isIdle() {
synchronized (this) {
final long now = SystemClock.uptimeMillis();
return mMessages == null || now < mMessages.when;
}
}
- IdleHandler用于在消息队列空闲的时候，执行任务；执行逻辑在next方法中
- IdleHandler.queueIdle：返回true表示保留任务，返回false表示删除任务只执行一次；
- 同步栅栏
private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
//同步栅栏Message（target = null）
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;
//插入同步栅栏Message
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
//用于删除同步栅栏
return token;
}
}
public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
//定义同步栅栏Message
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
//删除同步栅栏Message
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
//唤醒
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
- 同步栅栏Message，用于拖延同步Message；代码逻辑在next方法中；
- 添加同步栅栏，将同步栅栏Message插入到合适的位置，该位置之后的Message都被拖延，只有异步消息可以执行，直到该同步栅栏消息被移除；
- 删除同步栅栏，根据token删除对应的同步栅栏，如果需要，唤醒线程；
扩展
IdleHandler
- IdleHandler用于在线程空闲时（没有Message要执行时）执行任务；
- IdleHandler的应用场景
- UI更新后，获取对应的属性，通过IdleHandler在空闲时执行，空闲意味着表示UI测量/布局/绘制已经结束；
- Activity中初始化以及加载数据可以在IdleHandler中执行；
- 用于App启动时间优化；
- 预加载/预处理；
HandlerThread
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
- HandlerThread继承于Thread，但是多了消息队列；
- 在 run 方法中调用 Looper.prepare() 初始化消息队列，然后进入循环队列；
Messager
ANR
总结
- Message是消息的载体（包括Runnable）；Handler是消息的发送者（MessageQueue入列）和处理者；Looper负责循环分发消息（MessageQueue出列）；MessageQueue负责消息的入列，出列，同步栅栏的处理，IdleHandler的处理；
- MessageQueue是Java层和Native的纽带；Java层和Native层都有消息队列，优先处理Native层消息，再处理Java层消息，如果Java层没有可执行Message，则Native层进入休眠（超时休眠或者一直休眠）；入列或者删除栅栏可唤醒休眠；
存疑
- Messager跨进程通信；
- Native与Java层的消息队列的交互；
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