目录
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学习资料：
Android多线程：手把手带你深入Handler源码分析（上）
Android多线程：手把手带你深入Handler源码分析（下）
大纲
概念
Handler，英[ˈhændlə(r)] 美[ˈhændlər] ，译为处理器；句柄；处理程序；处理者；处理
概述
很多时候，耗时任务都是需要放到次线程去执行的，次线程任务执行完毕之后，主线程需要及时的进行一个UI的更新与反馈以告知用户，那么如何能够及时？主线程何时才知道次线程完成了任务？——Handler，就是主次线程沟通的一个桥梁。
详细描述
查阅官方对于Handler的描述可知，Handler 继承于Object，它允许我们发送和处理与线程MessageQueue关联的Message与Runnable对象。每个Handler都与一个线程以及该线程的消息队列关联。当我们创建一个新的Handler时，它被绑定到正在创建它的线程/该线程的消息队列——从那时起，它将消息和runnables传递给该消息队列并在消息从消息队列中取出时，执行它们。
Handler有两个主要用途：
（1）安排messages和runnables在将来的某个时刻被执行；
（2）将在不同的线程上执行的操作排入队列。
整个Handler的消息调度离不开以下几个方法：
（1）post(Runnable)
（2）postAtTime(java.lang.Runnable, long)
（3）postDelayed(Runnable, Object, long)
（4）sendEmptyMessage(int)
（5）sendMessage(Message)
（6）sendMessageAtTime(Message, long)
（7）sendMessageDelayed(Message, long)
post(Runnable)，将Runnable对象添加到消息队列message queue中，该Runnable对象将在该Handler所连接的线程上运行；sendMessage(Message)，它将在消息队列message queue的末尾添加一条消息，而这条消息，它是在当前时间之前的所有挂起消息之后。它将以handleMessage(Message)的形式在附加到此Handler的线程中接收。
当向Handler发布或发送消息时，可以允许在message queue准备好处理该项时立即处理该项，或者指定处理该项之前的延迟或处理该项的绝对时间。后两者允许实现超时、标记和其他基于时间的行为。
为应用程序创建进程时，其主线程专用于运行消息队列message queue，该队列负责管理*应用程序对象（活动，广播接收器等）及其创建的任何窗口。我们可以创建自己的线程，并通过Handler与主应用程序线程进行通信。这是通过调用post或sendMessage方法完成的，然后，将在Handler的消息队列message queue中调度给定的Runnable或Message，并在适当时进行处理。
执行流程
Handler执行流程如图所示：
4个关键词：
//1，Looper：循环器，用于管理MessageQueue，不断地循环消息，不断地从中取出Message分发给对应的Handler处理，每个线程只能够有一个Looper。
//2，Message：消息对象。
//3，MessageQueue：存放消息对象的消息队列，队列它是一种先进先出的数据结构。
//4，Handler：处理消息对象的处理器。
//注意：系统在创建主线程的时候，会初始化一个Looper对象，同时也会把它关联的MessageQueue创建好，所以我们的Handler在主线程中实例化时，不需要额外的对Looper进行操作（就可以进行信息的发送与处理了）。
初步使用
在主线程中使用
sendMessage(Message)
使用sendMessage(Message)，有如下几个步骤：
//1：在主线程创建一个Handler静态内部类
//2：在主线程实例化第1步中创建的静态内部类
//3：创建一个子线程
//4：在子线程中，通过Handler对象调用sendMessage(Message)方法，将消息发送到消息队列中
//请看代码：
public class HandlerSendMessageTest extends AppCompatActivity {
TextView test;
Handler myHandler;
//1:在主线程创建一个Handler静态内部类
// 静态内部类不会持有外部类的引用 再结合WeakReference使用 可以预防内存泄漏
private static class MyHandler extends Handler {
WeakReference<MainActivity> weakReferenceAct;
public MyHandler(MainActivity mainActivity) {
weakReferenceAct = new WeakReference<MainActivity>(mainActivity);
}
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what) {
case 1:
weakReferenceAct.get().test.setText("myHandler.sendMessage(msg)");
break;
default:
break;
}
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
test = findViewById(R.id.test);
//2:实例化内部类
myHandler = new MyHandler(this);
//3：举个栗子 创建一个线程
new Thread() {
@Override
public void run() {
super.run();
SystemClock.sleep(2000);
Message msg = Message.obtain();
msg.what = 1;
msg.obj = "nhan";
//4：在子线程中，通过Handler对象调用sendMessage(Message)方法，将消息发送到消息队列中
myHandler.sendMessage(msg);
}
}.start();
}
}
post(Runnable)
//1：在主线程中实例化一个Handler，得到一个Handler的对象
//2：创建一个子线程
//3：在子线程中，直接调用Handler对象.post(Runnable)方法传入一个Runnable，而后进行相关的UI操作
//请看代码：
public class HandlerPostMessageTest extends AppCompatActivity {
TextView test;
Handler handler;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
test = findViewById(R.id.test);
//1:在主线程中实例化一个Handler，得到一个Handler的对象
handler = new Handler();
//2:举个栗子 创建一个子线程
new Thread() {
@Override
public void run() {
super.run();
//3：在子线程中，直接调用Handler对象.post(Runnable)方法传入一个Runnable，而后进行相关的UI操作
handler.post(new Runnable() {
@Override
public void run() {
test.setText("handler.post(Runnable)");
}
});
}
}.start();
}
}
在次线程中使用
public class LooperPrepare extends AppCompatActivity {
Handler handler;
TextView test;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
test = findViewById(R.id.test);
//1：创建一个子线程
new Thread() {
@Override
public void run() {
super.run();
//2：在子线程中 准备Looper对象
Looper.prepare();
//3：实例化Handler 并重写handleMessage（）方法处理消息
handler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what) {
case 1:
test.setText("在子线程中使用Handler");
break;
default:
break;
}
}
};
//4：调用Looper.loop() 运行消息队列
Looper.loop();
}
}.start();
}
@Override
protected void onResume() {
super.onResume();
Message msg = Message.obtain();
msg.what = 1;
msg.obj = "nhan";
handler.sendMessage(msg);
}
}
我的Demo
先欠着吧？回头一定补上 .. - - ，
源码原理
Handler消息机制能够正常运作，如前面的执行流程图所示，它跟Looper、MessageQueue之间是密不可分的，所以对于Handler消息机制源码的分析，以Handler在次线程中的使用为栗子，从头到尾进行分析，那么就慢慢来吧 ——
在子线程中使用Handler，我们首先要能够获取到一个Looper的对象，对于Looper，它最重要的两块，就是prepare()和loop()两个方法：
prepare()源码如下：
/** 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) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
我们可以看到，prepare（）最终是返回一个sThreadLocal.set(new Looper(quitAllowed))：
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
而这个new Looper(quitAllowed)是作为参数，传入到sThreadLocal.set(new Looper(quitAllowed))当中，可以看到Looper操作中，有new一个消息队列MessageQueue：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
所以我们说，系统在创建主线程的时候，会初始化一个Looper对象，同时也会把它关联的MessageQueue创建好，所以我们的Handler在主线程中实例化时，不需要额外的对Looper进行操作。
那么在创建好Looper对象与MessageQueue之后，就需要在创建好的线程内对Handler进行实例化、重写handleMessage（）方法 .. 并开启消息队列的循环，进而去对消息队列中的消息进行轮询。
此时开启消息队列的循环，loop（）就派上了用场，代码有点长，可以慢慢看：
/**
* 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) {
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();
// 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;
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();
}
}
由代码我们也可以看到，在不断的轮询过程中，消息队列是通过next（）方法将消息从消息队列中移出来的：
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;
}
}
不断的轮询，不断的将消息取出，不断的去执行对应的消息，这就是我们的Handler消息机制了。
最后，我们分析sendMessage(Message)与post(Runnable)到底有什么差别？
如下图，是sendMessage(Message)与post(Runnable)发送消息，所经过的所有方法：
它们最大的差别就是：sendMessage(Message)需要传入Message，而post(Runnable)需要传入Runnable。
细细分析，post(Runnable)最终return的是sendMessageDelayed(getPostMessage(r), 0)：
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
而sendMessageDelayed(getPostMessage(r), 0)我们又可以看到，它需要传入一个参数getPostMessage(r)，这个参数，最终返回的也是一个Message类型的对象。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r;
return m;
}
接下来，post(Runnable)走的都是与sendMessage(Message)相同的方式，所以我们可以认为，它们的本质其实都是相同的，最终的最终，也只是为了把消息压入消息队列MessageQueue中。
自此，整个流程分析完毕。
相关面试题
面试题传送门：200斤牌面试必备Handler面试题 请安利（不停更）