概述
Android UI是线程不安全的，如果在子线程中尝试进行UI操作，程序就有可能会崩溃，因为在ViewRootImpl.checkThread对UI操作做了验证，导致必须在主线程中访问UI，但Android在主线程中进行耗时的操作会导致ANR，为了解决子线程无法访问UI的矛盾，提供了消息机制。
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
Android消息机制主要指Handler的运行机制，Handler的运行需要底层的MessageQueue和Looper的支撑。MQ即消息队列，存储消息的单元，但并不能处理消息，这时需要Looper，它会无限循环查找是否有新消息，有即处理消息，没有就等待。
Handler的创建方式很简单，只需要new一个实例即可，但是当前线程中没有Looper而创建Handler就会导致报错，下面来看下两个Handler的创建过程，看看有什么不一样。
private Handler handler1;
private Handler handler2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
handler1 = new Handler();
new Thread(new Runnable() {
@Override
public void run() {
handler2 = new Handler();
}
}).start();
}
运行下会发现handler2会报下面的错误“Can't create handler inside thread that has not called Looper.prepare()”
11-14 11:51:56.591 5751-5769/com.fomin.demo E/AndroidRuntime: FATAL EXCEPTION: Thread-642
Process: com.fomin.demo, PID: 5751
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
at android.os.Handler.<init>(Handler.java:200)
at android.os.Handler.<init>(Handler.java:114)
at com.fomin.demo.MainActivity$1.run(MainActivity.java:20)
at java.lang.Thread.run(Thread.java:818)
为什么handler1没有报错呢？因为Handler的创建时会采用当前线程的Looper来构建内部的消息循环系统，而handler1是在主线程创建的，而主线程已经默认调用Looper.prepareMainLooper()创建Looper，所以handler2创建时需要先调用Looper.prepare()创建Looper。
接下来看下整个Handler的处理流程并且会具体分析下ThreadLocal、Handler、MessageQueue和Looper，如图：
ThreadLocal工作原理
ThreadLocal是一个线程内部的的数据存储类，通过它可以在指定的线程中存储数据，存储以后，也只能在指定的线程中获取存储数据，对于其他线程来说则无法获取到数据。在Handler中，需要获取当前的线程的Looper，而Looper作用域就是线程并且不同线程具有不同的Looper，使用ThreadLocal可以轻松实现Looper在线程中的存取。
先看一个例子，分别在主线程、线程1和线程2设置和访问它的值，如下：
private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal<>();
Log.d(TAG, "Current Thread: mBooleanThreadLocal is : " + mBooleanThreadLocal.get());
new Thread("Thread#1") {
@Override
public void run() {
mBooleanThreadLocal.set(false);
Log.d(TAG, "Thread 1: mBooleanThreadLocal is : " + mBooleanThreadLocal.get());
}
}.start();
new Thread("Thread#2") {
@Override
public void run() {
Log.d(TAG, "Thread 2: mBooleanThreadLocal is : " + mBooleanThreadLocal.get());
}
}.start();
运行程序，日志如下：
11-14 14:18:41.731 7754-7754/com.fomin.demo D/MainActivity: Current Thread: mBooleanThreadLocal is : true
11-14 14:18:41.731 7754-7807/com.fomin.demo D/MainActivity: Thread 1: mBooleanThreadLocal is : false
11-14 14:18:41.731 7754-7808/com.fomin.demo D/MainActivity: Thread 2: mBooleanThreadLocal is : null
日志可以看出，不同线程访问同一个ThreadLocal对象，但是他们的值是不一样的。因为ThreadLocal会从各自的线程中取出一个数据，然后数组根据当前ThreadLocal的索引去查找对应的value值。可以先看下ThreadLocal的set方法：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
在看下get方法
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
ThreadLocal的get和set方法操作的对象都是当前线程ThreadLocalMap，读写操作仅限于各自线程的内部。这也是为什么ThreadLocal在多个线程中互不干扰的操作。
MessageQueue工作原理
MessageQueue只有两个操作：插入和读取。其内部是一个单链表的数据结构来维护消息列表，链表的节点就是 Message。它提供了 enqueueMessage() 来进行插入新的消息，提供next() 从链表中取出消息，值得注意的是next()会循环地从链表中取出 Message 交给 Handler，但如果链表为空的话会阻塞这个方法，直到有新消息到来。
boolean enqueueMessage(Message msg, long when) {
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) {
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) {
// New head, wake up the event queue if blocked.
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;
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;
}
enqueueMessage主要操作就是单链表的插入操作，在看下next方法
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);
}
...
// 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;
}
}
next方法是一个无线信息的方法，如果消息队列没有消息，会一直阻塞在这里。
Looper工作原理
Looper在Android的消息机制中扮演着消息循环的角色，它不停从MessageQueue查看是否有新消息，有会立即处理，否则会一直阻塞在那里。
Looper会在构造方法中构建一个MessageQueue和当前线程对象。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper提供了两个退出方法quit和quitSafely，区别是前个是直接退出，后一个把消息队列中已有的消息处理完毕后安全退出，均是调用MessageQueue中退出quit方法。
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
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);
}
}
Looper最重要的方法是loop方法，只有调用了loop后，消息系统才会真正的起作用，具体代码如下
/**
* 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();
}
}
loop方法是一个死循环，唯一跳出就是next返回null。如果next返回了新消息，会调用msg.target.dispatchMessage(msg)处理消息(即Handler处理)。
Handler工作原理
Handler的工作主要包含消息的发送和接收过程。消息发送通过post系列方法和send系列方法来实现，而post最终还是调用sendMessageAtTime方法来实现发送消息。
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);
}
可以发现，发送消息最终只是在向消息队列中插入了一条消息，流程MessageQueue——>Looper——>Handler，最终在dispatchMessage处理，由handleMessage消费。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}