Handler是消息机制的上层接口，开发中基本只用和Handler交互即可。Handler可以将一个任务切换到Handler指定的线程中执行。如在用Handler在子线程更新UI。
Android消息机制主要就是Handler的运行机制。Handler的运行还依赖MessageQueue、Looper，及Looper内部使用到的ThreadLocal。 MessageQueue是消息队列，用于存放Handler发送的消息，实际是单链表的结构。 Looper会在消息队列中无限循环的查找消息，有消息就取出，没有就等待。 ThreadLocal本质作用是在每个线程中存储数据。在Looper中的作用就是给每个线程存Looper实例。因为我们知道，创建Handler时是需要线程的Looper实例的，而非UI线程默认是没有Looper的。
一、Handler使用与概述
1.1使用步骤
-
在任务执行的线程，使用Looper.prepare()来给线程创建Looper实例。
-
在任务执行的线程，创建Handler实例。
-
在任务执行的线程，使用Looper.loop()开启消息循环。
-
在任务发出的线程，使用Handler实例发送消息。
举个例子🌰 如下所示，点击按钮，在主线程发送消息，就会在子线程执行。 （这个例子为了完整展示使用步骤，所以在子线程创建了handler，在主线程发送和消息。通常实际我们使用是在主线程创建handler，在子线程发送消息然后再主线程执行UI的更新，而主线程默认是有Looper并开启的，所以一般不需要第一步和第三部。）
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
testHandler();
}
private void testHandler() {
new Thread(new Runnable() {
@Override
public void run() {
//1、准备looper，即threadLocal<Looper>.set(new Looper())
Looper.prepare();
//2、创建handler实例
// 这个重写了handleMessage，handler是属于Handler的子类的实例
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.i(TAG, "child thread, handleMessage: what="+msg.what);
}
};
//3、looper启动,sThreadLocal.get()拿到looper，拿到queue，开始queue.next
Looper.loop();
}
}).start();
}
public void onClick(){
//4.2、handler.sendMessage发送消息，queue.enqueueMessage(msg),即消息入队列。
Log.i(TAG, "main thread, sendMessage");
Message message = Message.obtain();
message.what = 100;
mHandler.sendMessage(message);
}
1.2Handler的使用背景
Handler可以将子线程中更新UI的任务切换到主线程。为什么要切换呢？我们知道，UI的访问只能在主线程进行。子线程访问UI就会出现异常，因为在ViewRootImpl中对线程做了校验，只有创建了这个View树的线程，才能访问这个view。 一般情况创建View的线程就是主线程，即UI线程，所以子线程访问会异常。
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
而且，UI线程一般不能做耗时操作，不然会发生ANR。所以当 在子线程做完耗时操作后 又需要更新UI，这时就需要用到Handler了。那为啥一定要用checkThread()保证不让子线程访问UI呢？ 因为UI控件不是线程安全的。那为啥不加锁呢？一是加锁会让UI访问变得复杂；二是加锁会降低UI访问效率，会阻塞一些线程访问UI。所以干脆使用单线程模型处理UI操作，使用时用Handler切换即可。
二、Android消息机制分析
前面说了，Android消息机制包含几个概念：Handler、MessageQueue、Looper、Looper内部使用到的ThreadLocal。下面详细介绍下。
2.1 ThreadLocal
外界想要在不同thread中存值，就可以threadLocal = new ThreadLocal，然后在不同线程中threadLocal.set(value)就可以了，获取值用threadLocal.get() 。
举个例子🌰，下面例子中 先只看booleanThreadLocal，在主线程设置true，a线程设置false，b线程设置null，然后每个线程都打印 booleanThreadLocal.get()的结果，发现每个线程get的值是不同的，是在每个线程中set的值。这就是神奇之处，同样的booleanThreadLocal.get()，所在线程不同，结果就不同。
ThreadLocal<Boolean> booleanThreadLocal = new ThreadLocal<>();
ThreadLocal<Integer> integerThreadLocal = new ThreadLocal<>();
booleanThreadLocal.set(true);
integerThreadLocal.set(0);
Log.i(TAG, "testThreadLocal: main thread, boolean= "+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: main thread, int = "+integerThreadLocal.get());
new Thread(new Runnable() {
@Override
public void run() {
booleanThreadLocal.set(false);
integerThreadLocal.set(1);
Log.i(TAG, "testThreadLocal: a thread, boolean="+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: a thread, int = "+integerThreadLocal.get());
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
booleanThreadLocal.set(null);
integerThreadLocal.set(2);
Log.i(TAG, "testThreadLocal: b thread, boolean="+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: b thread, int = "+integerThreadLocal.get());
}
}).start();
结果：
2020-01-08 10:15:38.623 8976-8976/com.hfy.demo01 I/hfy: testThreadLocal: main thread, boolean= true
2020-01-08 10:15:38.623 8976-8976/com.hfy.demo01 I/hfy: testThreadLocal: main thread, int = 0
2020-01-08 10:15:38.624 8976-9226/com.hfy.demo01 I/hfy: testThreadLocal: a thread, boolean=false
2020-01-08 10:15:38.624 8976-9226/com.hfy.demo01 I/hfy: testThreadLocal: a thread, int = 1
2020-01-08 10:15:38.626 8976-9227/com.hfy.demo01 I/hfy: testThreadLocal: b thread, boolean=null
2020-01-08 10:15:38.626 8976-9227/com.hfy.demo01 I/hfy: testThreadLocal: b thread, int = 2
下面看下ThreadLocal的get()、set()方法。
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
get()：获取当前线程的ThreadLocalMap，这里可以先理解成普通 键值对的Map。然后传入threadLocal实例，获取键值对Entry，然后获取Entry的value。如果map为空或value为空则会初始化map、value。
/**
* 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);
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
set()中也是获取当前线程的ThreadLocalMap，然后ThreadLocal实例作为key， 和value一起设置给map。没有map就去创建并把value初始化进去。
我们再去看下Thread，有个默认为空的ThreadLocalMap实例threadLocals。
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
那ThreadLocalMap是啥呢？ThreadLocalMap是ThreadLocal的内部类，作用类似Map，内部有个Entry[]的属性table。所以上面看的get、set方法就是对ThreadLocalMap的Entry[]取和存 。下面详细看下。
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private Entry getEntry(ThreadLocal<?> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
使用Entry[] 存多个threadLocal-value键值对，数组下标index与是ThreadLocal 实例的hashCode相关。而ThreadLocalMap唯一实例是createMap(Thread t, T firstValue)赋给Thread的变量threadLocals。 例如 线程A threadLocalMap的table[] 可以存储 int、String、boolean类型的3个键值对threadLocal-int， threadLocal-String、threadLocal-Boolean。还是上面的例子。
（常规的HashMap的键值得类型是固定的；threadLocalMap的key是ThreadLocal，value是T，即可以存多种类型的value）
ThreadLocal<Boolean> booleanThreadLocal = new ThreadLocal<>();
ThreadLocal<Integer> integerThreadLocal = new ThreadLocal<>();
booleanThreadLocal.set(true);
integerThreadLocal.set(0);
Log.i(TAG, "testThreadLocal: main thread, boolean= "+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: main thread, int = "+integerThreadLocal.get());
new Thread(new Runnable() {
@Override
public void run() {
booleanThreadLocal.set(false);
integerThreadLocal.set(1);
Log.i(TAG, "testThreadLocal: a thread, boolean="+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: a thread, int = "+integerThreadLocal.get());
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
booleanThreadLocal.set(null);
integerThreadLocal.set(2);
Log.i(TAG, "testThreadLocal: b thread, boolean="+booleanThreadLocal.get());
Log.i(TAG, "testThreadLocal: b thread, int = "+integerThreadLocal.get());
}
}).start();
结果：
2020-01-08 10:15:38.623 8976-8976/com.hfy.demo01 I/hfy: testThreadLocal: main thread, boolean= true
2020-01-08 10:15:38.623 8976-8976/com.hfy.demo01 I/hfy: testThreadLocal: main thread, int = 0
2020-01-08 10:15:38.624 8976-9226/com.hfy.demo01 I/hfy: testThreadLocal: a thread, boolean=false
2020-01-08 10:15:38.624 8976-9226/com.hfy.demo01 I/hfy: testThreadLocal: a thread, int = 1
2020-01-08 10:15:38.626 8976-9227/com.hfy.demo01 I/hfy: testThreadLocal: b thread, boolean=null
2020-01-08 10:15:38.626 8976-9227/com.hfy.demo01 I/hfy: testThreadLocal: b thread, int = 2
到目前为止我们知道，ThreadLocal的作用，就是操作线程内部的threadLocals，存和取value。value的实际类型就是 实例化ThreadLocal时定义的泛型T。
2.2 messageQueue
messageQueue，消息队列，实际是单向链表。看下存、取消息。
enqueueMessage()，存消息，单链表的插入。
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;
}
next()：取一条消息，没有消息就无限循环，会阻塞。
Message next() {
//...
//有msg就return,没有消息就无限循环，会阻塞。如quit，return null。
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();
//有消息就return
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
//quit后返回null
return null;
}
// ...
}
2.3 Looper
looper，消息循环器。
先看静态方法prepare（）：
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/** 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));
}
/**
* 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();
}
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可见sThreadLocal是个静态常量，value类型是Looper。 prepare()方法调sThreadLocal.set(new Looper），创建looper实例，设置给当前线程ThreadLocalMap属性中的table[i]（i是threadLocal实例的hashCode相关）。
且创建looper实例时默认创建了对应的消息队列mQueue实例。另外，prepareMainLooper()是主线程，是给主线程创建looper实例。
再看下获取looper实例、queue实例的方法：
/**
* Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
/**
* 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();
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
myLooper() 方法，调用sThreadLocal.get()。就是上面讲解的ThreadLocal的使用方法。通过静态常量sThreadLocal获取对应每个线程的Looper实例。
looper的quit，两种，立即退出，执行完消息再退出。
/**
* Quits the looper.
* <p>
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
/**
* Quits the looper safely.
* <p>
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p>
*/
public void quitSafely() {
mQueue.quit(true);
}
静态方法loop()：用threadLocal.get()获取当前线程的Looper，然后拿到queue，循环取消息，给到handler的dispatchMessage方法-handleMessage方法。唯一跳出循环是取到null，null是因为调用了quit或quitSafly。 ==因为静态方法loop()是在线程中调用的，所以不论handler从哪里发送msg都会在loop的线程中执行==。
/**
* 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;
// ...
for (;;) {
//没有msg ，queue.next()阻塞，loop() 也就阻塞了。next有msg就处理，无限循环。
Message msg = queue.next(); // might block
if (msg == null) {
//调用quit()时才会 跳出循环
// No message indicates that the message queue is quitting.
return;
}
// ...
//用target（handler）处理消息，dispatchMessage执行在loop() 调用的地方，即looper所在线程。
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
}
}
流程 prepare（）-new hanler()- loop() 连续的在同个线程调用。保证handleMessage执行在当前线程。即使handler.sengMessage()在其他线程调用。
2.4 Handler
发送，处理消息。 先看Handler构造方法，可见调用了Looper.myLooper()，就是获取当前线程的looper，没有就会抛出异常。
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Callback callback, boolean async) {
...
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;
}
发送消息，就是把消息放入队列
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
处理消息，根据Handler的创建形式和使用方法对应处理。
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//msg.callback就是handler.post()发送的runable
handleCallback(msg);
} else {
if (mCallback != null) {
//mCallback是创建Handler时传入CallBack的情况。
if (mCallback.handleMessage(msg)) {
return;
}
}
//覆写handleMessage()创建handler的情况
handleMessage(msg);
}
}
三、主线程的消息机制
主线程的消息
Looper中：
/**
* 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();
}
}
ActivityThread的静态方法main：
final H mH = new H();
public static void main(String[] args) {
...
//1、准备主线程的Looper
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
//这里实例化ActivityThread，也就实例化了上面的mH，就是handler。
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
//获取handler
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
...
//主线程looper开启
Looper.loop();
//因为主线程的Looper是不能退出的，退出就无法接受事件了。一旦意外退出，会抛出异常
throw new RuntimeException("Main thread loop unexpectedly exited");
}
H处理了四大组件的启动停止等。ActivityThread通过ApplicationThread和AMS进行进程间通信，AMS完成ActivityThread的请求后，回调到ApplicationThread中的binder方法，然后ApplicationThread使用H发送消息，然后就把此消息切换到ActivityThread中执行，即在主线程执行。这就是主线程的消息循环。
.
好了，今天就到这里，欢迎留言讨论~
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