Android Handler:全面,详细解读
有时我们需要在子线程中进行耗时的I/O操作,可能是读取文件或者访问网络等,当耗时操作完成后可能需要UI上做一些改变,由于android的开发规范限制,我们不能在子线程中访问UI控件,否则会触发异常,这个时候通过Handler就可以将更新UI的操作切换到主线程中。
概述
Handler的运行需要底层的MessageQueue和Looper的支撑。MessageQueue的中文翻译是消息队列,它的内部存储了一组消息,以队列的形式对外提供插入和删除工作。虽然叫消息队列,但内部存储结构并不是真正的队列,而是采用单链表的数据结构来存储消息列表。Looper的中文翻译为循环,由于MessageQueue只是一个消息的存储器,它不处理消息,而Looper填补了这功能。Looper中还有一个特殊的概念,那就是ThreadLocal,ThreadLocal并不是线程,它的作用是可以在每个线程中存储数据。我们知道Handler创建的时候会采用当前线程的Looper来构造消息循环系统,那么Handler内部如何获取当前线程Looper呢,这就要用到ThreadLocal了,ThreadLocal可以在不同线程中互不干扰地存储并提供数据,通过ThreadLocal可以轻松获得每个线程的Looper。当然需要注意的是,线程默认没有Looper,如果需要使用Handler就必须为线程创建Looper。我们的UI线程,它就是ActivityThread,ActivityThread被创建时就会初始化Looper,这也是在主线程中默认可以使用Handler的原因。如果需要在子线程中使用Handler,需如下操作: 创建Looper
new Thread(new Runnable() {
public void run() {
Looper.prepare();
Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show();
}
};
handler.sendEmptyMessage(1);
Looper.loop();
};
}).start();
Handler创建完毕后,这个时候其内部的Looper以及MessageQueue就可以和Handler一起协同工作了,然后通过Handler的Handler的send方法发送一个消息到Looper中去处理,也可以通过post方法将一个Runnable投递到Looper中。其实post方法最终也是通过send方法调用的,它会调用MessageQueue的enqueueMessage方法将这个消息放入消息队列中,然后Looper发现有新消息到来时,就会处理这个消息,最终消息中的Runnable或者Handler的handleMessage方法就会调用。注意Looper是运行在创建Handler所在的线程中的,这样一来Handler的业务逻辑就被切换到创建Handler所在的线程中去执行了,这个过程可以用下图表示。
相关概念
关于 Handler 异步通信机制中的相关概念如下:
ThreadLocal,Message、Message Queue、Looper,接下来结合源码分析它们的工作原理。
ThreadLocal的工作原理
ThreadLocal是一个线程内部的数据存储类,通过它可以在指定的线程中存储数据,数据存储后,只有在指定的线程中可以获取存储的数据,对于其他线程则无法获取到数据。 在日常开发中用到ThreadLocal的地方较少, 一般来说,当某些数据是以线程为作用域并且不同线程具有不同数据副本时,可以考虑采用ThreadLocal。比如对于Handler来说,它需要获取当前线程的Looper,很显然Looper的作用域就是线程并且不同线程具有不同的Looper,这个时候通过ThreadLocal就可以轻松实现Looper在线程中存取。下面通过实际的例子来演示ThreadLocal的真正含义。首先定义一个ThreadLocal对象,选择Boolean类型,如下所示。
private ThreadLocal<Boolean> mBooleanThreadLocal=new ThreadLocal<Boolean>();
然后分别在主线程,子线程1和子线程2中设置和访问它的值,代码如下:
mBooleanThreadLocal.set(true);
Log.d("ThreadLocal","mainThread="+mBooleanThreadLocal.get());
new Thread("Thread1"){
@Override
public void run() {
mBooleanThreadLocal.set(false);
Log.d("ThreadLocal","thread1="+mBooleanThreadLocal.get());
}
}.start();
new Thread("Thread2"){
@Override
public void run() {
Log.d("ThreadLocal","thread2="+mBooleanThreadLocal.get());
}
}.start();
上述代码中,子线程设置为true,子线程1中设置false,子线程2中不设置值,日志如下:
ThreadLocal: mainThread=true
ThreadLocal: thread1=false
ThreadLocal: thread2=null
从上面日志看,不同线程中访问的同一个ThreadLocal对象,获取值却不一样。不同线程访问同一个ThreadLocal的get方法,ThreadLocal内部会从各自线程中取出一个数组,然后再从数组中根据当前ThreadLocal的索引去查找对应的value值。下面我们来看看set和get方法,首先看ThreadLocal的set方法,如下:
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
从上面的set方法中,首先通过getMap方法获取当前线程中的ThreadLocalMap数据,如果为空就对其初始化。我们再看看 map.set方法
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();
}
我们再来看看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方法,同样是取出当前线程的ThreadLocalMap对象,如果这个对象为null就返回初始值,初始值由ThreadLocal的initialValue方法来描述,默认为null。
MessageQueue的工作原理
主要包含插入和读取操作,对应的方法分别为enqueueMessage和next。enqueueMessage源码如下:
boolean enqueueMessage(Message msg, long when) {
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;
}
从enqueueMessage的实现来看,主要是单链表的插入操作,下面看一下next的源码:
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;
}
}
可以发现next方法是一个无限循环的方法,如果消息队列中没有消息,那么next方法一直阻塞在这里,当有新消息时,next方法会返回这条消息。
Looper的工作原理
Looper会不停地从MessageQueue中查看是否有新消息,如果有新消息就立即处理,否则就一直阻塞在那里,首先看下它的构造方法,在构造方法中会创建一个MessageQueue,然后将当前线程的对象保存起来。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
我们知道Handler的工作需要Looper,没有Looper的线程会报错,上面也讲述了如何为线程创建Looper,通过Looper.prepaer()即可为当前线程创建一个Looper,通过Looper.loop()来开启消息循环,如下:
new Thread("Thread1"){
@Override
public void run() {
Looper.prepare();
Handler handler=new Handler();
Looper.loop();
}
}.start();
Looper除了prepare方法外,还提供了prepareMainLooper()方法,这个方法主要是给主线程创建Looper使用的,其本质也是通过prepare方法。由于主线程的Looper比较特殊,所以Looper提供了一个getMainLooper()方法,通过它可以在任何地方获取主线程的Looper。Looper最重要的一个方法是loop方法,只有调用了loop后,消息循环系统才会真正起作用,如下:
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();
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
Looper的loop方法是一个死循环,会调用MessageQueue的next方法来获取新消息,而next是一个阻塞操作,当没有消息时,next方法会一直阻塞在那里,这也导致loop方法会一直阻塞。如果MessageQueue的next方法返回了新消息,Looper会处理这条消息: msg.target.dispatchMessage(msg),这里的 msg.target是发送这条消息的Handler对象,这样Handler发送的消息最终又交给它的dispatchMessage方法来处理了,最终回调复写的handleMessage(Message msg)。
Handler的工作原理
Handler的工作主要包含消息的发送和接收过程。消息的发送可以通过send,post的一系列方法实现。post的方法最终也是通过send来实现,发送一条消息的典型过程如下:
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
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,Handler发送消息就是向消息队列插入一条消息,MessageQueue的next方法就会返回这条消息给Looper,Looper收到消息后就开始处理,最终消息由Looper交由Handler处理,即上面说的dispatchMessage方法被调用,代码如下:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Handler处理消息的过程如下:
首先检查Message的callback是否为mCallbacknull,不为null就通过handleCallback来处理消息,Message的callback是一个Runnable对象,实际是Handler的post方法传递的Runnable参数,handleCallback逻辑如下:
private static void handleCallback(Message message) {
message.callback.run();
}
其次,检查mCallback是否为null,不为null就调用mCallback的handleMessage方法处理消息,mCallback是个接口,定义如下:
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}
通过Callback可以采用如下方式来创建Handler对象:Handler handler=new Handler(callback)。那么Callback的意义是什么呢,源码的注释说明了,可以用来创建一个Handler的实例但并不需要派生Handler的子类。日常开发中,创建Handler,最常见是派生一个Handler的子类并重写handlerMessage方法来处理具体消息,而Callback给我们提供了另一种使用Handler的方式。
最后,调用Handler的handleMessage方法来处理消息。
Handler还有一个构造方法,就是通过一个特定的Looper来构造Handler,实现如下:
public Handler(Looper looper) {
this(looper, null, false);
}
下面看下Handler的默认构造方法
public Handler() {
this(null, false);
}
this(null, false)实现如下,很明显,如果当前线程没有创建Looper,就会抛出 “Can’t create handler inside thread that has not called Looper.prepare()”这个异常。
public Handler(Callback callback, boolean async) {
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
以上就是Handler相关内容
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