一、android中需要另开线程处理耗时、网络的任务,但是有必须要在UI线程中修改组件。这样做是为了:
①只能在UI线程中修改组件,避免了多线程造成组件显示混乱
②不使用加锁策略是为了提高性能,因为android中经常使用多线程。
handler就是为了解决在多个线程之间的通信问题。
二、基本使用:
1 package com.dqxst.first.multithread;
2
3 import android.app.Activity;
4 import android.os.Bundle;
5 import android.os.Handler;
6 import android.os.Handler.Callback;
7 import android.os.Message;
8 import android.util.Log;
9 import android.view.View;
10 import android.widget.TextView;
11
12 import com.dqxst.first.R;
13
14 public class HandlerActivity extends Activity {
15 private TextView tv,msg_tv;
16 private Handler handler = new Handler();
17 final Handler msgHandler=new Handler(new Callback() {
18 @Override
19 public boolean handleMessage(Message msg) {
20 //！！！在这里可以进行一些操作,返回true则将msg进行拦截
21 return false;
22 }
23 }){
24 @Override
25 public void handleMessage(Message msg) {
26 switch(msg.what){
27 case 200:
28 msg_tv.setText("网络请求成功.");
29 break;
30 case 404:
31 msg_tv.setText("not found!");
32 break;
33 }
34 super.handleMessage(msg);
35 }
36
37 };
38 private MyRunnable runnable = new MyRunnable();
39
40 @Override
41 protected void onCreate(Bundle savedInstanceState) {
42 super.onCreate(savedInstanceState);
43 setContentView(R.layout.activity_handler);
44 init();
45
46 Log.i("main", "主线程=" + Thread.currentThread().toString());
47 }
48
49 class MyRunnable implements Runnable {
50 @Override
51 public void run() {
52 int time = Integer.parseInt(tv.getText().toString());
53 int now = time + 1;
54 Log.i("main", "处理UI更新线程=" + Thread.currentThread().toString());
55 tv.setText(now + "");
56 //这里是将Runnable间隔1秒重新发送到消息队列中,所以是定时操作的效果
57 handler.postDelayed(runnable, 1000);
58 }
59
60 }
61
62 //下面主要是在子线程中调用post将消息加入到消息队列中
63 public void startCallback(View view) {
64 new Thread() {
65 public void run() {
66 try {
67 Thread.sleep(1000);
68 } catch (InterruptedException e) {
69 e.printStackTrace();
70 }
71
72 Log.i("main", "新线程=" + Thread.currentThread().toString());
73
74 handler.post(runnable);
75 }
76 }.start();
77 }
78
79 //这里是移除消息中的执行体部分,就是那个Runnable
80 public void stopCallback(View view){
81 handler.removeCallbacks(runnable);
82 }
83
84 public void msgChange(View view){
85 new Thread(new Runnable() {
86 @Override
87 public void run() {
88 Message message=Message.obtain();
89 if(Math.random()>0.3){
90 message.what=200;
91 }else{
92 message.what=404;
93 }
94 msgHandler.sendMessage(message);
95 }
96 }).start();
97 }
98
99 private void init() {
100 tv = (TextView) findViewById(R.id.handler_tv);
101 tv.setText("0");
102 msg_tv=(TextView) findViewById(R.id.handler_tv_message);
103 }
104 }handler基本使用
由上例可以看到,handler使用有两种形式:
①post/postDelayed:这时传递的是一个Runnable对象,会封装到一个Message对象中发送到消息队列中
handler的handleMessage或者在构造时传入一个实现Callback接口的对象(该对象的方法也是前面重写的方法名一样,但是先执行,可以用来对消息进行拦截)
三、源码分析:与handler机制有关的类有Looper、MessageQueue、Message以及Handler本身。
1、Looper：主要有两个作用
①创建一个与当前线程相关的Looper实例,通过调用prepare()
1 public static void prepare() {
2 prepare(true);
3 }
4
5 private static void prepare(boolean quitAllowed) {
6 if (sThreadLocal.get() != null) {
7 throw new RuntimeException("Only one Looper may be created per thread");
8 }
9 sThreadLocal.set(new Looper(quitAllowed));
10 }prepare
②不断的读取MessageQueue中的消息,通过调用loop(),MessageQueue是和Looper相关的,由其创建和管理。注意:这个过程是阻塞的。
1 public static void loop() {
2 final Looper me = myLooper();
3 if (me == null) {
4 throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
5 }
6 final MessageQueue queue = me.mQueue;
7
8 // Make sure the identity of this thread is that of the local process,
9 // and keep track of what that identity token actually is.
10 Binder.clearCallingIdentity();
11 final long ident = Binder.clearCallingIdentity();
12
13 //这里是一个死循环,读取message
14 for (;;) {
15 Message msg = queue.next(); // might block
16 //！！！这里注意:通常不会执行这一句,除非使用quit退出Looper
17 if (msg == null) {
18 // No message indicates that the message queue is quitting.
19 return;
20 }
21
22 // This must be in a local variable, in case a UI event sets the logger
23 Printer logging = me.mLogging;
24 if (logging != null) {
25 logging.println(">>>>> Dispatching to " + msg.target + " " +
26 msg.callback + ": " + msg.what);
27 }
28
29 //这里是一个重点,说明了消息的具体执行方法
30 msg.target.dispatchMessage(msg);
31
32 if (logging != null) {
33 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
34 }
35
36 // Make sure that during the course of dispatching the
37 // identity of the thread wasn't corrupted.
38 final long newIdent = Binder.clearCallingIdentity();
39 if (ident != newIdent) {
40 Log.wtf(TAG, "Thread identity changed from 0x"
41 + Long.toHexString(ident) + " to 0x"
42 + Long.toHexString(newIdent) + " while dispatching to "
43 + msg.target.getClass().getName() + " "
44 + msg.callback + " what=" + msg.what);
45 }
46
47 msg.recycleUnchecked();
48 }
49 }loop
2、Message:就是消息对象,一般使用都只是用来传递数据(从网络获取的)等,常用属性有
①what :int类型,用于标识不同的消息,在handler中使用
②arg1,arg2 :int类型,用于传递int型的数据
③obj :Object类型,用于传递对象
④data :Bundle类型,
创建Message有一些需要注意的点:
①使用Message.obtain()或者Handler.obtainMessage()进行创建而不是new,可以利用公共池中对象避免重新分配空间。
一个Message是和一个handler关联的,表现为Message中的target属性,但是通常这个属性是由系统自动赋值的,就是那个发送该消Message到MessageQueue的handler。
3、Handler:主要作用有以下三点
①关联所在线程的Looper对象和其所管理的MessageQueue对象,这是在构造函数中体现的,最终都会调用以下两个构造函数
1 public Handler(Callback callback, boolean async) {
2 if (FIND_POTENTIAL_LEAKS) {
3 final Class<? extends Handler> klass = getClass();
4 if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
5 (klass.getModifiers() & Modifier.STATIC) == 0) {
6 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
7 klass.getCanonicalName());
8 }
9 }
10
11 mLooper = Looper.myLooper();
12 if (mLooper == null) {
13 throw new RuntimeException(
14 "Can't create handler inside thread that has not called Looper.prepare()");
15 }
16 mQueue = mLooper.mQueue;
17 mCallback = callback;
18 mAsynchronous = async;
19 }
20
21
22 public Handler(Looper looper, Callback callback, boolean async) {
23 mLooper = looper;
24 mQueue = looper.mQueue;
25 mCallback = callback;
26 mAsynchronous = async;
27 }Handler构造函数
！！！注意:在构造函数中,必须有一个Looper对象与其绑定,因为handler要将Message发送到MessageQueue中,而MessageQueue是由Looper创建的。
②发送消息到上面关联的消息队列,可以使用post和sendMessage两种类型方法。可以看到他们内部的调用过程是一样的。最终都是调用MessageQueue中的方法将消息绑定到一个链表上。
1 public final boolean post(Runnable r)
2 {
3 return sendMessageDelayed(getPostMessage(r), 0);
4 }
5
6 public final boolean postDelayed(Runnable r, long delayMillis)
7 {
8 return sendMessageDelayed(getPostMessage(r), delayMillis);
9 }
10 //这是上面用到的将Runnable封装到Message中的方法
11 private static Message getPostMessage(Runnable r) {
12 Message m = Message.obtain();
13 m.callback = r;
14 return m;
15 }
16
17
18 public final boolean sendMessage(Message msg)
19 {
20 return sendMessageDelayed(msg, 0);
21 }发送消息
1 public final boolean sendMessageDelayed(Message msg, long delayMillis)
2 {
3 if (delayMillis < 0) {
4 delayMillis = 0;
5 }
6 return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
7 }
8
9
10 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
11 MessageQueue queue = mQueue;
12 if (queue == null) {
13 RuntimeException e = new RuntimeException(
14 this + " sendMessageAtTime() called with no mQueue");
15 Log.w("Looper", e.getMessage(), e);
16 return false;
17 }
18 return enqueueMessage(queue, msg, uptimeMillis);
19 }
20
21
22 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
23 msg.target = this;
24 if (mAsynchronous) {
25 msg.setAsynchronous(true);
26 }
27 return queue.enqueueMessage(msg, uptimeMillis);
28 }进一步的调用过程
msg.target.dispatchMessage(msg);交由Handler中的dispatchMessage()进行处理。
1 public void dispatchMessage(Message msg) {
2 if (msg.callback != null) {
3 handleCallback(msg);
4 } else {
5 if (mCallback != null) {
6 if (mCallback.handleMessage(msg)) {
7 return;
8 }
9 }
10 handleMessage(msg);
11 }
12 }dispatchMessage
这里就说明了Handler处理消息有三种方式，需要说明的是:
a、msg.callback就是通过post/postDelayed或者构造函数传入的Runnable对象
b、mCallback就是通过构造函数传入的实现其内部接口的对象,该方法返回true则退出,否则向下执行
四、扩展:
该类的实现是在子线程中开启一个Looper,然后在使用时将handler与该子线程中的Looper/MessageQueue进行绑定，即可通过handler将消息发送到子线程中进行处理,避免了主线程的阻塞。通过上述的描述,该类的使用只需简单的两步即可：
①实例化一个线程对象并开启该线程:HandlerThread downThread=new HandlerThread("ThreadName");downThread.start();
②将Handler与该线程的Looper绑定:Handler handler=new Handler(downThread.getLooper());
Handler创建之前必须有一个该线程的Looper对象被创建。在主线程中直接使用是因为已经由系统进行创建了,此时在创建则会出错。下面是android中给出的标准使用方法:
1 class LooperThread extends Thread {
2 public Handler mHandler;
3
4 public void run() {
5 Looper.prepare();
6
7 mHandler = new Handler() {
8 public void handleMessage(Message msg) {
9 // process incoming messages here
10 }
11 };
12
13 Looper.loop();
14 }
15 }LooperThread
2、上面说主线程不能再创建Looper,这是因为主线程被初始化是就是一个Looper线程,各种消息都是通过Handler机制处理的。
3、AsyncTask:异步消息处理机制,在内部封装了Handler处理流程。
execute()即可
1 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
2 protected Long doInBackground(URL... urls) {
3 int count = urls.length;
4 long totalSize = 0;
5 for (int i = 0; i < count; i++) {
6 totalSize += Downloader.downloadFile(urls[i]);
7 publishProgress((int) ((i / (float) count) * 100));
8 // Escape early if cancel() is called
9 if (isCancelled()) break;
10 }
11 return totalSize;
12 }
13
14 protected void onProgressUpdate(Integer... progress) {
15 setProgressPercent(progress[0]);
16 }
17
18 protected void onPostExecute(Long result) {
19 showDialog("Downloaded " + result + " bytes");
20 }
21 }DownloadFilesTask
这里简要说明三个泛型参数的作用:
a、Params:就是后台执行需要的参数,例如下载时需要url
b、Progress:就是任务执行的进度,在onProgressUpdate(Progress...)中会被使用,
c、Result:就是任务完成返回的结果,在onPostExecute(Result)中进行处理。
②源码解析:首先说明,Handler机制就是在子线程中处理任务,通过Handler将需要处理的消息传递到与其绑定的Looper的消息队列中,再由Looper取出交由Handler处理。
a、任务在子线程中处理,这里就是通过线程池处理,而任务就是doInBackground()中的部分,该函数被放在线程池的工作任务之中
1 public AsyncTask() {
2 mWorker = new WorkerRunnable<Params, Result>() {
3 public Result call() throws Exception {
4 mTaskInvoked.set(true);
5
6 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
7 //noinspection unchecked
8 //！！！重点:doInBackground()在这里被封装
9 Result result = doInBackground(mParams);
10 Binder.flushPendingCommands();
11 return postResult(result);
12 }
13 };
14
15 mFuture = new FutureTask<Result>(mWorker) {
16 @Override
17 protected void done() {
18 try {
19 postResultIfNotInvoked(get());
20 } catch (InterruptedException e) {
21 android.util.Log.w(LOG_TAG, e);
22 } catch (ExecutionException e) {
23 throw new RuntimeException("An error occurred while executing doInBackground()",
24 e.getCause());
25 } catch (CancellationException e) {
26 postResultIfNotInvoked(null);
27 }
28 }
29 };
30 }AsyncTask构造函数
execute()就是用默认线程池来进行处理任务
1 @MainThread
2 public final AsyncTask<Params, Progress, Result> execute(Params... params) {
3 return executeOnExecutor(sDefaultExecutor, params);
4 }
5
6
7 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
8 Params... params) {
9 if (mStatus != Status.PENDING) {
10 switch (mStatus) {
11 case RUNNING:
12 throw new IllegalStateException("Cannot execute task:"
13 + " the task is already running.");
14 case FINISHED:
15 throw new IllegalStateException("Cannot execute task:"
16 + " the task has already been executed "
17 + "(a task can be executed only once)");
18 }
19 }
20
21 mStatus = Status.RUNNING;
22
23 onPreExecute();
24
25 mWorker.mParams = params;
26 exec.execute(mFuture);
27
28 return this;
29 }线程池处理
execute()时的默认处理,默认处理是指:1、通过执行器模仿单一线程池的效果,即多个任务(一个AsyncTask实例的一次execute()创建一个任务)时只能有一个在线程中执行,2、而且线程中只能有128个任务(这是由阻塞队列的大小限制的)。
我们可以通过执行executeOnExecutor(Executor exec, Params... params)而不是execute()传入定制的线程池来解决上面问题。
1 //这是一个执行器,负责添加任务到一个ArrayDeque队列中,并且将第一个任务分配给线程池处理。注意下面的execute是synchronized的,说明一次只能有一个线程在处理一个任务。
2 private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
3 public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
4 private static class SerialExecutor implements Executor {
5 final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
6 Runnable mActive;
7
8 public synchronized void execute(final Runnable r) {
9 mTasks.offer(new Runnable() {
10 public void run() {
11 try {
12 r.run();
13 } finally {
14 scheduleNext();
15 }
16 }
17 });
18 if (mActive == null) {
19 scheduleNext();
20 }
21 }
22
23 protected synchronized void scheduleNext() {
24 if ((mActive = mTasks.poll()) != null) {
25 THREAD_POOL_EXECUTOR.execute(mActive);
26 }
27 }
28 }
29
30 //这是上面用到的线程池的设置
31 private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
32 private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
33 private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
34 private static final int KEEP_ALIVE = 1;
35
36 private static final ThreadFactory sThreadFactory = new ThreadFactory() {
37 private final AtomicInteger mCount = new AtomicInteger(1);
38
39 public Thread newThread(Runnable r) {
40 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
41 }
42 };
43
44 private static final BlockingQueue<Runnable> sPoolWorkQueue =
45 new LinkedBlockingQueue<Runnable>(128);
46
47 public static final Executor THREAD_POOL_EXECUTOR
48 = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
49 TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);执行器和线程池
可以看到构造函数的任务对象中的最后一句是postResult(result),而该方法就是向AsyncTask内部的Handler发送消息。
b、下面就到了Handler的部分,
1 //获取内部Handler
2 private static Handler getHandler() {
3 synchronized (AsyncTask.class) {
4 if (sHandler == null) {
5 sHandler = new InternalHandler();
6 }
7 return sHandler;
8 }
9 }
10
11 private static class InternalHandler extends Handler {
12 public InternalHandler() {
13 super(Looper.getMainLooper());
14 }
15
16 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
17 @Override
18 public void handleMessage(Message msg) {
19 AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
20 switch (msg.what) {
21 case MESSAGE_POST_RESULT:
22 // There is only one result
23 result.mTask.finish(result.mData[0]);
24 break;
25 case MESSAGE_POST_PROGRESS:
26 result.mTask.onProgressUpdate(result.mData);
27 break;
28 }
29 }
30 }AsyncTask内部Handler
可以看到,这个handler处理两种类型的消息,一种是MESSAGE_POST_PROGRESS,由publishProgress发送,触发;一种是MESSAGE_POST_RESULT,由postResult发送。
1 //发送MESSAGE_POST_PROGRESS消息,触发onProgressUpdate()
2 protected final void publishProgress(Progress... values) {
3 if (!isCancelled()) {
4 getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
5 new AsyncTaskResult<Progress>(this, values)).sendToTarget();
6 }
7 }
8 //所以重写这个方法可以在主UI中更新界面
9 protected void onProgressUpdate(Progress... values) {
10 }
11
12
13 //发送MESSAGE_POST_RESULT消息,触发finish()
14 private Result postResult(Result result) {
15 @SuppressWarnings("unchecked")
16 Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
17 new AsyncTaskResult<Result>(this, result));
18 message.sendToTarget();
19 return result;
20 }
21 //！！！这里的结束有两种情况，一种是被取消,一种是执行完成。
22 private void finish(Result result) {
23 if (isCancelled()) {
24 onCancelled(result);
25 } else {
26 onPostExecute(result);
27 }
28 mStatus = Status.FINISHED;
29 }消息的发送和处理
有关AsyncTask的使用还有一个问题就是如果在执行耗时任务时用户退出Activity,此时该任务线程可能没有执行完所以不会退出,这是可能造成内存泄露,需要在onPause中进行判断。还有其他问题参见http://droidyue.com/blog/2014/11/08/bad-smell-of-asynctask-in-android/index.html
参考:
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