Handler:是一个消息分发对象,进行发送和处理消息,并且其 Runnable 对象与一个线程的 MessageQueue 关联。
作用:调度消息,将一个任务切换到某个指定的线程中去执行。
假若子线程允许访问 UI,则在多线程并发访问情况下,会使得 UI 控件处于不可预期的状态。
传统解决办法:加锁,但会使得UI访问逻辑变的复杂,其次降低 UI 访问的效率。
采用单线程模型处理 UI 操作,通过 Handler 切换到 UI 线程,解决子线程中无法访问 UI 的问题。
创建一个 handler,通过 handler.post/postDelay,投递创建的 Runnable,在 run 方法中进行更新 UI 操作。
new Thread(new Runnable() {
@Override
public void run() {
/**
* 耗时操作
*/
handler.post(new Runnable() {
@Override
public void run() {
/**
* 更新UI
*/
}
});
}
}).start();
创建一个handler,重写 handleMessage 方法,根据 msg.what 信息判断,接收对应的信息,再在这里更新 UI。
private Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what) { //判断标志位
case 1:
/**
* 获取数据,更新UI
*/
break;
}
}
};
public class WorkThread extends Thread {
@Override
public void run() {
super.run();
/**
* 耗时操作
*/
Message msg =Message.obtain(); //从全局池中返回一个message实例,避免多次创建message(如new Message)
msg.obj = data;
msg.what=1; //标志消息的标志
handler.sendMessage(msg);
}
new WorkThread().start();
Handler 被作为 Activity 引用,如果为非静态内部类,则会引用外部类对象。当 Activity finish 时,Handler可能并未执行完,从而引起 Activity 的**内存泄漏**。故而在所有调用 Handler 的地方,都用静态内部类。
当 Activity finish 时,在 onDestroy 方法中释放了一些资源。此时 Handler 执行到 handlerMessage 方法,但相关资源已经被释放,从而引起**空指针的异常**。
避免
则使用如下方式创建 handler 对象:
/**
* 为避免handler造成的内存泄漏
* 1、使用静态的handler,对外部类不保持对象的引用
* 2、但Handler需要与Activity通信,所以需要增加一个对Activity的弱引用
*/
private static class MyHandler extends Handler {
private final WeakReference<Activity> mActivityReference;
MyHandler(Activity activity) {
this.mActivityReference = new WeakReference<Activity>(activity);
}
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
MainActivity activity = (MainActivity) mActivityReference.get(); //获取弱引用队列中的activity
switch (msg.what) { //获取消息,更新UI
case 1:
byte[] data = (byte[]) msg.obj;
activity.threadIv.setImageBitmap(activity.getBitmap(data));
break;
}
}
}
并在 onDesotry 中销毁:
@Override
protected void onDestroy() {
super.onDestroy();
//避免activity销毁时,messageQueue中的消息未处理完;故此时应把对应的message给清除出队列
handler.removeCallbacks(postRunnable); //清除runnable对应的message
//handler.removeMessage(what) 清除what对应的message
}
public class MainActivity extends AppCompatActivity {
ImageView threadIv;
ImageView runnableIv;
SendThread sendThread;
PostRunnable postRunnable;
private final MyHandler handler = new MyHandler(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
threadIv = (ImageView) findViewById(R.id.thread_iv);
runnableIv = (ImageView) findViewById(R.id.runnable_iv);
sendThread = new SendThread(handler);
sendThread.start();
postRunnable = new PostRunnable(handler);
postRunnable.setRefreshUI(new PostRunnable.RefreshUI() {
@Override
public void setImage(byte[] data) {
runnableIv.setImageBitmap(getBitmap(data));
}
});
new Thread(postRunnable).start();
}
/**
* 为避免handler造成的内存泄漏
* 1、使用静态的handler,对外部类不保持对象的引用
* 2、但Handler需要与Activity通信,所以需要增加一个对Activity的弱引用
*/
private static class MyHandler extends Handler {
private final WeakReference<Activity> mActivityReference;
MyHandler(Activity activity) {
this.mActivityReference = new WeakReference<Activity>(activity);
}
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
MainActivity activity = (MainActivity) mActivityReference.get(); //获取弱引用队列中的activity
switch (msg.what) { //获取消息,更新UI
case 1:
byte[] data = (byte[]) msg.obj;
activity.threadIv.setImageBitmap(activity.getBitmap(data));
break;
}
}
}
private Bitmap getBitmap(byte[] data) {
return BitmapFactory.decodeByteArray(data, 0, data.length);
}
@Override
protected void onDestroy() {
super.onDestroy();
//避免activity销毁时,messageQueue中的消息未处理完;故此时应把对应的message给清除出队列
handler.removeCallbacks(postRunnable); //清除runnable对应的message
//handler.removeMessage(what) 清除what对应的message
}
}
public class PostRunnable implements Runnable {
private Handler handler;
private RefreshUI refreshUI;
byte[] data = null;
public PostRunnable(Handler handler) {
this.handler = handler;
}
@Override
public void run() {
/**
* 耗时操作
*/
final Bitmap bitmap = null;
HttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet("http://i3.17173cdn.com/2fhnvk/YWxqaGBf/cms3/FNsPLfbkmwgBgpl.jpg");
HttpResponse httpResponse = null;
try {
httpResponse = httpClient.execute(httpGet);
if (httpResponse.getStatusLine().getStatusCode() == 200) {
data = EntityUtils.toByteArray(httpResponse.getEntity());
}
} catch (IOException e) {
e.printStackTrace();
}
//返回结果给UI线程
handler.post(new Runnable() {
@Override
public void run() {
refreshUI.setImage(data);
}
});
}
public interface RefreshUI {
public void setImage(byte[] data);
}
public void setRefreshUI(RefreshUI refreshUI) {
this.refreshUI = refreshUI;
}
}
public class SendThread extends Thread {
private Handler handler;
public SendThread(Handler handler) {
this.handler = handler;
}
@Override
public void run() {
super.run();
/**
* 耗时操作
*/
byte[]data=null;
HttpClient httpClient = new DefaultHttpClient();
HttpGet httpGet = new HttpGet("https://d36lyudx79hk0a.cloudfront.net/p0/descr/pc27/3095587d8c4560d8.png");
HttpResponse httpResponse = null;
try {
httpResponse = httpClient.execute(httpGet);
if(httpResponse.getStatusLine().getStatusCode()==200){
data= EntityUtils.toByteArray(httpResponse.getEntity());
}
} catch (IOException e) {
e.printStackTrace();
}
//返回结果给UI线程
doTask(data);
}
/**
* 通过handler返回消息
* @param data
*/
private void doTask(byte[] data) {
Message msg =Message.obtain(); //从全局池中返回一个message实例,避免多次创建message(如new Message)
msg.obj = data;
msg.what=1; //标志消息的标志
handler.sendMessage(msg);
}
}
Handler 通信机制
Handler及其关联的类图
以上类图可以快速帮助我们理清Handler与Looper、MessageQueue的关系,以下从源码的角度慢慢分析:
上一段很熟悉的代码:
Message msg =Message.obtain(); //从全局池中返回一个message实例,避免多次创建message(如new Message)
msg.obj = data;
msg.what=1; //标志消息的标志
handler.sendMessage(msg);
从sendMessageQueue开始追踪,函数调用关系:sendMessage -> sendMessageDelayed ->sendMessageAtTime,在sendMessageAtTime中,携带者传来的message与Handler的mQueue一起通过enqueueMessage进入队列了。
对于postRunnable而言,通过post投递该runnable,调用getPostMessage,通过该runnable构造一个message,再通过 sendMessageDelayed投递,接下来和sendMessage的流程一样了。
在enqueueMessage中,通过MessageQueue入队列,并为该message的target赋值为当前的handler对象,记住msg.target
很重要,之后Looper取出该消息时,还需要由msg.target.dispatchMessage
回调到该handler中处理消息。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在MessageQueue中,由Message的消息链表进行入队列
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;
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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));
}
这里创建Looper,并把Looper对象保存在sThreadLocal中,那sThreadLocal是什么呢?
static final ThreadLocal sThreadLocal = new ThreadLocal();
它是一个保存Looper的TheadLocal实例,而ThreadLocal是线程私有的数据存储类,可以来保存线程的Looper对象,这样Handler就可以通过ThreadLocal来保存于获取Looper对象了。
TheadLocal 如何保存与获取Looper?
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}
return (T) values.getAfterMiss(this);
}
在 set 中都是通过 `values.put` 保存当前线程的 Looper 实例,通过 `values.getAfterMiss(this)`获取,其中`put`和`getAfterMiss`都有`key`和`value`,都是由Value对象的table数组保存的,那么在table数组里怎么存的呢?
table[index] = key.reference;
table[index + 1] = value;
很显然在数组中,前一个保存着ThreadLocal对象引用的索引,后一个存储传入的Looper实例。
在loop
中,一个循环,通过next
取出MessageQueue中的消息
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(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();
}
}
Looper把消息回调到handler的dispatchMessage中进行消息处理:
runnable
时,把runnable
对象赋值给了message的callback
。callback
创建handler方式去处理。 public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
以一个时序图来总结handler的消息机制,包含上述如何关联Looper和MessageQueue的过程。
Handler-Looper-MessageQueue时序图