一、HandlerThread简介
-
HandlerThread是什么鬼？
HandlerThread本身是继承自Thread，所以HandlerThread还是一个Thread - 既然还是一个
Thread，那HandlerThread还有存在的必要么？
其实HandlerThread的出现是为了减少对Thread的频繁创建销毁而诞生的，毕竟线程的创建还是很耗资源的，而且频繁的创建销毁也会带来内存的抖动； -
HandlerThread如何可以减少Thread频繁创建销毁
其实HandlerThread中帮我们封装了一个Looper对象，通过Looper的机制再配合Handler，就可以减少频繁的使用new Thread()
如果想了解Looper和Handler是如何配合的？
可以看下我之前写的两个文章
Handler、Looper、messagequeue源码分析及使用（1）
Handler、Looper、messagequeue源码分析及使用（2）
其中在讲解handler的使用 中 如何创建非UI线程的handler，实现UI线程发送消息通知非UI线程做耗时操作？ 这部分完全可以使用HandlerThread搞定。
二、HandlerThread使用
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HandlerThread使用很简单，用法也和Thread一直，但需要额外创建一个Handler对象【或者调用HandlerThread.getThreadHandler()获取】，通过重写Handler的handleMessage，或者post(Runnable r)来实现耗时操作，但值得注意的是，在调用HandlerThread的getLooper()和getThreadHandler()之前一定要先start() -
HandlerThread一个简单的使用案例
public class HandlerThreadTest {
private final String TAG = this.getClass().getSimpleName();
private static final int MSG_WHAT = 0x100;
private HandlerThread handlerThread = null;
private Handler handler = null;
public HandlerThreadTest(String name) {
// 注意顺序
handlerThread = new HandlerThread(name);
handlerThread.start();
handler = new MyHandler(handlerThread.getLooper());
}
public void excute() {
Log.d(TAG, "主线程id=" + Looper.getMainLooper().getThread().getId());
handler.sendEmptyMessage(MSG_WHAT);
}
private class MyHandler extends Handler {
private MyHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
if (msg.what == MSG_WHAT) {
try {
// 模拟耗时操作
handlerThread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
Log.d(TAG, "当前运行线程id=" + Thread.currentThread().getId());
}
}
}
上面通过自己创建一个Handler来实现耗时操作，你也可以调用getThreadHandler()获得一个Handler对象，运行结果：
02-08 15:31:51.387 16111-16111/com.ktln.must D/HandlerThreadTest: 主线程id=1
02-08 15:31:54.397 16111-16370/com.ktln.must D/HandlerThreadTest: 当前运行线程id=22899
三、HandlerThread源码分析
关键代码已添加注释
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
private @Nullable Handler mHandler;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* {@link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority; // 设置优先级
}
/**
* 在执行Looper.loop()前调用此方法的重载方法，可以用来执行一个初始化操作
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
// 创建Looper对象，以及Looper内的消息队列
Looper.prepare();
synchronized (this) {
// 获取创建的Looper对象，并赋值给mLooper
mLooper = Looper.myLooper();
// 通知处于wait的方法，继续执行
notifyAll();
}
Process.setThreadPriority(mPriority);
// 预留方法，可以执行自己的一些初始化操作
onLooperPrepared();
// 开启对消息队列的轮询
Looper.loop();
mTid = -1;
}
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
* 这个方法会返回一个跟当前线程关联的Looper对象. 如果当前线程没有调用start方法，或者其他原因使
* isAlive()返回false, 这个方法将返回null.
* 如果当前线程已经start了, 这个方法将堵塞，直到Looper对象被初始化完成。
* @return 返回 looper 对象.
*/
public Looper getLooper() {
if (!isAlive()) {
// 如果线程不是活动状态，则返回null
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
// 如果线程是活着的，并且mLooper == null，那就处于堵塞状态
// 此处是等待run方法中的mLooper = Looper.myLooper()赋值完成，并且调用notifyAll()才会激活
// 被激活后，会再一次执行while的判断，因为这时候mLooper不为null,则跳出循环
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
/**
* @return a shared {@link Handler} associated with this thread
* @hide
*/
@NonNull
public Handler getThreadHandler() {
if (mHandler == null) {
// 如果没有handler对戏，创建一个绑定当前Looper的handler对象。
// 即：创建了一个为非UI线程服务的handler
mHandler = new Handler(getLooper());
}
return mHandler;
}
/**
* Quits the handler thread's looper.
* <p>
* Causes the handler thread's looper 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>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*
* @see #quitSafely
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
/**
* Quits the handler thread's looper safely.
* <p>
* Causes the handler thread's looper to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* Pending delayed messages with due times in the future will not be delivered.
* </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>
* If the thread has not been started or has finished (that is if
* {@link #getLooper} returns null), then false is returned.
* Otherwise the looper is asked to quit and true is returned.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*/
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}
HandlerThread的精髓代码是执行在run()中的，在run()中创建了Looper对象，Looper.loop()开启循环，在通过mLooper创建一个Handler来服务此线程。
总结:
HandlerThread可以帮助我们创建一个服务于子线程的Handler，通过消息机制的方式，来处理耗时操作；这样就可以避免Thread运行完就被销毁的命运。
