android的UI操作不是线程安全的，同时也只有主线程才能够操作UI，同时主线程对于UI操作有一定的时间限制（最长5秒）。为了能够做一些比较耗时的操作（比如下载、打开大文件等），android提供了一些列机制。《android基础知识02线程安全》系列文章就是参考了网上许多网友的文章后，整理出来的一个系列，介绍了主要的方法。分别如下：
android基础知识02线程安全1：定义及例子
android基础知识02线程安全2：handler、message、runnable
android基础知识02线程安全3：Message，MessageQueue，Handler，Looper
android基础知识02线程安全4：HandlerThread
android基础知识02线程安全5： AsyncTask
在前面介绍的线程安全中，为了操作主线程的UI，使用子线程进行处理。在android开发中，还可以使用另外两种方法进行处理：
HandlerThread
AsyncTask
本文先介绍HandlerThread。
HandlerThread继承于Thread，所以它本质就是个Thread。与普通Thread的差别就在于，它有个Looper成员变量。这个Looper其实就是对消息队列以及队列处理逻辑的封装，简单说就是 消息队列+消息循环。
当我们需要一个工作者线程，而不是把它当作一次性消耗品，用过即废弃的话，就可以使用它。
其使用方法为：
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
HandlerThread hThread=new HandlerThread("myThread");
hThread.start();
myhandler myhandler=new myhandler(hThread.getLooper());
Message msg=myhandler.obtainMessage();
msg.sendToTarget();//把 Message发送到目标对象，目标对象就是生成msg的目标对象。
}
class myhandler extends Handler{
public myhandler(Looper looper){
super(looper);
}
public void handleMessage(Message msg) {
Log.e("这是新线程", "》》》》》》》》》》》》》》》》》新线程的测试");
}
}
这里通过HandlerThread启动一个新线程。
注：这里需要handlerThread.start();先启动线程 才能 handlerThread.getLooper() 获取当前线程的Looper。
HandlerThread使用是比较简单的，我们翻翻它的源码，也是很简单的。
/**
* Handy class for starting a new thread that has a looper. The looper can then be
* used to create handler classes. Note that start() must still be called.
*/
public class HandlerThread extends Thread {
private int mPriority;
private int mTid = -1;
private Looper mLooper;
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;
}
/**
* Call back method that can be explicitly over ridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
Process.setThreadPriority(mPriority);
notifyAll();
}
onLooperPrepared();
Looper.loop();
mTid = -1;
}
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason is 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.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
/**
* Ask the currently running looper to quit.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.
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}
