android基础知识02——线程安全4：HandlerThread

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;
    }
}