关于HandlerThread的分析

Android中的Thread没有对java中的Thread做任何封装，而Android提供了一个遍历方法HandlerThread，他继承于Thread，实现了对遍历系统的一些封装，下面研究一下HandlerThread的源码：

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

/**
 * 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 {
    int mPriority;
    int mTid = -1;
    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 overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        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;
    }

    /**
     * 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;
    }
}

下面来分析一下该类的实现原理：

1、首先该类有两个构造方法：一个传入一个字符串名称，任意选取，优先级设置为默认的，第二个多了设置进程优先级的参数

2、

实际上简单理解就是：HandlerThread可以让Handler运行于其他线程中，在平常应用中，以Activity为例，新建一个Handler对象，默认是运行于UI线程中的，如果需要在Handler中执行一些耗时的任务，可以重新开启一个子线程来执行，而不可以直接在handler中执行；

那具体用途在于哪呢？

如果需要在子线程中使用Handler类，首先需要创建Looper类实例，这时可以通过Looper.prepare()和Looper.loop()函数来实现的。阅读Framework层源码发现，Android为我们提供了一个HandlerThread类，该类继承Thread类，并使用上面两个函数创建Looper对象，而且使用wait/notifyAll解决了多线程中子线程1获取子线程2的Looper对象为空的问题

为了方便描述，这里附上一段demo：

void useHanlerThread() {
 
    // two ways to construct HandlerThread
    HandlerThread hThread = new HandlerThread(”AnyName“,
 
            // a property in android.os.Process
            Process.THREAD_PRIORITY_BACKGROUND);
 
    // HandlerThread hThread2 = new HandlerThread(”AnyName“);
    hThread.start();
 
    // get Looper object of hThread(HandlerThread)
    Looper looper = hThread.getLooper();
 
    Handler h = new Handler(looper) {
        @Override
        public void handleMessage(Message msg) {
            // TODO Auto-generated method stub
            super.handleMessage(msg);
        }
    };
 
}

由上面代码第4行可以看到，这里采用HandlerThread新建一个thread对象，实际上他就是一个Thread，只不过经过了一些封装，它封装了looper等，所以该实例也就是一个worker thread，也即新建了一个线程，只是不需要维护Looper，内部封装好了，当调用HandlerThread的start方法时，就会执行重写的run方法，进而可以直接通过第13行代码的getLooper获取，然后根据获取的这个looper来实例化Handler，这样这个Handler就工作于前面创建的HandlerThread实例中，也就是说相比传统的方式，这里可以直接处理耗时逻辑而无需再新建线程

 这里再附上一段样例代码来辅助理解：

public void startWorkerHandler(){
        HandlerThread handlerThread = new HandlerThread("handler-thread");
        handlerThread.start();

        final Handler handler = new Handler(handlerThread.getLooper());
        handler.post(new Runnable() {
            @Override
            public void run() {

                npb.increaseProgressBy(1);
                handler.postDelayed(this, 100);
            }
        });
    }

 实例：

package com.example.randy.helloworld;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Looper;

/**
 * Created by randy on 2015/12/7.
 */
public class HandlerDemo extends Activity {
    private Handler handler1;

    private Handler handler2;

    private HandlerThread handlerThread;
    private Handler handler3;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        handler1 = new Handler();
        new Thread(new Runnable() {
            @Override
            public void run() {
                Looper.prepare();
                handler2 = new Handler();
                Looper.loop();
            }
        }).start();

       /* new Thread(new Runnable(){
            @Override
            public void run(){
                handlerThread = new HandlerThread("handlerThread");
                handlerThread.start();
            }
        }).start();*/

        new HandlerThread("handlerThread"){

            @Override
            public void run(){

                handler3 = new Handler();
            }
        }.start();
    }
}