概要介紹

HandlerThread是一種特殊的Thread叶摄，也就是有Looper的thread狈究，既然有Looper的話碗淌，那我們就可以用此Looper來(lái)創(chuàng)建一個(gè)Handler，從而實(shí)現(xiàn)和它的交互抖锥。比如你可以通過(guò)與它關(guān)聯(lián)的Handler對(duì)象在UI線程中發(fā)消息給它處理亿眠。HandlerThread一般可以用來(lái)執(zhí)行某些background的操作，比如讀寫文件磅废、進(jìn)行一些計(jì)算（在此HandlerThread而非UI線程中）纳像。既然還是一個(gè)Thread系瓢，那么和一般的Thread一樣溶浴，也要通過(guò)調(diào)用其start()方法來(lái)啟動(dòng)它。它只是Android替我們封裝的一個(gè)Helper類，其源碼相當(dāng)簡(jiǎn)潔荆忍，有一些經(jīng)典的標(biāo)準(zhǔn)寫法禁灼，我們一起來(lái)看看昵仅。

源碼分析

和以往一樣退渗，我們先來(lái)看看其字段和構(gòu)造函數(shù)：

    int mPriority; // 線程優(yōu)先級(jí)
    int mTid = -1; // 線程id
    Looper mLooper; // 與此線程關(guān)聯(lián)的Looper

    public HandlerThread(String name) { // 提供個(gè)名字，方便debug
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT; // 沒(méi)提供犀勒，則使用默認(rèn)優(yōu)先級(jí)
    }
    
    /**
     * 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; // 使用用戶提供的優(yōu)先級(jí)屎飘，取Process.java中定義的常量
    }

代碼很簡(jiǎn)單，相關(guān)的分析都直接寫在代碼的注釋里了贾费，值得注意的是這里的priority是基于Linux優(yōu)先級(jí)的钦购，而不是Java Thread類里的MIN_PRIORITY，NORM_PRIORITY铸本，MAX_PRIORITY之類肮雨，請(qǐng)注意區(qū)分（其實(shí)認(rèn)真閱讀方法的doc即可）遵堵。

接下來(lái)看看此類的3個(gè)關(guān)鍵方法：

    /**
     * Call back method that can be explicitly overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() {
        // callback方法箱玷，如果你愿意可以O(shè)verride放自己的邏輯；其在looper開(kāi)始前執(zhí)行
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare(); // 此方法我們前面介紹過(guò)陌宿，會(huì)創(chuàng)建與線程關(guān)聯(lián)的Looper對(duì)象
        synchronized (this) { // 進(jìn)入同步塊锡足，當(dāng)mLooper變的可用的使用，調(diào)用notifyAll通知其他可能block在當(dāng)前對(duì)象上的線程
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority); // 設(shè)置線程優(yōu)先級(jí)
        onLooperPrepared(); // 調(diào)用回調(diào)函數(shù)
        Looper.loop(); // 開(kāi)始loop
        mTid = -1; // reset為invalid值
    }
    
    /**
     * 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()) { // 如果線程不是在alive狀態(tài)則直接返回null壳坪，有可能是你忘記調(diào)start方法了舶得。。爽蝴。
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) { // 進(jìn)入同步塊沐批，當(dāng)條件不滿足時(shí)無(wú)限等待，
                try {                              // 直到mLooper被設(shè)置成有效值了才退出while（當(dāng)然也可能是線程狀態(tài)不滿足）蝎亚；
                    wait();                        // run方法里的notifyAll就是用來(lái)喚醒這里的
                } catch (InterruptedException e) { // 忽略InterruptedException
                }
            }
        }
        return mLooper; // 最后返回mLooper九孩，此時(shí)可以保證是有效值了。
    }

當(dāng)你new一個(gè)HandlerThread的對(duì)象時(shí)記得調(diào)用其start()方法发框，然后你可以接著調(diào)用其getLooper()方法來(lái)new一個(gè)Handler對(duì)象躺彬，最后你就可以利用此Handler對(duì)象來(lái)往HandlerThread發(fā)送消息來(lái)讓它為你干活了。
　　
最后來(lái)看2個(gè)退出HandlerThread的方法梅惯，其實(shí)對(duì)應(yīng)的是Looper的2個(gè)退出方法：

    /**
     * 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(); // 注意這里是調(diào)用getLooper而不是直接使用mLooper宪拥，
        if (looper != null) {        // 因?yàn)閙Looper可能還沒(méi)初始化完成，而調(diào)用方法可以
            looper.quit();           // 等待初始化完成铣减。
            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;
    }

通過(guò)代碼我們可以看到其內(nèi)部都是delegate給了Looper對(duì)象她君，而Looper我們?cè)谇懊嬉步榻B過(guò)了，感興趣的同學(xué)可以翻看前面的分析或者查看這2個(gè)方法的doc葫哗，寫的都很詳細(xì)犁河。
　　至此這個(gè)簡(jiǎn)單的Handy class就算分析完畢了鳖枕。在實(shí)際的開(kāi)發(fā)中，如果你只是要做某些后臺(tái)的操作（短暫的桨螺，比如把某些設(shè)置文件load到內(nèi)存中）宾符，而不需要更新UI的話，那你可以優(yōu)先使用HandlerThread而不是AsyncTask灭翔。