Android SDK里面很多類名都起的讓人傻傻分不清楚，本篇文章就是從IntentService這個(gè)組件的生命周期出發(fā)，深度剖析ThreadLocal、HandlerThread以及IntentService之間錯(cuò)綜復(fù)雜的關(guān)系。

首先了解概念

ThreadLocal：線程的本地變量。該變量不與其他線程共享，只能在本線程內(nèi)操作。

HandlerThread：一種經(jīng)過包裝的Thread。隨著該thread啟動(dòng)，其內(nèi)部的Looper會(huì)自動(dòng)開始循環(huán)。

IntentService：一種經(jīng)過包裝的Service。該Service內(nèi)部會(huì)自動(dòng)開啟一個(gè)線程用來執(zhí)行任務(wù)，并在任務(wù)結(jié)束后自動(dòng)結(jié)束。

敲黑板！記住結(jié)論：

1. HandlerThread本質(zhì)上仍是一個(gè)Thread。
2. IntentService本質(zhì)上仍是一個(gè)Service。

IntentService的生命周期

既然IntentService也是個(gè)Service，那么就從它的onCreate方法看起。

IntentService的onCreate方法.png

1. 實(shí)例化了一個(gè)HandlerThread，并啟動(dòng)該線程。
2. 獲取該HandlerThread中自帶的Looper對(duì)象。
3. 將獲取到的Looper對(duì)象作為參數(shù)用來實(shí)例化ServiceHandler。

HandlerThread部分源碼.png

首先，通過閱讀HandlerThread的源碼，可以發(fā)現(xiàn)HandlerThread在run方法中會(huì)幫我們創(chuàng)建好一個(gè)Looper并讓該Looper執(zhí)行l(wèi)oop方法。在深入理解Handler、Looper與MessageQueue之間的關(guān)系一文中，我們詳細(xì)的描述了Handler、Looper以及MesageQueue三者之間的關(guān)系。Looper循環(huán)是Handler能在線程中運(yùn)行的前提條件。
所以我們可以理解為，IntentService的onCreate方法中所用到的HandlerThread，相當(dāng)于是為IntentService快速的構(gòu)建并啟動(dòng)了一個(gè)帶有Looper的工作線程。

Looper部分源碼.png

最后一點(diǎn)，就是ServiceHandler。

ServiceHandler源碼.png

下面放上IntentService的完整代碼：

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;

    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public IntentService(String name) {
        super();
        mName = name;
    }

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     * with your preferred semantics.
     *
     * <p>If enabled is true,
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
     * {@link #onHandleIntent(Intent)} returns, the process will be restarted
     * and the intent redelivered.  If multiple Intents have been sent, only
     * the most recent one is guaranteed to be redelivered.
     *
     * <p>If enabled is false (the default),
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     */
    public void setIntentRedelivery(boolean enabled) {
        mRedelivery = enabled;
    }

    @Override
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    /**
     * You should not override this method for your IntentService. Instead,
     * override {@link #onHandleIntent}, which the system calls when the IntentService
     * receives a start request.
     * @see android.app.Service#onStartCommand
     */
    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        mServiceLooper.quit();
    }

    /**
     * Unless you provide binding for your service, you don't need to implement this
     * method, because the default implementation returns null.
     * @see android.app.Service#onBind
     */
    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     * This method is invoked on the worker thread with a request to process.
     * Only one Intent is processed at a time, but the processing happens on a
     * worker thread that runs independently from other application logic.
     * So, if this code takes a long time, it will hold up other requests to
     * the same IntentService, but it will not hold up anything else.
     * When all requests have been handled, the IntentService stops itself,
     * so you should not call {@link #stopSelf}.
     *
     * @param intent The value passed to {@link
     *               android.content.Context#startService(Intent)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

最后結(jié)合代碼完整的梳理一遍IntentService的執(zhí)行過程：
①首先IntentService執(zhí)行onCreate方法，該方法中會(huì)創(chuàng)建一個(gè)HandlerThread線程并實(shí)例化一個(gè)內(nèi)部類ServiceHandler。
②隨后，onStartCommand方法會(huì)調(diào)用onStart方法，在onStart方法內(nèi)部，會(huì)構(gòu)造一個(gè)Message，然后用ServiceHandler來發(fā)送該Message。
③再之后，ServiceHandler的handleMessage方法會(huì)對(duì)發(fā)來的Mesage進(jìn)行處理，具體的處理邏輯則是交給抽象方法onHandleIntent來完成，而這個(gè)方法正是使用者使用IntentService必須要實(shí)現(xiàn)的方法，使用者應(yīng)該將該Service要執(zhí)行的任務(wù)寫在該抽象方法的實(shí)現(xiàn)中。
④在onHandleIntent方法完成后，便會(huì)執(zhí)行Service的stopSelf(int id)方法，來結(jié)束該IntentService。
⑤最后，在IntentService的onDestroy方法里，通過執(zhí)行mServiceLooper的quit()方法，終止掉該Looper循環(huán)，也順帶結(jié)束了該IntentService對(duì)應(yīng)的工作線程。

通過整體流程分析，我們可以發(fā)現(xiàn)，IntentService為我們封裝好了工作線程的初始化、啟動(dòng)以及結(jié)束相關(guān)的代碼，使得開發(fā)者能專注于業(yè)務(wù)邏輯的編寫，減少了忘記結(jié)束Service所帶來的內(nèi)存泄漏的威脅。