安卓IntentService 源码分析

今天偶然看到IntentService 源码，看了一会觉得很好玩也有很多启动知识点，遂记录下。

Handler用法
volatile 修饰符
abstract 抽象类的加深复习
Handler.obtainMessage() 的意义
synchronized 同步锁的使用

源码如下

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.     *     * 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.     *     * 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);}

Handler用法

内部类继承了Handler，并且持有了一个Handler 对象。onHandleIntent()方法是IntentService 抽象类的抽象方法，继承了IntentService 就必须实现该方法，并且在handleMessage 里调用了该方法。

private volatile ServiceHandler mServiceHandler;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);        }    }

看到这里 Handler.obtainMessage()

      @Override    public void onStart(@Nullable Intent intent, int startId) {        Message msg = mServiceHandler.obtainMessage();        msg.arg1 = startId;        msg.obj = intent;        mServiceHandler.sendMessage(msg);    }    public final Message obtainMessage(){        return Message.obtain(this);    }     public static Message obtain(Handler h) {        Message m = obtain();        m.target = h;        return m;    }    /**     * Return a new Message instance from the global pool. Allows us to     * avoid allocating new objects in many cases.     */    public static Message obtain() {        synchronized (sPoolSync) {            if (sPool != null) {                Message m = sPool;                sPool = m.next;                m.next = null;                m.flags = 0; // clear in-use flag                sPoolSize--;                return m;            }        }        return new Message();    }

从全局池中返回一个新的Message实例，允许我们避免在很多情况下分配新的对象。
这里是从MessagePool 中取一个Message，区分于我们自己 new Message

volatile

static也是各个业务方可以去全局修改；
volatile是处理多线程锁的替代方案，对应有时需要实时的修改共享资源的变量，被volatile修复的变量的值可以立刻被业务方取得最新的值。

不过，猛地感觉，nnd，这不是一样么，static是静态的，所以理论上也可以在不同线程去访问，能访问也就是能修改，所以这里老穆在网上搜了搜相关的资料，把这个知识点在加强下：

变量放在主存区上，使用该变量的每个线程，都将从主存区拷贝一份到自己的工作区上进行操作。

volatile, 声明这个字段易变（可能被多个线程使用），Java内存模型负责各个线程的工作区与主存区的该字段的值保持同步，即一致性。

static, 声明这个字段是静态的（可能被多个实例共享），在主存区上该类的所有实例的该字段为同一个变量，即唯一性。

volatile, 声明变量值的一致性；static,声明变量的唯一性。

此外，volatile同步机制不同于synchronized, 前者是内存同步，后者不仅包含内存同步（一致性），且保证线程互斥（互斥性）。
static 只是声明变量在主存上的唯一性，不能保证工作区与主存区变量值的一致性；除非变量的值是不可变的，即再加上final的修饰符，否则static声明的变量，不是线程安全的。

下面摘自Java语言规范(Java Language Specification)的官方解释：

1) If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created.

2) A field may be declared volatile, in which case the Java Memory Model ensures that all threads see a consistent value for the variable。

——转自 http://blog.sina.com.cn/s/blog_4e1e357d0101i486.html

synchronized 同步锁

—— 待更新

安卓IntentService 源码启迪

安卓IntentService 源码分析

目录

源码如下

Handler用法

volatile

synchronized 同步锁

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