安卓IntentService 源码启迪

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

目录

安卓IntentService 源码分析

目录

源码如下
Handler用法

volatile

synchronized 同步锁

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.
*
* <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);

}

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