服务（Service）是Android中实现程序后台运行的解决方案，它的运行不依赖于任何用户界面，适合去执行那些不需要和用户交互且需要长期运行的任务。
注意
- Service并不是运行在一个独立的进程中，而是依赖于创建服务时所在的应用程序进程。当某个应用程序进程被杀掉时，所有依赖于该进程的服务也会停止运行。
- 服务并不会自动开启线程，所有的代码都默认运行在主线程中。所以，我们需要在服务内部手动创建子线程，并在这里执行具体的任务，避免出现主线程阻塞的情况。
生命周期
onCreate()：服务第一次创建的时候调用的
onStartCommand()：每次启动服务的时候都会调用
onBind()：绑定服务的时候调用
onUnBind()：解绑服务的时候调用
onDestory()：销毁服务的时候调用
补充：
- 每个服务都只会存在一个实例，不管调用了多少次startService or bindService方法，只需要调用一次stopService（或stopSelf）or unbindService方法，服务就会停止。
- 一个服务只要被启动或被绑定之后，就会一直处于运行状态，必须让上述两种条件同时不满足，服务才能被销毁。
启动方式对比
| 启动方式 | 终止方式 | 特点 |
|---|---|---|
| startService() | 启动服务的控件调用stopService or 服务内部自己调用stopSelf or 依赖的进程销毁 | 不需要获取服务的数据，服务不随控件的销毁而终止 |
| bindService() | 绑定服务的控件调用unbindService or 绑定服务的控件销毁 or 依赖的进程销毁 | 需要获取服务的数据，服务随着控件的销毁而终止 |
注意：
- 没有调用bindService直接调用unbindService会出现异常：
java.lang.IllegalArgumentException: Service not registered；
没有调用startService直接调用stopService则没事
服务的分类
前台服务了解
前台服务会一直有一个正在运行的图标在系统的状态栏显示，下拉状态栏可以看到更加详细的信息，类似于通知的效果。使用前台服务，可以防止服务由于系统内存不足等原因被回收掉，而且可以满足一些特殊需求显示信息。
public class MyService extends Service {
@Override
public void onCreate() {
super.onCreate();
Intent intent = new Intent(this, MainActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
Notification notification = new NotificationCompat.Builder(this)
.setContentTitle("this is title")
.setContentText("I'm SO HAPPY")
.setWhen(System.currentTimeMillis())
.setSmallIcon(R.mipmap.smile)
.setContentIntent(pendingIntent)
.build();
startForeground((int) (Math.random() * 100), notification);
}
}
IntentService了解
IntentService是一个用于处理异步请求的基类，客户端通过startService发送请求。该服务按需启动，用子线程依次处理每个Intent，并且当它执行完成时自动停止。内部实现机制为Handler。
/**
* IntentService is a base class for {@link Service}s that handle asynchronous
* requests (expressed as {@link Intent}s) on demand. Clients send requests
* through {@link android.content.Context#startService(Intent)} calls; the
* service is started as needed, handles each Intent in turn using a worker
* thread, and stops itself when it runs out of work.
*
* This "work queue processor" pattern is commonly used to offload tasks
* from an application's main thread. The IntentService class exists to
* simplify this pattern and take care of the mechanics. To use it, extend
* IntentService and implement {@link #onHandleIntent(Intent)}. IntentService
* will receive the Intents, launch a worker thread, and stop the service as
* appropriate.
*
*
All requests are handled on a single worker thread -- they may take as
* long as necessary (and will not block the application's main loop), but
* only one request will be processed at a time.
*
* @see android.os.AsyncTask
*/
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);
}