AndroidManifest.xml. Services
* can be started with
* {@link android.content.Context#startService Context.startService()} and
* {@link android.content.Context#bindService Context.bindService()}.
*
* Note that services, like other application objects, run in the main * thread of their hosting process. This means that, if your service is going * to do any CPU intensive (such as MP3 playback) or blocking (such as * networking) operations, it should spawn its own thread in which to do that * work. More information on this can be found in * Application Fundamentals: * Processes and Threads. The {@link IntentService} class is available * as a standard implementation of Service that has its own thread where it * schedules its work to be done.
* *The Service class is an important part of an * application's overall lifecycle.
* *Topics covered here: *
Most confusion about the Service class actually revolves around what * it is not:
* *Thus a Service itself is actually very simple, providing two main features:
* *When a Service component is actually created, for either of these reasons, * all that the system actually does is instantiate the component * and call its {@link #onCreate} and any other appropriate callbacks on the * main thread. It is up to the Service to implement these with the appropriate * behavior, such as creating a secondary thread in which it does its work.
* *Note that because Service itself is so simple, you can make your * interaction with it as simple or complicated as you want: from treating it * as a local Java object that you make direct method calls on (as illustrated * by Local Service Sample), to providing * a full remoteable interface using AIDL.
* * *There are two reasons that a service can be run by the system. If someone * calls {@link android.content.Context#startService Context.startService()} then the system will * retrieve the service (creating it and calling its {@link #onCreate} method * if needed) and then call its {@link #onStartCommand} method with the * arguments supplied by the client. The service will at this point continue * running until {@link android.content.Context#stopService Context.stopService()} or * {@link #stopSelf()} is called. Note that multiple calls to * Context.startService() do not nest (though they do result in multiple corresponding * calls to onStartCommand()), so no matter how many times it is started a service * will be stopped once Context.stopService() or stopSelf() is called; however, * services can use their {@link #stopSelf(int)} method to ensure the service is * not stopped until started intents have been processed. * *
For started services, there are two additional major modes of operation * they can decide to run in, depending on the value they return from * onStartCommand(): {@link #START_STICKY} is used for services that are * explicitly started and stopped as needed, while {@link #START_NOT_STICKY} * or {@link #START_REDELIVER_INTENT} are used for services that should only * remain running while processing any commands sent to them. See the linked * documentation for more detail on the semantics. * *
Clients can also use {@link android.content.Context#bindService Context.bindService()} to * obtain a persistent connection to a service. This likewise creates the * service if it is not already running (calling {@link #onCreate} while * doing so), but does not call onStartCommand(). The client will receive the * {@link android.os.IBinder} object that the service returns from its * {@link #onBind} method, allowing the client to then make calls back * to the service. The service will remain running as long as the connection * is established (whether or not the client retains a reference on the * service's IBinder). Usually the IBinder returned is for a complex * interface that has been written * in aidl. * *
A service can be both started and have connections bound to it. In such * a case, the system will keep the service running as long as either it is * started or there are one or more connections to it with the * {@link android.content.Context#BIND_AUTO_CREATE Context.BIND_AUTO_CREATE} * flag. Once neither * of these situations hold, the service's {@link #onDestroy} method is called * and the service is effectively terminated. All cleanup (stopping threads, * unregistering receivers) should be complete upon returning from onDestroy(). * * *
Global access to a service can be enforced when it is declared in its * manifest's {@link android.R.styleable#AndroidManifestService <service>} * tag. By doing so, other applications will need to declare a corresponding * {@link android.R.styleable#AndroidManifestUsesPermission <uses-permission>} * element in their own manifest to be able to start, stop, or bind to * the service. * *
In addition, a service can protect individual IPC calls into it with * permissions, by calling the * {@link #checkCallingPermission} * method before executing the implementation of that call. * *
See the Security and Permissions * document for more information on permissions and security in general. * * *
The Android system will attempt to keep the process hosting a service * around as long as the service has been started or has clients bound to it. * When running low on memory and needing to kill existing processes, the * priority of a process hosting the service will be the higher of the * following possibilities: * *
If the service is currently executing code in its * {@link #onCreate onCreate()}, {@link #onStartCommand onStartCommand()}, * or {@link #onDestroy onDestroy()} methods, then the hosting process will * be a foreground process to ensure this code can execute without * being killed. *
If the service has been started, then its hosting process is considered * to be less important than any processes that are currently visible to the * user on-screen, but more important than any process not visible. Because * only a few processes are generally visible to the user, this means that * the service should not be killed except in extreme low memory conditions. *
If there are clients bound to the service, then the service's hosting * process is never less important than the most important client. That is, * if one of its clients is visible to the user, then the service itself is * considered to be visible. *
A started service can use the {@link #startForeground(int, Notification)} * API to put the service in a foreground state, where the system considers * it to be something the user is actively aware of and thus not a candidate * for killing when low on memory. (It is still theoretically possible for * the service to be killed under extreme memory pressure from the current * foreground application, but in practice this should not be a concern.) *
Note this means that most of the time your service is running, it may * be killed by the system if it is under heavy memory pressure. If this * happens, the system will later try to restart the service. An important * consequence of this is that if you implement {@link #onStartCommand onStartCommand()} * to schedule work to be done asynchronously or in another thread, then you * may want to use {@link #START_FLAG_REDELIVERY} to have the system * re-deliver an Intent for you so that it does not get lost if your service * is killed while processing it. * *
Other application components running in the same process as the service * (such as an {@link android.app.Activity}) can, of course, increase the * importance of the overall * process beyond just the importance of the service itself. * * *
One of the most common uses of a Service is as a secondary component * running alongside other parts of an application, in the same process as * the rest of the components. All components of an .apk run in the same * process unless explicitly stated otherwise, so this is a typical situation. * *
When used in this way, by assuming the * components are in the same process, you can greatly simplify the interaction * between them: clients of the service can simply cast the IBinder they * receive from it to a concrete class published by the service. * *
An example of this use of a Service is shown here. First is the Service * itself, publishing a custom class when bound: * * {@sample development/samples/ApiDemos/src/com/example/android/apis/app/LocalService.java * service} * *
With that done, one can now write client code that directly accesses the * running service, such as: * * {@sample development/samples/ApiDemos/src/com/example/android/apis/app/LocalServiceActivities.java * bind} * * *
If you need to be able to write a Service that can perform complicated * communication with clients in remote processes (beyond simply the use of * {@link Context#startService(Intent) Context.startService} to send * commands to it), then you can use the {@link android.os.Messenger} class * instead of writing full AIDL files. * *
An example of a Service that uses Messenger as its client interface * is shown here. First is the Service itself, publishing a Messenger to * an internal Handler when bound: * * {@sample development/samples/ApiDemos/src/com/example/android/apis/app/MessengerService.java * service} * *
If we want to make this service run in a remote process (instead of the
* standard one for its .apk), we can use android:process in its
* manifest tag to specify one:
*
* {@sample development/samples/ApiDemos/AndroidManifest.xml remote_service_declaration}
*
*
Note that the name "remote" chosen here is arbitrary, and you can use * other names if you want additional processes. The ':' prefix appends the * name to your package's standard process name. * *
With that done, clients can now bind to the service and send messages * to it. Note that this allows clients to register with it to receive * messages back as well: * * {@sample development/samples/ApiDemos/src/com/example/android/apis/app/MessengerServiceActivities.java * bind} */ public abstract class Service extends ContextWrapper implements ComponentCallbacks { private static final String TAG = "Service"; public Service() { super(null); } /** Return the application that owns this service. */ public final Application getApplication() { return mApplication; } /** * Called by the system when the service is first created. Do not call this method directly. */ public void onCreate() { } /** * @deprecated Implement {@link #onStartCommand(Intent, int, int)} instead. */ @Deprecated public void onStart(Intent intent, int startId) { } /** * Bits returned by {@link #onStartCommand} describing how to continue * the service if it is killed. May be {@link #START_STICKY}, * {@link #START_NOT_STICKY}, {@link #START_REDELIVER_INTENT}, * or {@link #START_STICKY_COMPATIBILITY}. */ public static final int START_CONTINUATION_MASK = 0xf; /** * Constant to return from {@link #onStartCommand}: compatibility * version of {@link #START_STICKY} that does not guarantee that * {@link #onStartCommand} will be called again after being killed. */ public static final int START_STICKY_COMPATIBILITY = 0; /** * Constant to return from {@link #onStartCommand}: if this service's * process is killed while it is started (after returning from * {@link #onStartCommand}), then leave it in the started state but * don't retain this delivered intent. Later the system will try to * re-create the service. Because it is in the started state, it will * guarantee to call {@link #onStartCommand} after creating the new * service instance; if there are not any pending start commands to be * delivered to the service, it will be called with a null intent * object, so you must take care to check for this. * *
This mode makes sense for things that will be explicitly started * and stopped to run for arbitrary periods of time, such as a service * performing background music playback. */ public static final int START_STICKY = 1; /** * Constant to return from {@link #onStartCommand}: if this service's * process is killed while it is started (after returning from * {@link #onStartCommand}), and there are no new start intents to * deliver to it, then take the service out of the started state and * don't recreate until a future explicit call to * {@link Context#startService Context.startService(Intent)}. The * service will not receive a {@link #onStartCommand(Intent, int, int)} * call with a null Intent because it will not be re-started if there * are no pending Intents to deliver. * *
This mode makes sense for things that want to do some work as a * result of being started, but can be stopped when under memory pressure * and will explicit start themselves again later to do more work. An * example of such a service would be one that polls for data from * a server: it could schedule an alarm to poll every N minutes by having * the alarm start its service. When its {@link #onStartCommand} is * called from the alarm, it schedules a new alarm for N minutes later, * and spawns a thread to do its networking. If its process is killed * while doing that check, the service will not be restarted until the * alarm goes off. */ public static final int START_NOT_STICKY = 2; /** * Constant to return from {@link #onStartCommand}: if this service's * process is killed while it is started (after returning from * {@link #onStartCommand}), then it will be scheduled for a restart * and the last delivered Intent re-delivered to it again via * {@link #onStartCommand}. This Intent will remain scheduled for * redelivery until the service calls {@link #stopSelf(int)} with the * start ID provided to {@link #onStartCommand}. The * service will not receive a {@link #onStartCommand(Intent, int, int)} * call with a null Intent because it will will only be re-started if * it is not finished processing all Intents sent to it (and any such * pending events will be delivered at the point of restart). */ public static final int START_REDELIVER_INTENT = 3; /** * This flag is set in {@link #onStartCommand} if the Intent is a * re-delivery of a previously delivered intent, because the service * had previously returned {@link #START_REDELIVER_INTENT} but had been * killed before calling {@link #stopSelf(int)} for that Intent. */ public static final int START_FLAG_REDELIVERY = 0x0001; /** * This flag is set in {@link #onStartCommand} if the Intent is a * a retry because the original attempt never got to or returned from * {@link #onStartCommand(Intent, int, int)}. */ public static final int START_FLAG_RETRY = 0x0002; /** * Called by the system every time a client explicitly starts the service by calling * {@link android.content.Context#startService}, providing the arguments it supplied and a * unique integer token representing the start request. Do not call this method directly. * *
For backwards compatibility, the default implementation calls * {@link #onStart} and returns either {@link #START_STICKY} * or {@link #START_STICKY_COMPATIBILITY}. * *
If you need your application to run on platform versions prior to API
* level 5, you can use the following model to handle the older {@link #onStart}
* callback in that case. The handleCommand method is implemented by
* you as appropriate:
*
* {@sample development/samples/ApiDemos/src/com/example/android/apis/app/ForegroundService.java
* start_compatibility}
*
*
Note that the system calls this on your * service's main thread. A service's main thread is the same * thread where UI operations take place for Activities running in the * same process. You should always avoid stalling the main * thread's event loop. When doing long-running operations, * network calls, or heavy disk I/O, you should kick off a new * thread, or use {@link android.os.AsyncTask}.
* * @param intent The Intent supplied to {@link android.content.Context#startService}, * as given. This may be null if the service is being restarted after * its process has gone away, and it had previously returned anything * except {@link #START_STICKY_COMPATIBILITY}. * @param flags Additional data about this start request. Currently either * 0, {@link #START_FLAG_REDELIVERY}, or {@link #START_FLAG_RETRY}. * @param startId A unique integer representing this specific request to * start. Use with {@link #stopSelfResult(int)}. * * @return The return value indicates what semantics the system should * use for the service's current started state. It may be one of the * constants associated with the {@link #START_CONTINUATION_MASK} bits. * * @see #stopSelfResult(int) */ public int onStartCommand(Intent intent, int flags, int startId) { onStart(intent, startId); return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY; } /** * Called by the system to notify a Service that it is no longer used and is being removed. The * service should clean up an resources it holds (threads, registered * receivers, etc) at this point. Upon return, there will be no more calls * in to this Service object and it is effectively dead. Do not call this method directly. */ public void onDestroy() { } public void onConfigurationChanged(Configuration newConfig) { } public void onLowMemory() { } /** * Return the communication channel to the service. May return null if * clients can not bind to the service. The returned * {@link android.os.IBinder} is usually for a complex interface * that has been described using * aidl. * *Note that unlike other application components, calls on to the * IBinder interface returned here may not happen on the main thread * of the process. More information about this can be found * in Application Fundamentals: * Processes and Threads.
* * @param intent The Intent that was used to bind to this service, * as given to {@link android.content.Context#bindService * Context.bindService}. Note that any extras that were included with * the Intent at that point will not be seen here. * * @return Return an IBinder through which clients can call on to the * service. */ public abstract IBinder onBind(Intent intent); /** * Called when all clients have disconnected from a particular interface * published by the service. The default implementation does nothing and * returns false. * * @param intent The Intent that was used to bind to this service, * as given to {@link android.content.Context#bindService * Context.bindService}. Note that any extras that were included with * the Intent at that point will not be seen here. * * @return Return true if you would like to have the service's * {@link #onRebind} method later called when new clients bind to it. */ public boolean onUnbind(Intent intent) { return false; } /** * Called when new clients have connected to the service, after it had * previously been notified that all had disconnected in its * {@link #onUnbind}. This will only be called if the implementation * of {@link #onUnbind} was overridden to return true. * * @param intent The Intent that was used to bind to this service, * as given to {@link android.content.Context#bindService * Context.bindService}. Note that any extras that were included with * the Intent at that point will not be seen here. */ public void onRebind(Intent intent) { } /** * Stop the service, if it was previously started. This is the same as * calling {@link android.content.Context#stopService} for this particular service. * * @see #stopSelfResult(int) */ public final void stopSelf() { stopSelf(-1); } /** * Old version of {@link #stopSelfResult} that doesn't return a result. * * @see #stopSelfResult */ public final void stopSelf(int startId) { if (mActivityManager == null) { return; } try { mActivityManager.stopServiceToken( new ComponentName(this, mClassName), mToken, startId); } catch (RemoteException ex) { } } /** * Stop the service if the most recent time it was started was * startId. This is the same as calling {@link * android.content.Context#stopService} for this particular service but allows you to * safely avoid stopping if there is a start request from a client that you * haven't yet seen in {@link #onStart}. * *Be careful about ordering of your calls to this function.. * If you call this function with the most-recently received ID before * you have called it for previously received IDs, the service will be * immediately stopped anyway. If you may end up processing IDs out * of order (such as by dispatching them on separate threads), then you * are responsible for stopping them in the same order you received them.
* * @param startId The most recent start identifier received in {@link * #onStart}. * @return Returns true if the startId matches the last start request * and the service will be stopped, else false. * * @see #stopSelf() */ public final boolean stopSelfResult(int startId) { if (mActivityManager == null) { return false; } try { return mActivityManager.stopServiceToken( new ComponentName(this, mClassName), mToken, startId); } catch (RemoteException ex) { } return false; } /** * @deprecated This is a now a no-op, use * {@link #startForeground(int, Notification)} instead. This method * has been turned into a no-op rather than simply being deprecated * because analysis of numerous poorly behaving devices has shown that * increasingly often the trouble is being caused in part by applications * that are abusing it. Thus, given a choice between introducing * problems in existing applications using this API (by allowing them to * be killed when they would like to avoid it), vs allowing the performance * of the entire system to be decreased, this method was deemed less * important. */ @Deprecated public final void setForeground(boolean isForeground) { Log.w(TAG, "setForeground: ignoring old API call on " + getClass().getName()); } /** * Make this service run in the foreground, supplying the ongoing * notification to be shown to the user while in this state. * By default services are background, meaning that if the system needs to * kill them to reclaim more memory (such as to display a large page in a * web browser), they can be killed without too much harm. You can set this * flag if killing your service would be disruptive to the user, such as * if your service is performing background music playback, so the user * would notice if their music stopped playing. * *If you need your application to run on platform versions prior to API
* level 5, you can use the following model to call the the older {@link #setForeground}
* or this modern method as appropriate:
*
* {@sample development/samples/ApiDemos/src/com/example/android/apis/app/ForegroundService.java
* foreground_compatibility}
*
* @param id The identifier for this notification as per
* {@link NotificationManager#notify(int, Notification)
* NotificationManager.notify(int, Notification)}.
* @param notification The Notification to be displayed.
*
* @see #stopForeground(boolean)
*/
public final void startForeground(int id, Notification notification) {
try {
mActivityManager.setServiceForeground(
new ComponentName(this, mClassName), mToken, id,
notification, true);
} catch (RemoteException ex) {
}
}
/**
* Remove this service from foreground state, allowing it to be killed if
* more memory is needed.
* @param removeNotification If true, the notification previously provided
* to {@link #startForeground} will be removed. Otherwise it will remain
* until a later call removes it (or the service is destroyed).
* @see #startForeground(int, Notification)
*/
public final void stopForeground(boolean removeNotification) {
try {
mActivityManager.setServiceForeground(
new ComponentName(this, mClassName), mToken, 0, null,
removeNotification);
} catch (RemoteException ex) {
}
}
/**
* Print the Service's state into the given stream. This gets invoked if
* you run "adb shell dumpsys activity service