java.lang.Object | ||||
android.content.Context | ||||
android.content.ContextWrapper | ||||
android.app.Service | ComponentCallbacks |
A Service is an application component that runs in the background, not
interacting with the user, for an indefinite period of time. Each service
class must have a corresponding
<service>
declaration in its package's AndroidManifest.xml
. Services
can be started with
Context.startService() and
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 the Threading section of the Application Model overview.
The Service class is an important part of an application's overall lifecycle.
Topics covered here:
There are two reasons that a service can be run by the system. If someone calls Context.startService() then the system will retrieve the service (creating it and calling its onCreate() method if needed) and then call its onStart(Intent, int) method with the arguments supplied by the client. The service will at this point continue running until Context.stopService() or stopSelf() is called. Note that multiple calls to Context.startService() do not nest (though they do result in multiple corresponding calls to onStart()), so no matter how many times it is started a service will be stopped once Context.stopService() or stopSelf() is called.
Clients can also use Context.bindService() to obtain a persistent connection to a service. This likewise creates the service if it is not already running (calling onCreate() while doing so), but does not call onStart(). The client will receive the IBinder object that the service returns from its onBind(Intent) 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 Context.BIND_AUTO_CREATE flag. Once neither of these situations hold, the service's 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 <service> tag. By doing so, other applications will need to declare a corresponding <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 checkCallingPermission(String) method before executing the implementation of that call.
See the Security Model 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 onCreate(), onStart(), or 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.
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 onStart() to schedule work to be done asynchronously or in another thread, then you may want to write information about that work into persistent storage during the onStart() call so that it does not get lost if the service later gets killed.
Other application components running in the same process as the service (such as an Activity) can, of course, increase the importance of the overall process beyond just the importance of the service itself.
Service() |
final | Application | getApplication() | ||||
Return the application that owns this service. | ||||||
abstract | IBinder | onBind(Intent intent) | ||||
Return the communication channel to the service. | ||||||
void | onConfigurationChanged(Configuration newConfig) | |||||
Called by the system when the device configuration changes while your component is running. | ||||||
void | onCreate() | |||||
Called by the system when the service is first created. | ||||||
void | onDestroy() | |||||
Called by the system to notify a Service that it is no longer used and is being removed. | ||||||
void | onLowMemory() | |||||
This is called when the overall system is running low on memory, and would like actively running process to try to tighten their belt. | ||||||
void | onRebind(Intent intent) | |||||
Called when new clients have connected to the service, after it had previously been notified that all had disconnected in its onUnbind(Intent). | ||||||
void | onStart(Intent intent, int startId) | |||||
Called by the system every time a client explicitly starts the service by calling startService(Intent), providing the arguments it supplied and a unique integer token representing the start request. | ||||||
boolean | onUnbind(Intent intent) | |||||
Called when all clients have disconnected from a particular interface published by the service. | ||||||
final | void | setForeground(boolean isForeground) | ||||
Control whether this service is considered to be a foreground service. | ||||||
final | void | stopSelf() | ||||
Stop the service, if it was previously started. | ||||||
final | void | stopSelf(int startId) | ||||
Stop the service, if the most recent time it was started was startId. |
void | dump(FileDescriptor fd, PrintWriter writer, String[] args) | |||||
Print the Service's state into the given stream. | ||||||
void | finalize() | |||||
Called by the virtual machine when there are no longer any (non-weak) references to the receiver. |
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 the Threading section of the Application Model overview.
intent | The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here. |
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At the time that this function has been called, your Resources object will have been updated to return resource values matching the new configuration.
Applications that want to be nice can implement this method to release any caches or other unnecessary resources they may be holding on to. The system will perform a gc for you after returning from this method.
intent | The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here. |
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intent | The Intent supplied to startService(Intent), as given. |
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startId | A unique integer representing this specific request to start. Use with stopSelf(int). |
intent | The Intent that was used to bind to this service, as given to Context.bindService. Note that any extras that were included with the Intent at that point will not be seen here. |
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isForeground | Determines whether this service is considered to be foreground (true) or background (false). |
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startId | The most recent start identifier received in onStart(Intent, int). |
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fd | The raw file descriptor that the dump is being sent to. |
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writer | The PrintWriter to which you should dump your state. This will be closed for you after you return. |
args | additional arguments to the dump request. |
Note: The virtual machine assumes that the implementation in class Object is empty.
Throwable |
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Copyright 2007 Google Inc. | Build 0.9_r1-98467 - 14 Aug 2008 18:56 |