Android的Service

 

关于Service的一些介绍参见：

http://blog.sina.com.cn/s/blog_5f8817250100mis2.html

Service的使用参见：

http://www.pin5i.com/showtopic-use-android-service.html

1.Context作用的说明

Interface to global information about an application environment.  This is an abstract class whose implementation is provided by the Android system.  It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.

2.Service的说明

 

A Service is an application component representing either an application's desire to perform a longer-running operation while not interacting with the user or to supply functionality for other applications to use.  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  Processes and Threads.  The IntentService class is available as a standard implementation of Service that has its own thread where it schedules its work to be done.

Topics covered here: 

1. What is a Service? 
2. Service Lifecycle 
3. Permissions 
4. Process Lifecycle 
5. Local Service Sample 
6. Remote Messenger Service Sample 

Developer Guides

For a detailed discussion about how to create services, read the  Services developer guide.

 

What is a Service?

Most confusion about the Service class actually revolves around what it is not:

• A Service is not a separate process.  The Service object itself does not imply it is running in its own process; unless otherwise specified, it runs in the same process as the application it is part of. 
• A Service is not a thread.  It is not a means itself to do work off of the main thread (to avoid Application Not Responding errors). 

Thus a Service itself is actually very simple, providing two main features:

• A facility for the application to tell the system about something it wants to be doing in the background (even when the user is not directly interacting with the application).  This corresponds to calls to Context.startService(), which ask the system to schedule work for the service, to be run until the service or someone else explicitly stop it. 
• A facility for an application to expose some of its functionality to other applications.  This corresponds to calls to Context.bindService(), which allows a long-standing connection to be made to the service in order to interact with it. 

When a Service component is actually created, for either of these reasons, all that the system actually does is instantiate the component and call itsonCreate() 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 byLocal Service Sample), to providing a full remoteable interface using AIDL.

 

Service Lifecycle

There are two reasons that a service can be run by the system.  If someone callsContext.startService() then the system will retrieve the service (creating it and calling itsonCreate() method if needed) and then call its onStartCommand(android.content.Intent,int,int) method with the arguments supplied by the client.  The service will at this point continue running untilContext.stopService() or 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 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():START_STICKY is used for services that are explicitly started and stopped as needed, while START_NOT_STICKY orSTART_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 Context.bindService() to obtain a persistent connection to a service.  This likewise creates the service if it is not already running (callingonCreate() while doing so), but does not call onStartCommand().  The client will receive the android.os.IBinder object that the service returns from its  onBind(android.content.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 startedor there are one or more connections to it with the Context.BIND_AUTO_CREATE flag.  Once neither of these situations hold, the service'sonDestroy() method is called and the service is effectively terminated.  All cleanup (stopping threads, unregistering receivers) should be complete upon returning from onDestroy().   

Permissions

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. 

As of android.os.Build.VERSION_CODES.GINGERBREAD, when using  Context.startService(Intent), you can also set Intent.FLAG_GRANT_READ_URI_PERMISSION and/orIntent.FLAG_GRANT_WRITE_URI_PERMISSION on the Intent.  This will grant the Service temporary access to the specific URIs in the Intent.  Access will remain until the Service has calledstopSelf(int) for that start command or a later one, or until the Service has been completely stopped. This works for granting access to the other apps that have not requested the permission protecting the Service, or even when the Service is not exported at all. 

In addition, a service can protect individual IPC calls into it with permissions, by calling the android.content.ContextWrapper.checkCallingPermission(java.lang.String) method before executing the implementation of that call.  

See the Security and Permissions document for more information on permissions and security in general.   

Process Lifecycle

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(), onStartCommand(), 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. 

• A started service can use the startForeground(int,android.app.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 implementonStartCommand() to schedule work to be done asynchronously or in another thread, then you may want to useSTART_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 anActivity) can, of course, increase the importance of the overall process beyond just the importance of the service itself.   

Local Service Sample

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:   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:   development/samples/ApiDemos/src/com/example/android/apis/app/LocalServiceActivities.java      bind   

Remote Messenger Service Sample

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 Context.startService to send commands to it), then you can use theandroid.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:   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 useandroid:process in its manifest tag to specify one:   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:   development/samples/ApiDemos/src/com/example/android/apis/app/MessengerServiceActivities.java      bind

 

 

android.app.Service是Context的一个派生类

public abstract class Service extends ContextWrapper implements ComponentCallbacks2

 启动的时候是通过：

调用Context->startService(Intent service)来启动

 

Request that a given application service be started.  The Intent can either contain the complete class name of a specific service implementation to start, or an abstract definition through the action and other fields of the kind of service to start.  If this service is not already running, it will be instantiated and started (creating a process for it if needed); if it is running then it remains running. 

Every call to this method will result in a corresponding call to the target service'sandroid.app.Service.onStartCommand(android.content.Intent,int,int) method, with the intent given here.  This provides a convenient way to submit jobs to a service without having to bind and call on to its interface. 

Using startService() overrides the default service lifetime that is managed bybindService(android.content.Intent,android.content.ServiceConnection,int): it requires the service to remain running until stopService(android.content.Intent) is called, regardless of whether any clients are connected to it.  Note that calls to startService() are not nesting: no matter how many times you call startService(), a single call tostopService(android.content.Intent) will stop it. 

The system attempts to keep running services around as much as possible.  The only time they should be stopped is if the current foreground application is using so many resources that the service needs to be killed.  If any errors happen in the service's process, it will automatically be restarted. 

This function will throw java.lang.SecurityException if you do not have permission to start the given service. 

Parameters:

service Identifies the service to be started.  The Intent may      specify either an explicit component name to start, or a logical      description (action, category, etc) to match an      IntentFilter published by a service.  Additional values      may be included in the Intent extras to supply arguments along with      this specific start call.

Returns:

If the service is being started or is already running, the  ComponentName of the actual service that was started is returned; else if the service does not exist null is returned.

Throws:

java.lang.SecurityException

See also:

stopService(android.content.Intent)

bindService(android.content.Intent,android.content.ServiceConnection,int)

    public abstract ComponentName startService(Intent service);

所以为了了解这种启动方式，还要了解下Intent的本质，这个有时间了再看吧