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In the Androird,
service is a none-UI background process that is doing some specific jobs. 6.1 Example Program : Understanding of Android Service. 6.2 Classification of Android Services * Android Service * System Service * Java System Service * Core Platform Service * Hardware Service * Native System Service * Application Service * Local Service * Remote Service 6.3 Android Application Service * It's one of the android application components. * Usage * Service start/end : Start/End a service on the background. * Remote control through binding : If a service client binds to the service, the client can control functions of the service through an interface provided by service while the binding is maintained.
 6.3.1 Separation of Application Service ※ The example of this chapter is in the API demos of Android 2.2. A basis of this separation is whether an application and a service that is created by application is in same process or separate. * The local service ends when application ends. but the remote service don't end though application ends. * The difference between local service and remote service is the binding way for the service control. - Local Service : Because the client program and service are on the same process, so local service binding needs only reference of local service that is bound by client. - Remote Service : For using a service, Activity must use IPC mechanism. In this case, binder IPC is used. When it sends or receives between service and activity in the binder IPC communication, it must go through marshalling or unmarshalling, for this process it uses the AIDL. ※ AIDL(Android Interface Definition Language)
 (1) Try to local binding through bindSercive() API. * bindService(Intent, ServiceConnection, int) - Intent : An intent for local service. - ServiceConnection : On the service client side, it processes the binding connection with service. - int : Context.BIND_AUTO_CREATE is a flag that creates a service automatically, if there is not service to bind. (2) For the binding processing, android calls the onBind() callback method in service. * the onBind() method returns a LocalBinder object that is extended for connecting with activity and itself. (3) Android framework calls the onServiceConnected(ComponentName, IBinder) method in the side of service client. * mBoundService = ((LocalService.LocalBinder)service).getService()
- The IBinder argument is returned from onBinder() method.
(4) Save a reference value of the LocalService into the mBoundService
member field in activity.  (1) Binding Activity : Requesting to connect with RemoteService * Using the bindService() API for the control. * The different part with local binding is in manifest file that includes processing about com.example.android.apis.app.ISecondary action. (2) RemoteService Service : Implementing the Actual Service Method Function and Providing the Binder Object for communicating with Service. * Main roles of the onBind() method is for creating the service binder object for processing the binder IPC and returning it. * The binder service object is created through the automatically created ISecondary.stub class of ISecondary.java file. And when the binder service object is created, implement the getPid() method(or whatever) as the actual code. (3) Binding Activity : Create the Proxy Object for Processing the Service and the Binder IPC. * When the onServiceConnected() call-back method of ServiceConnection object is called, the second IBinder typed argument is passed to ISecondary.Stub.asInterface() function. And then created and returned ISecondary.Stub.Proxy proxy service object is saved into mSecondaryService member variable. * It's the end of binding process. And after this process, a activity can call methods of RemoteService like it owns them. (4) Binding Activity : Through the Using of Proxy Object, Call The Service Proxy Method in the Remote Service. (5) Binder IPC : Pass the Binder IPC Data from the Service Proxy Object to Service Binder Object. (6) RemoteService Service : Call the Stub Method of RemoteService Service. ※ The Creation Classification of Local Service With Remote Service. 6.4 Android System Service * Android system services are in the application framework layer and libraries layer. 6.4.1 Classification of System Service (1) Native System Service * Representatively, audio flinger service and surface flinger service are exist. (2) Java System Service A. Core Platform Service B. Hardware Service
* Can use the system service directly with getSystemService() method. Because, the system service is launched by init process on the booting process. * The system service is started by the media server and the system server. - The media server process : Execute the native services like audio server or media player server. - The system server process : This process is on the java based, and created first by Zygote. : Execute native system services like the surface flinger and etc. * Creation Process on the Android System Booting (1) Start the Media Server Processes. (2) Start the Zygote. (3) Start the System Server. (4) Start the Surface Flinger Services. (5) Start the Java System Services. 6.5.1 Media Server Execution Code Analysis  * It makes the native services(Audio Flinger, Media Player Service, Camera Service, Audio Policy Service, etc) started by the media server. (2) Create and Initialize the Native Service Instance. * Do this in the main function of media server. (3) Look up the Initializing Code in Each System Services. * Using binder IPC. * Service provider like system service have to register information to manifest manager. * Each initializing codes are shaped of same form. just using new and addService() each services' functions to context manager. * defaultServiceManager() function returns an object, service manager is sort of proxy object doing binder communication with the context manager. * It has simple mechanism. Just create each instance of service, and register to context manager. 6.5.2 Analysis of Running Code of Server (1) Created by Zygote. (2) Load android_servers library. * In the main() method of SystemServer class, system_init() JNI function is called by the init1() method. Because, SystemServer is a Java process. So can't call the c++ based custom service. * After start a service, call the callStatic() function to call the init2() method. (3) Initialize and Register the Java System Service. * In the init2() method, Create and run the ServerThread that creates all of the Java system service. * Register to ContextManager as like native system services, but use the addService() static method in the ServiceManager class. |