Android-Adding SystemService

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This wiki page will demonstrate - "How to add system service to android framework". Example - "Adding a Bluetooth HID service" - taken as reference of understanding.This will also help to add support for more bluetooth profiles into android framework.

What is service?

As per the definition given at http://developer.android.com/guide/topics/fundamentals/services.html

A Service is an application component that can perform long-running operations in the background and does not provide a user interface. Another application component can start a service and it will continue to run in the background even if the user switches to another application. Additionally, a component can bind to a service to interact with it and even perform interprocess communication (IPC). For example, a service might handle network transactions, play music, perform file I/O, or interact with a content provider, all from the background.

Service layer

Customservice.png

Create service

  • Add your code to frameworks/base/services/java/com/android/server/
/*TestService.java */
package com.android.server;
import android.content.Context;
import android.os.Handler;
import android.os.ITestService;
import android.os.Looper;
import android.os.Message;
import android.os.Process;
import android.util.Log;
public class TestService extends ITestService.Stub {
    private static final String TAG = "TestService";
    private TestWorkerThread mWorker;
    private TestWorkerHandler mHandler;
    private Context mContext;
    public TestService(Context context) {
        super();
        mContext = context;
        mWorker = new TestWorkerThread("TestServiceWorker");
        mWorker.start();
        Log.i(TAG, "Spawned worker thread");
    }
 
    public void setValue(int val) {
        Log.i(TAG, "setValue " + val);
        Message msg = Message.obtain();
        msg.what = TestWorkerHandler.MESSAGE_SET;
        msg.arg1 = val;
        mHandler.sendMessage(msg);
    }
 
    private class TestWorkerThread extends Thread {
        public TestWorkerThread(String name) {
            super(name);
        }
        public void run() {
            Looper.prepare();
            mHandler = new TestWorkerHandler();
            Looper.loop();
        }
    }
 
    private class TestWorkerHandler extends Handler {
        private static final int MESSAGE_SET = 0;
        @Override
        public void handleMessage(Message msg) {
            try {
                if (msg.what == MESSAGE_SET) {
                    Log.i(TAG, "set message received: " + msg.arg1);
                }
            } catch (Exception e) {
                // Log, don't crash!
                Log.e(TAG, "Exception in TestWorkerHandler.handleMessage:", e);
            }
        }
    }
}

Register service

  • Register service in SystemServer.java
/*
 * go to function "@Override public void run()"
 * ........
 * Add following block after line "if (factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {"
 */
 
try {
    Slog.i(TAG, "Test Service");
    ServiceManager.addService(“Test”, new TestService(context));
} catch (Throwable e) {
    Slog.e(TAG, "Failure starting TestService Service", e);
}

Expose service

  • A service can expose set of functions that can be access by other process/application. Exposed functions are required to be declared in .aidl file at following location

frameworks/base/core/java/android/os/[server].aidl

/*
* aidl file : frameworks/base/core/java/android/os/ITestService.aidl
* This file contains definitions of functions which are exposed by service
*/
package android.os;
interface ITestService {
/**
* {@hide}
*/
	void setValue(int val);
}

Add [service].aidl for build

/*
 * open frameworks/base/Android.mk and add following line
 */
...
core/java/android/os/IPowerManager.aidl \
core/java/android/os/ITestService.aidl \
core/java/android/os/IRemoteCallback.aidl \
...
  • Rebuild the framework/base or android system.Service is now ready to use by other application/process.

Using service

To use service

  • first get service handle using "ServiceManager.getService()" api
  • use service handle to call set of functions exposed by service

Below is the sample code to use service.

/*
 * HelloServer.java
 */
package com.Test.helloserver;
import android.app.Activity;
import android.os.Bundle;
import android.os.ServiceManager;
import android.os.ITestService;
import android.util.Log;
public class HelloServer extends Activity {
    private static final String DTAG = "HelloServer";
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
 
        ITestService om = ITestService.Stub.asInterface(ServiceManager.getService("Test"));
        try {
            Log.d(DTAG, "Going to call service");
            om.setValue(20);
            Log.d(DTAG, "Service called succesfully");
        }
        catch (Exception e) {
            Log.d(DTAG, "FAILED to call service");
            e.printStackTrace();
        }
    }
}

References

Support

For community support join http://groups.google.com/group/rowboat
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