Android-Adding SystemService

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.

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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

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

/*
 * 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
For IRC #rowboat on irc.freenode.net

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