As per the definition given at http://developer./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

1. 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();
        }
    }
}