Services typically are required to run for a long time and hence should run in their own thread. Such services can be invoked by any number of clients who want to connect to the service, invoke a few methods on the service and finally release the service, probably to serve more clients or close down.
Here, I would like to introduce you to the concept of connecting to a remote service and the kind of support provided by the android platform for the same.
We have earlier seen how local services can be created and used. The difference between the two mainly is that the local service runs in the same process as the application that started it and hence the life of the local service is dependent on the life of the said application while remote service can run in its own process. This causes a challenge of inter-process communication. If one process wants to communicate with another process, the object that is passed between the two needs to be marshaled.
For this purpose, Android provides the AIDL (Android Interface Definition Language) tool that handles the marshaling as well as the communication.
The service has to declare a service interface in an aidl file and the AIDL tool will automatically create a java interface corresponding to the aidl file. The AIDL tool also generates a stub class that provides an abstract implementation of the service interface methods. The actual service class will have to extend this stub class to provide the real implementation of the methods exposed through the interface.
The service clients will have to invoke the onBind() method on the service to be able to connect to the service. The onBind() method returns an object of the stub class to the client. Here are the code related code snippets:
The AIDL file:
package com.collabera.labs.sai;
interface IMyRemoteService {
int getCounter();
}
Once you write this AIDL file (.aidl) in eclipse, it will automatically generate the Remote interface corresponding to this file. The remote interface will also provide a stub inner class which has to have an implementation provided by the RemoteService class. The stub class implementation within the service class is as given here:
private IMyRemoteService.Stub myRemoteServiceStub = newIMyRemoteService.Stub() {
public int getCounter() throws RemoteException {
return counter;
}
};
The onBind() method in the service class:
public IBinder onBind(Intent arg0) {
Log.d(getClass().getSimpleName(), "onBind()");
return myRemoteServiceStub;
}
Now, let us quickly look at the meat of the service class before we move on to how the client connects to this service class. My RemoteService class is just incrementing a counter in a separate thread. This thread is created in the onStart()method as this gets certainly called whether the service is connected to by a call to startService(intent).Please read the lifecycle of a service if this needs more clarity. Here are the over-ridden onCreate(), onStart()and onDestroy()methods. Note that the resources are all released in the onDestroy()method.
public void onCreate() {
super.onCreate();
Log.d(getClass().getSimpleName(),"onCreate()");
}
public void onStart(Intent intent, int startId) {
super.onStart(intent, startId);
serviceHandler = new Handler();
serviceHandler.postDelayed(myTask, 1000L);
Log.d(getClass().getSimpleName(), "onStart()");
}
public void onDestroy() {
super.onDestroy();
serviceHandler.removeCallbacks(myTask);
serviceHandler = null;
Log.d(getClass().getSimpleName(),"onDestroy()");
}
A little explanation: In the onStart() method, I created a new Handler object that will spawn out a new task that implements the Runnableinterface. This thread does the job of incrementing the counter. Here is the code for the Task class – an inner class of the RemoteServiceclass.
class Task implements Runnable {
public void run() {
++counter;
serviceHandler.postDelayed(this,1000L);
Log.i(getClass().getSimpleName(), "Incrementing counter in the run method");
}
}
An object of this Taskclass is passed to the serviceHandler object as a message that needs to be executed after 1 second. The Taskclass implements the run() method in which we repeatedly post the same message to the serviceHandler. Thus, this becomes a repeated task till all the messages in the serviceHandlerqueue are deleted by calling the removeCallbacks()method on the serviceHandler in the destroy()method of the RemoteService class.
Note that the onDestroy()method thus stops this thread and set the serviceHandlerto null. This completes the implementation of the RemoteServiceclass. The complete code is downloadable here.
Now coming to the client class - Here, for simplicity sake, I have put the start, stop, bind, release and invoke methods all in the same client. While in reality, one client may start and another can bind to the already started service.
There are 5 buttons one each for start, stop, bind, release and invoke actions. A client needs to bind to a service before it can invoke any method on the service.
Here are the start and the bind methods.
private void startService(){
if (started) {
Toast.makeText(RemoteServiceClient.this, "Service already started", Toast.LENGTH_SHORT).show();
} else {
Intent i = new Intent();
i.setClassName("com.collabera.labs.sai", "com.collabera.labs.sai.RemoteService");
startService(i);
started = true;
updateServiceStatus();
Log.d( getClass().getSimpleName(), "startService()" );
}
}
Rest of the code is to update some status on the UI. There is nothing specific to a remote service invocation here. It is on the bindService()method that we see the difference from a local service.
private void bindService() {
if(conn == null) {
conn = newRemoteServiceConnection();
Intent i = new Intent();
i.setClassName("com.collabera.labs.sai", "com.collabera.labs.sai.RemoteService");
bindService(i, conn, Context.BIND_AUTO_CREATE);
updateServiceStatus();
Log.d( getClass().getSimpleName(), "bindService()" );
} else {
Toast.makeText(RemoteServiceClient.this, "Cannot bind - service already bound", Toast.LENGTH_SHORT).show();
}
}
Here we get a connection to the remote service through the RemoteServiceConnection class which implements ServiceConnection Interface. The connection object is required by the bindService()method – an intent, connection object and the type of binding are to be specified. So, how do we create a connection to the RemoteService? Here is the implementation:
class RemoteServiceConnection implements ServiceConnection {
public voidonServiceConnected(ComponentName className,
IBinder boundService ) {
remoteService = IMyRemoteService.Stub.asInterface((IBinder)boundService);
Log.d( getClass().getSimpleName(), "onServiceConnected()" );
}
public voidonServiceDisconnected(ComponentName className) {
remoteService = null;
updateServiceStatus();
Log.d( getClass().getSimpleName(), "onServiceDisconnected" );
}
};
The Context.BIND_AUTO_CREATE ensures that a service is created if one did not exist although the onstart() will be called only on explicit start of the service.
Once the client is bound to the service and the service has already started, we can invoke any of the methods that are exposed by the service. Here we have only one method and that is getCounter(). In this example, the invocation is done by clicking the invoke button. That would update the counter text that is below the button.
Let us see the invoke method:
private void invokeService() {
if(conn == null) {
Toast.makeText(RemoteServiceClient.this, "Cannot invoke - service not bound", Toast.LENGTH_SHORT).show();
} else {
try {
int counter = remoteService.getCounter();
TextView t = (TextView)findViewById(R.id.notApplicable);
t.setText( "Counter value: "+Integer.toString( counter ) );
Log.d( getClass().getSimpleName(), "invokeService()" );
} catch (RemoteException re) {
Log.e( getClass().getSimpleName(), "RemoteException" );
}
}
}
Once we use the service methods, we can release the service. This is done as follows (by clicking the release button):
private void releaseService() {
if(conn != null) {
unbindService(conn);
conn = null;
updateServiceStatus();
Log.d( getClass().getSimpleName(), "releaseService()" );
} else {
Toast.makeText(RemoteServiceClient.this, "Cannot unbind - service not bound", Toast.LENGTH_SHORT).show();
}
}
Finally we can stop the service by clicking the stop button. After this point no client can invoke this service.
private void stopService() {
if (!started) {
Toast.makeText(RemoteServiceClient.this, "Service not yet started", Toast.LENGTH_SHORT).show();
} else {
Intent i = new Intent();
i.setClassName("com.collabera.labs.sai", "com.collabera.labs.sai.RemoteService");
stopService(i);
started = false;
updateServiceStatus();
Log.d( getClass().getSimpleName(), "stopService()" );
}
}
These are the basics of working with a remote service on Android platform. All the best!
Addendum (updated on 6 Jan 2011) – based on many questions related to Remote services. If the client accessing the remote service is in a separate package or application, it has to include the .aidl file along with the package structure as in the Service provider.
I have written a sample remote client in a completely different application, while the remote service is the same one provided above. You may download the client app here.
