Android Services
Android services are system component which allows performing a
longer-running operation while not interacting with the user.
Just like Activities, Service runs on Main Thread and has a life cycle but unlike Activity, Services do not have a UI.
Anyone can use & create Service or its direct subclasses IntentServcice in their Apps and it works pretty well.
Common usages are: Playing Media, Downloading files from the Internet, etc.
System Services
System services are a direct derivative of SystemService class. They reside in com.android.server packagein AOSP tree
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/services/
System Services also run in Main Thread so if you want to do some CPU intensive task then you much reuse HandlerThread architecture of AOSP.
What makes System service different from Android Services?
System Services are started by SystemServer hence they run as a System process which gives them additional privileges which normal Android Service will never get hence they are named name SystemService.
Example: You can't use Instrumentation to inject events in App other than yours because you will need INJECT_EVENT permission which is not granted to normal apps. SystemService can do that because they have elevated access.
The simple example of System Service :
package com.android.server.appwidget;
import android.content.Context;
import com.android.server.AppWidgetBackupBridge;
import com.android.server.FgThread;
import com.android.server.SystemService;
/**
* SystemService that publishes an IAppWidgetService.
*/
public class AppWidgetService extends SystemService {
private final AppWidgetServiceImpl mImpl;
public AppWidgetService(Context context) {
super(context);
mImpl = new AppWidgetServiceImpl(context);
}
@Override
public void onStart() {
mImpl.onStart();
publishBinderService(Context.APPWIDGET_SERVICE, mImpl);
AppWidgetBackupBridge.register(mImpl);
}
@Override
public void onBootPhase(int phase) {
if (phase == PHASE_ACTIVITY_MANAGER_READY) {
mImpl.setSafeMode(isSafeMode());
}
}
@Override
public void onStopUser(int userHandle) {
mImpl.onUserStopped(userHandle);
}
@Override
public void onSwitchUser(int userHandle) {
mImpl.reloadWidgetsMaskedStateForGroup(userHandle);
}
}
