How to start android service from another Android app

Start your service like this

Intent intent = new Intent();
intent.setComponent(new ComponentName("pkg", "cls"));
ComponentName c = getApplicationContext().startForegroundService(intent);

btw you actually need to use the applicationId, instead of the pkg. it can be found in the app gradle. I was struggling with that mistake for hours!

   defaultConfig {
        applicationId "com.xxx.zzz"
}

the cls is the name of your service declared in the manifest. example: com.xxx.yyy.yourService.

 <service android:name="com.xxx.yyy.yourService"
android:exported="true"/>

You should be able to start your service like this:

Intent i = new Intent();
i.setComponent(new ComponentName("com.xxx.yyy", "com.xxx.yyy.SyncService"));
ComponentName c = ctx.startService(i);

You don't need to set ACTION or CATEGORY if you are specifying a specific component. Make sure that your service is properly defined in the manifest.

As an addition to David Wasser's answer to make it work when targeting API 30 and above you also have to add either:

Required package name in queries tag in manifest:

<queries>
        <package android:name="com.example.service.owner.app" />
</queries>

or permission

<uses-permission android:name="android.permission.QUERY_ALL_PACKAGES" />

Additional info on package visibility changes here