How CallSuper annotation evaluates whether overridden method is calling the super method or not?

Question

I just found curious how my andorid studio is able to notify me when in my onCreate() method of Activity when I fail to call super.onCreate() then came to know that Android (too smart) implemented annotation CallSuper to make sure that the overridden method is calling the super method.

I just went into CallSuper to see the magic of what's happening internally,

package androidx.annotation;

import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.CLASS;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

/**
 * Denotes that any overriding methods should invoke this method as well.
 * <p>
 * Example:
 * <pre><code>
 *  &#64;CallSuper
 *  public abstract void onFocusLost();
 * </code></pre>
 */
@Documented
@Retention(CLASS)
@Target({METHOD})
public @interface CallSuper {
}

Source code reference : https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/annotation/CallSuper.java But to my surprise it does nothing. Where does it evaluate the method against this annotation? How compiler is able to do that? I'm quite curious.

Answer 1

It works via a lint rule bundled into the android developer tools.

It's not just the @CallSuper annotation acting alone. The annotation works as an identifier for the accompanying lint rule. Check out the source of the CallSuperDetector.