Writing custom lint warning to check for custom an

I have written the following annotation:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.SOURCE)
@Target({ElementType.METHOD})
public @interface Warning {

}

Which is intended to annotate methods which can cause problems if called carelessly. I added an annotation processor to my project, but this only provides the warning in the log output of the javac command. I want this warning to appear in Android Studio along with the other lint warnings anywhere a method with this annotation is called. This is why I am trying to write a custom lint rule. I have the basic skeleton of the lint rule:

import com.android.tools.lint.detector.api.Category;
import com.android.tools.lint.detector.api.Detector;
import com.android.tools.lint.detector.api.Implementation;
import com.android.tools.lint.detector.api.Issue;
import com.android.tools.lint.detector.api.Scope;
import com.android.tools.lint.detector.api.Severity;

public class CaimitoDetector extends Detector implements Detector.JavaScanner {

  public static final Issue ISSUE = Issue.create(
      "WarningAnnotation",
      "This method has been annotated with @Warning",
      "This method has special conditions surrounding it's use, be careful when using it and refer to its documentation.",
      Category.USABILITY, 7, Severity.WARNING,
      new Implementation(CaimitoDetector.class, Scope.JAVA_FILE_SCOPE));

  @Override
  public void visitMethod(JavaContext context, AstVisitor visitor, MethodInvocation node) {

  }

}

import com.android.tools.lint.client.api.IssueRegistry;
import com.android.tools.lint.detector.api.Issue;

import java.util.Collections;
import java.util.List;

public class CaimitoIssueRegistry extends IssueRegistry {

  @Override
  public List<Issue> getIssues() {
    return Collections.singletonList(CaimitoDetector.ISSUE);
  }

}

But I do not know how to proceed from here. How can I check if an annoation exists on a method, and raise a warning such that it will be visible in Android Studio?

But I do not know how to proceed from here

I suggest to write a test for your Detector first. Here is an example project which demonstrates how to write Detector tests [1]. That way you can try and adjust your Detector as you like.

How can I check if an annoation exists on a method

I suggest to have a look at Android's default detectors [2]. There you'll most probably find a good point to start. E.g. the AnnotationDetector.

and raise a warning such that it will be visible in Android Studio?

If you integrate your custom rules correctly into your project, then Lint will raise the warning for you. Please have a look here [3] for different options on how to integrate custom rules in your project. Note: AFAIK warnings of custom rules will only reported when running the corresponding Gradle task. The "auto-highlight" of Android Studio does not work with custom rules.

https://github.com/a11n/CustomLintRules
https://android.googlesource.com/platform/tools/base/+/master/lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks
https://github.com/a11n/android-lint/tree/master/6_application