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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.common.reflection;

import com.sun.source.tree.Tree;
import java.util.List;
import javax.lang.model.element.AnnotationMirror;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.common.basetype.BaseTypeValidator;
import org.checkerframework.common.basetype.BaseTypeVisitor;
import org.checkerframework.common.reflection.qual.ClassBound;
import org.checkerframework.common.reflection.qual.ClassVal;
import org.checkerframework.framework.source.Result;
import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;

public class ClassValVisitor extends BaseTypeVisitor {
    public ClassValVisitor(BaseTypeChecker checker) {
        super(checker);
    }

    @Override
    protected ClassValAnnotatedTypeFactory createTypeFactory() {
        return new ClassValAnnotatedTypeFactory(checker);
    }

    @Override
    protected BaseTypeValidator createTypeValidator() {
        return new ClassNameValidator(checker, this, atypeFactory);
    }
}

class ClassNameValidator extends BaseTypeValidator {

    public ClassNameValidator(
            BaseTypeChecker checker,
            BaseTypeVisitor visitor,
            AnnotatedTypeFactory atypeFactory) {
        super(checker, visitor, atypeFactory);
    }

    /**
     * Reports an "illegal.classname" error if the type contains a classVal annotation with
     * classNames that cannot possibly be valid class annotations.
     */
    @Override
    public boolean isValid(AnnotatedTypeMirror type, Tree tree) {
        AnnotationMirror classVal = type.getAnnotation(ClassVal.class);
        classVal = classVal == null ? type.getAnnotation(ClassBound.class) : classVal;
        if (classVal != null) {
            List classNames =
                    ClassValAnnotatedTypeFactory.getClassNamesFromAnnotation(classVal);
            for (String className : classNames) {
                if (!isLegalClassName(className)) {
                    checker.report(Result.failure("illegal.classname", className, type), tree);
                }
            }
        }
        return super.isValid(type, tree);
    }

    /**
     * A string is a legal binary name if it has the following form: ((Java identifier)\.)*(Java
     * identifier)([])* https://docs.oracle.com/javase/specs/jls/se10/html/jls-13.html#jls-13.1
     *
     * @param className string to check
     * @return true if className is a legal class name
     */
    private boolean isLegalClassName(String className) {
        int lastBracket = className.lastIndexOf("]");
        if (lastBracket != -1 && lastBracket != className.length() - 1) {
            return false;
        }
        className = className.replaceAll("\\[\\]", "");
        String[] identifiers = className.split("(\\.)");
        for (String identifier : identifiers) {
            if (!isJavaIdentifier(identifier)) {
                return false;
            }
        }
        return true;
    }

    /**
     * Whether the given string is a Java Identifier. (This method returns true if the Identifier is
     * a keyword, boolean literal, null literal.
     */
    private boolean isJavaIdentifier(String identifier) {
        char[] identifierChars = identifier.toCharArray();
        if (!(identifierChars.length > 0 && Character.isJavaIdentifierStart(identifierChars[0]))) {
            return false;
        }
        for (int i = 1; i < identifierChars.length; i++) {
            if (!Character.isJavaIdentifierPart(identifierChars[i])) {
                return false;
            }
        }
        return true;
    }
}