In ASM, a ClassVisitor is a class that can be used to visit the elements of a Java class and generate an in-memory representation of the class.

Here is an example of how you might use a ClassVisitor to read and print the fields of a Java class:

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;

public class Example {
    public static void main(String[] args) throws Exception {
        // Read the class file as a byte array
        byte[] bytes = readClassFile();

        // Use the ClassReader to parse the bytecode
        ClassReader cr = new ClassReader(bytes);

        // Create a ClassVisitor that will be used to visit the fields of the class
        ClassVisitor cv = new ClassVisitor(Opcodes.ASM8) {
            @Override
            public FieldVisitor visitField(int access, String name, String desc, String signature, Object value) {
                // Print the field information
                System.out.println("Field: " + name + " " + desc + " " + value);

                // Return a FieldVisitor that will be used to visit the metadata of the field
                return new FieldVisitor(Opcodes.ASM8) {
                    // Override the visitAnnotation method to print the field annotations
                    @Override
                    public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
                        System.out.println("  Annotation: " + desc + " " + visible);
                        return null;
                    }
                };
            }
        };

        // Have the ClassReader accept the ClassVisitor, which will cause it to call the appropriate methods
        cr.accept(cv, ClassReader.SKIP_DEBUG);
    }
}

In this example, the ClassReader is used to parse the bytecode of the class and the ClassVisitor is used to visit the fields of the class. The FieldVisitor is used to visit the metadata of each field, and the visitAnnotation method is overridden to print the field annotations. When the accept method is called on the ClassReader, it will call the appropriate methods on the ClassVisitor, which will in turn call the appropriate methods on the FieldVisitor.