如何检查自定义注释?

今天我创建了一个hello world应用程序。 这不是一个简单的hello world应用程序。 它使用类、自定义异常、线程、泛型、自定义注释等。 这里的问题是,检查注释是否应用良好的AnnotationApplyer类什么也不做。

我试图创建一个注释,说明该类是用户定义的异常(扩展了java.lang.exception类)。 这不起作用,我在检查时犯了错误还是别的什么?

import java.lang.annotation.*;

@UserException // wrong placed annotation for testing
@UserDefined(user = "iasonas")
public abstract class Main {

    public static void main(String[] args) {
        // check annotations
        AnnotationApplyer.check();

        // start the program
        new Greeter().greet();
    }
    
    @UserDefined(user = "iasonas")
    private static class Greeter {
        private final String MSG = "Hello, World!";
        private Printer ms = new MessageService<String>(MSG);
        
        public Greeter () {
        }
        
        public void greet () {
            try {
                ms.print();
            } catch (InvalidMessageException e) {
                System.out.println("Cannot greet. "+e.toString());
            }
        }
    };
};

@UserException // wrong placed annotation for testing
@UserDefined(user = "iasonas")
class AnnotationApplyer {
    public static void check () {
        checkUserException(Main.class, AnnotationApplyer.class, MessageService.class, UserDefined.class);
        checkUserException(UserException.class, Printer.class, InvalidMessageException.class);
        
    }

    public static void checkUserException (Class<?>... classes) {
        for (Class<?> clazz : classes) {
            if (!clazz.isAnnotationPresent(UserException.class)) {
                continue;
            }

            if (!Exception.class.isAssignableFrom(clazz)) {
                System.out.println("Error: "+clazz.getName()+" does not extend java.lang.Exception");
            } else {
                System.out.println("Class "+clazz.getName()+" extends java.lang.Exception!");
            }
        }
    }
}

@Target(ElementType.TYPE)
@interface UserException {

}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface UserDefined {
    String user() default "Not specified";
}

class MessageService <T> implements Runnable, Printer {
    private T message;
    private final Thread thread = new Thread(this);
    private int printCounter;
    
    public MessageService () {   
    }
    
    public MessageService (T message) {
        this.message = message;
    }

    public T getMessage () {
        return message;
    }
    
    public void setMessage (T aMsg) {
        this.message = aMsg;
    }

    public int getTimes () {
        return printCounter;
    }
    
    @Override
    public void run () {
        System.out.println(message);
        printCounter++;
    }
    
    @Override
    public synchronized void print () throws InvalidMessageException {
        if (message==null) {
            throw new InvalidMessageException("message is null");
        }
        thread.start();
    }
};

interface Printer {
    void print() throws InvalidMessageException;
};

@UserException // well placed annotation for checking
class InvalidMessageException extends Exception {
    public InvalidMessageException (String m) {
        super(m);
        System.exit(0);
    }
}