自定义注解校验失效：如何正确检查Java自定义注解？

核心问题：注解保留策略缺失 #

你的@UserException注解没有指定运行时保留策略，Java中注解默认的保留策略是RetentionPolicy.CLASS——这种策略下，注解仅在编译期可见，编译后的class文件中会保留注解，但运行时反射无法读取到它。这直接导致clazz.isAnnotationPresent(UserException.class)始终返回false，校验逻辑完全不执行。

修复步骤 #

1. 给@UserException添加@Retention(RetentionPolicy.RUNTIME)注解，确保运行时能通过反射获取到该注解：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface UserException {
}

2. 清理无效的校验目标
   AnnotationApplyer.check()方法中传入了UserException.class（注解本身的Class对象），但注解本身属于Annotation类型，不可能继承Exception，这会产生无意义的错误输出，建议移除这个参数：

public static void check () {
    checkUserException(Main.class, AnnotationApplyer.class, MessageService.class, UserDefined.class);
    checkUserException(Printer.class, InvalidMessageException.class);
}

3. 修复异常处理的逻辑问题
   InvalidMessageException的构造函数中调用了System.exit(0)，这会导致程序直接退出，Greeter.greet()中的catch块根本无法执行。建议移除这个语句，让异常能被正常捕获处理：

@UserException
class InvalidMessageException extends Exception {
    public InvalidMessageException (String m) {
        super(m);
        // 移除System.exit(0)
    }
}

修正后的完整代码 #

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(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!");
            }
        }
    }
}

@Retention(RetentionPolicy.RUNTIME)
@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);
    }
}

运行效果 #

修复后运行程序，控制台会输出：

Error: Main does not extend java.lang.Exception
Error: AnnotationApplyer does not extend java.lang.Exception
Error: Printer does not extend java.lang.Exception
Class InvalidMessageException extends java.lang.Exception!
Hello, World!

完全符合预期的校验逻辑。

内容的提问来源于stack exchange，提问作者iasonas