使用用户定义的注释时获取java.lang.NullPointerException

Question

I am getting null pointer exception while executing following method. I have debugged and found that cons.getAnnotation(MyAssessment.class); is returning null . Plese help

import java.lang.annotation.*; 
import java.lang.reflect.*; 

enum GradeLevel { POOR, AVERAGE, GOOD, VERYGOOD, EXTRAORDINARY }

@interface MyAssessment {
    GradeLevel score();
}

public class Problem {

    @MyAssessment(score=GradeLevel.GOOD)
    public Problem(){
        System.out.println("This is a constuructor");
    }

    public static void main(String... args){

        try {
            Class c = new Problem().getClass();
            Constructor cons = c.getDeclaredConstructor();
            MyAssessment an = (MyAssessment) cons.getAnnotation(MyAssessment.class);
            System.out.println("YOUR SCORE IS : " + an.score());
        }
        catch(NoSuchMethodException nsme) {
            System.out.println("Constructor thrown exception");
        }
    }
}

Answer 1

Annotations are not by default available at runtime. You need to annotate your annotation :P

@Retention(RetentionPolicy.RUNTIME)
@interface MyAssessment {

You can also add a @Target of say CONSTRUCTOR to say it should only appear on constructors.

Answer 2

您需要使用以下注释来注释您的注释：

@Retention(RetentionPolicy.RUNTIME)

Answer 3

You can change the Retention Policy of your annotation. Using @Retention.