java - 使用java注释和反射获取java.lang.NullPointerException

Question

我编写了 3 个类“PrintDevice、Printer、Test”如下

首先，PrintDevice.java

public @interface PrintDevice{
    String defaultPrint();
    int defaultNumber();
}

二、Printer.java

@PrintDevice(defaultPrint="print",defaultNumber=5)
public class Printer{
    public Printer(){
        //empty Construcutor
    }
    @PrintDevice(defaultPrint="print",defaultNumber=5)
    public void print(int number){
        System.out.println(number);
    }
}

三、Test.java

import java.lang.reflect.*;
public class Test{
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException{
        Printer printer = new Printer();
        PrintDevice anno = printer.getClass().getAnnotation(PrintDevice.class);
        Method m = printer.getClass().getMethod(anno.defaultPrint(),int.class);
        m.invoke(printer,anno.defaultNumber());
    }
}

编译它们没有任何问题，但是当我尝试运行 Test.java 时，我得到 NullPointerExcetion 如下:-

Exception in thread "main" java.lang.NullPointerException at Test.main(Test.java:7)

Answer 1

您需要带有运行时保留策略的@Retention

@Retention(RetentionPolicy.RUNTIME)
public @interface PrintDevice{
    String defaultPrint();
    int defaultNumber();
}

否则注释在运行时不可用。