使用java注释和反射获取java.lang.NullPointerException
我写了三个类“PrintDevice,Printer,Test”,如下所示 首先是PrintDevice.java使用java注释和反射获取java.lang.NullPointerException,java,reflection,nullpointerexception,annotations,Java,Reflection,Nullpointerexception,Annotations,我写了三个类“PrintDevice,Printer,Test”,如下所示 首先是PrintDevice.java public @interface PrintDevice{ String defaultPrint(); int defaultNumber(); } 第二,Printer.java @PrintDevice(defaultPrint="print",defaultNumber=5) public class Printer{ public Printe
public @interface PrintDevice{
String defaultPrint();
int defaultNumber();
}
第二,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);
}
}
第三,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());
}
}
编译它们时没有任何问题,但是当我尝试运行Test.java时,我得到了NullPointerExcetion,如下所示:-
线程“main”java.lang.NullPointerException中出现异常
at Test.main(Test.java:7)
您需要具有运行时保留策略的
@Retention
@Retention(RetentionPolicy.RUNTIME)
public @interface PrintDevice{
String defaultPrint();
int defaultNumber();
}
否则,批注在运行时不可用。您需要使用运行时保留策略的
@Retention
@Retention(RetentionPolicy.RUNTIME)
public @interface PrintDevice{
String defaultPrint();
int defaultNumber();
}
否则,批注在运行时不可用。@OsamaElsayed您可能还希望向批注添加
@Target
批注,以限制批注的应用位置。@OsamaElsayed您可能还希望向批注添加@Target
批注,以限制批注的应用位置。