注释看不见？ [英] annotation invisible?

问题描述

这让我为难。我有一个自定义注解类，我似乎无法验证注解是present。我在做什么错在这里？如果我运行 MyOperationTest （见下文），我得到这个结果：

 实现Library.Operation：真
有@ Library.Marker：假的
田田！
 

---

Library.java：

 包com.example.gotchas;公共类库{
    私人图书馆（）{}    公共@interface标记{}    公共接口操作{
        公共无效的execute（）;
    }
}
 

MyOperation.java：

 包com.example.gotchas;@ Library.Marker
公共类MyOperation实现Library.Operation {
    @覆盖公共无效的execute（）{
        的System.out.println（田田！）;
    }
}
 

MyOperationTest.java：

 包com.example.gotchas;公共类MyOperationTest {
    静态公共无效的主要（字串[] args）
    {
        尝试{
            类＆LT;＆GT; CL =的Class.forName（com.example.gotchas.MyOperation）;
            布尔implementsLibraryOperation =
                Library.Operation.class.isAssignableFrom（CL）;
            布尔hasLibraryMarker =
                cl.isAnnotation present（Library.Marker.class）;
            的System.out.println（实现Library.Operation：
                    + implementsLibraryOperation）;
            的System.out.println（有@ Library.Marker：+ hasLibraryMarker）;
            如果（implementsLibraryOperation）
            {
                类＆LT ;?扩展Library.Operation＆GT;的opclass =
                    cl.asSubclass（Library.Operation.class）;
                Library.Operation运算= opClass.newInstance（）;
                op.execute（）;
            }
        }
        捕捉（ClassNotFoundException的E）{
            e.printStackTrace（）;
        }
        赶上（InstantiationException E）{
            e.printStackTrace（）;
        }
        赶上（IllegalAccessException E）{
            e.printStackTrace（）;
        }
    }
}
 

解决方案

您必须定义<一个href=\"http://download.oracle.com/javase/6/docs/api/java/lang/annotation/Retention.html\"><$c$c>@Retention您的注释：

  @Retention（RetentionPolicy.RUNTIME）
公共@interface标记{}
 

否则，注释在运行时没有保留。

（你还可以添加明确的 @Target（ElementType.TYPE）如果你想限制你只能在类注解的使用）

作为旁注 - 这不是一种常见的做法来定义注解内部类

This puzzles me. I have a class with a custom annotation and I can't seem to verify that the annotation is present. What am I doing wrong here? If I run MyOperationTest (see below), I get this as a result:

implements Library.Operation: true
has @Library.Marker: false
Tada!

---

Library.java:

package com.example.gotchas;

public class Library {
    private Library() {}

    public @interface Marker {}

    public interface Operation {
        public void execute(); 
    }
}

MyOperation.java:

package com.example.gotchas;

@Library.Marker
public class MyOperation implements Library.Operation {
    @Override public void execute() {
        System.out.println("Tada!");
    }
}

MyOperationTest.java:

package com.example.gotchas;

public class MyOperationTest {
    static public void main(String[] args)
    {
        try {
            Class<?> cl = Class.forName("com.example.gotchas.MyOperation");
            boolean implementsLibraryOperation =
                Library.Operation.class.isAssignableFrom(cl);
            boolean hasLibraryMarker =
                cl.isAnnotationPresent(Library.Marker.class);
            System.out.println("implements Library.Operation: "
                    +implementsLibraryOperation);
            System.out.println("has @Library.Marker: "+hasLibraryMarker);
            if (implementsLibraryOperation)
            {
                Class<? extends Library.Operation> opClass = 
                    cl.asSubclass(Library.Operation.class); 
                Library.Operation op = opClass.newInstance();
                op.execute();
            }
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        catch (InstantiationException e) {
            e.printStackTrace();
        }
        catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

解决方案

You have to define @Retention for your annotation:

@Retention(RetentionPolicy.RUNTIME)
public @interface Marker {}

Otherwise the annotation is not retained at runtime.

(You can also add explicit @Target(ElementType.TYPE) if you want to limit the usage of your annotation only on classes)

As a sidenote - it is not a common practice to define annotations as inner classes.

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