Java 注释不可见?
这让我困惑。我有一个带有自定义注释的类,我似乎无法验证注释是否存在。我做错了什么?如果我运行Java 注释不可见?,java,reflection,annotations,Java,Reflection,Annotations,这让我困惑。我有一个带有自定义注释的类,我似乎无法验证注释是否存在。我做错了什么?如果我运行MyOperationTest(见下文),我会得到以下结果: implements Library.Operation: true has @Library.Marker: false Tada! Library.java: package com.example.gotchas; public class Library { private Library() {} public
MyOperationTest
(见下文),我会得到以下结果:
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();
}
}
}
package com.example.gotchas;
公开课考试{
静态公共void main(字符串[]args)
{
试一试{
Class cl=Class.forName(“com.example.gotchas.MyOperation”);
布尔运算=
Library.Operation.class.isAssignableFrom(cl);
布尔hasLibraryMarker=
cl.isAnnotationPresent(Library.Marker.class);
System.out.println(“实现库操作:”
+实施图书馆业务;
System.out.println(“has@Library.Marker:+hasLibraryMarker”);
if(实施图书馆操作)
{
类必须为注释定义:
@Retention(RetentionPolicy.RUNTIME)
public @interface Marker {}
否则,在运行时不会保留注释
(如果要限制注释仅在类上使用,还可以添加显式的@Target(ElementType.TYPE)
)
作为旁注-将注释定义为内部类不是一种常见做法。默认情况下,注释保留策略为CLASS
,这不会使您的注释对反射可用。您应该在声明注释时对其进行限定。同意。默认情况下,注释不会在运行时…在c之后保留ompile time已删除。因此,当您运行测试代码时,注释不再是已编译代码的一部分。更多详细信息,请参阅:运行时-注释将由编译器记录在类文件中,并在运行时由VM保留,因此可以反射地读取它们。