java - 为什么 RecordComponent 没有在 Java 17 的 Records 类中定义的注释信息？

Question

我玩唱片，发现一些对我来说看起来不合逻辑的东西：

记录：

record R(@NotEmpty Integer i) {}

代码：

RecordComponent[] recordComponents = R.class.getRecordComponents();
System.out.println(recordComponents[0].isAnnotationPresent(NotEmpty.class));
//prints false

但是，如果我这样做：

System.out.println(R.class.getDeclaredFields()[0].isAnnotationPresent(NotEmpty.class));
//prints true

这是预期的吗？因为RecordComponent implements AnnotatedElement，所以我认为RecordComponent应该有关于注释的信息。我的期望错了吗？

Answer 1

这取决于注释的指定允许目标。记录组件具有关联的构造函数参数、字段、访问器方法和类型。如果允许这些位置中的至少一个，编译器将接受注释，但注释只会与记录的允许位置相关联。

所以下面的代码

public class RecordAnnotations {
    public static void main(String[] args) {
        Class<?> cl = Test.class;
        for(var r: cl.getRecordComponents()) {
            System.out.println("record component "
                + Arrays.toString(r.getAnnotations()) + " " + r);
            System.out.println("\tof type " + r.getAnnotatedType());
            System.out.println("\taccessed by " +
                Arrays.toString(r.getAccessor().getAnnotations())+" "+r.getAccessor());
            System.out.println("\twith return type "
                + r.getAccessor().getAnnotatedReturnType());
        }
        System.out.println();
        for(var r: cl.getDeclaredFields()) {
            System.out.println("field " + Arrays.toString(r.getAnnotations())+" "+r);
            System.out.println("\tof type " + r.getAnnotatedType());
        }
        System.out.println();

        for(var c: cl.getDeclaredConstructors()) {
            System.out.println("constructor " + c);
            for(var r: c.getParameters()) {
                System.out.println("\tparameter "
                    + Arrays.toString(r.getAnnotations()) + " " + r);
                System.out.println("\t\tof type " + r.getAnnotatedType());
            }
        }
        //System.out.println();
        //for(var r: cl.getRecordComponents()) System.out.println(r);
    }
}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.RECORD_COMPONENT)
@interface WithRecord {}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD)
@interface WithField {}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.PARAMETER)
@interface WithParameter {}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE_USE)
@interface WithTypeUse {}

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
@interface WithMethod {}

record Test(@WithRecord @WithField @WithParameter @WithTypeUse @WithMethod
            int component) {
}

印刷

record component [@WithRecord()] int component
        of type @WithTypeUse() int
        accessed by [@WithMethod()] public int Test.component()
        with return type @WithTypeUse() int

field [@WithField()] private final int Test.component
        of type @WithTypeUse() int

constructor Test(int)
        parameter [@WithParameter()] int component
                of type @WithTypeUse() int

显示如何将每个注释复制到其允许的位置。只有具有RECORD_COMPONENT允许目标的注释才能通过RecordComponent'sgetAnnotation或isAnnotationPresent.

当然，@Retention(RetentionPolicy.RUNTIME)也是必须的。