如何在注释处理器中获取注释参数

Question

我正在编写我自己的注解处理器，我正在尝试获取我的注解参数，就像下面代码中的处理方法：

roundEnv.getElementsAnnotatedWith(annotation).forEach {
        val annotation = it.getAnnotation(annotation)
        annotation.interfaces
}

我在构建过程中得到的是 An exception occurred: javax.lang.model.type.MirroredTypesException: Attempt to access Class objects for TypeMirrors []。有人知道如何获取注释数据吗？

Answer 1

关于 getAnnotation 方法的文档解释了为什么 Class<?> 对象对于注释处理器有问题：

The annotation returned by this method could contain an element whose value is of type Class. This value cannot be returned directly: information necessary to locate and load a class (such as the class loader to use) is not available, and the class might not be loadable at all. Attempting to read a Class object by invoking the relevant method on the returned annotation will result in a MirroredTypeException, from which the corresponding TypeMirror may be extracted. Similarly, attempting to read a Class[]-valued element will result in a MirroredTypesException.

要访问类等注释元素，您需要改为使用 Element.getAnnotationMirrors() 并手动查找感兴趣的注释。这些注释镜像将包含表示实际值的元素，但不需要存在所讨论的类。

Answer 2

这个 blog post 似乎是如何做到这一点的规范来源。它引用了一个 Sun 论坛讨论，并在许多注释处理器中被引用。

对于以下注释：

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Action {
    Class<?> value();
}

类型Class<?>的字段可以通过此代码访问：

for (ExecutableElement ee : ElementFilter.methodsIn(te.getEnclosedElements())) {
    Action action = ee.getAnnotation(Action.class);
    if (action == null) {
        // Look for the overridden method
        ExecutableElement oe = getExecutableElement(te, ee.getSimpleName());
        if (oe != null) {
            action = oe.getAnnotation(Action.class);
        }
    }

    TypeMirror value = null;
    if (action != null) {
        try {
            action.value();
        } catch (MirroredTypeException mte) {
            value = mte.getTypeMirror();
        }
    }

    System.out.printf(“ % s Action value = %s\n”,ee.getSimpleName(), value);
}

如何在注释处理器中获取注释参数

How to get annotation parameter in annotation processor

java

kotlin

annotations

annotation-processing