java - 关于Processor接口(interface)的process(...)方法中定义的参数

Question

在 javax.annotation.processing包有接口(interface)Processor其中有一个功能:

/**
     * Processes a set of annotation types on type elements
     * originating from the prior round and returns whether or not
     * these annotation types are claimed by this processor.  If {@code
     * true} is returned, the annotation types are claimed and subsequent
     * processors will not be asked to process them; if {@code false}
     * is returned, the annotation types are unclaimed and subsequent
     * processors may be asked to process them.  A processor may
     * always return the same boolean value or may vary the result
     * based on chosen criteria.
     *
     * <p>The input set will be empty if the processor supports {@code
     * "*"} and the root elements have no annotations.  A {@code
     * Processor} must gracefully handle an empty set of annotations.
     *
     * @param annotations the annotation types requested to be processed
     * @param roundEnv  environment for information about the current and prior round
     * @return whether or not the set of annotation types are claimed by this processor
     */
    boolean process(Set<? extends TypeElement> annotations,
                    RoundEnvironment roundEnv);

Java API AbstractProcessor实现上述接口(interface)。现在我创建了自己的处理器类:

public class MyProcessor extends AbstractProcessor {
   ...
   @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {

        for (TypeElement annotation: annotations) {
            // How can I get the class of the annotation ?
        }
    }
}

我的问题:

API 文档告诉我 annotations在处理函数中是

the the annotation types requested to be processed

那么，为什么是类型TypeElement不是 java.lang.annotation.Annotation ?我对此感到困惑，因为我不确定 annotations实际上是指被注释的元素或真正的注释注释元素。

由于我上面的第一个问题，如何从 TypeElement 中获取每个注释的类?

Answer 1

有包 javax.lang.model 它遵循 Mirrors: Design Principles for Meta-level Facilities of Object-Oriented Programming Languages 中描述的基于镜像的设计.它是在(但不限于)注释处理框架中使用的经过精心设计的抽象。

Then, why is it with type TypeElement not java.lang.annotation.Annotation?

设计框架时，保留它很重要open (可扩展)。你永远不知道该语言的 future 版本会带来什么。假设可能出现新形式的元数据，您需要保持灵 active 。

I get confused by this because I am not sure whether the annotations actually mean the elements that being annotated or the real annotations annotating elements.

因为您正在处理注释类型，所以它是“真正的注释元素”。

for (TypeElement typeElement : annotations) {
    // it's likely the ElementKind.ANNOTATION_TYPE
    typeElement.getKind();

    // elements annotated with the annotation
    environment.getElementsAnnotatedWith(typeElement);
}

Because of my 1st question above, how can I get each annotation's class from the TypeElement?

TypeElement代表您的注释类。

读书:

https://bracha.org/mirrors.pdf (处理需要很长时间)

https://www.baeldung.com/java-annotation-processing-builder (它是一个简单的注释处理器)

https://github.com/rzwitserloot/lombok/blob/master/src/core/lombok/core/AnnotationProcessor.java (这是一个很好的 practical 示例)

the package 的 Javadoc以及 Processor 等核心类, AnnotatedConstruct , Element , TypeElement , TypeMirror .