如何从注释处理器中的AnnotationValue捕获枚举

Question

我试图使用注释处理器和注释镜像读取注释中枚举的值,但我得到了返回null.我认为这与将Enum包装为VariableElement的AnnotationValue有关.VariableElement的文档#getConstantValue()表示"如果这是一个初始化为编译时常量的最终字段,则返回此变量的值." 好的,但是final不是注释成员的有效修饰符.另外值得注意的是,我可以轻松阅读其他注释值,只需阅读Enums.

我做了一些调查,看起来AnnotationValue在运行时被实例化为Symbol.VarSymbol,但是Symbol.VarSymbol #getConstantValue()看起来应该只返回对象.

最后,如果我在AnnotationValue上执行toString(),我会得到正确的值.

注释:

package annotation;
public @interface AnAnnotation
{
    String value();
    Behavior defaultBehavior() default Behavior.NEW;

    public static enum Behavior
    {
        NEW, NULL;
    }
}

我的处理器的一部分并嵌套在过多的循环中以获得正确的AnnotaionMirror:

Map<? extends ExecutableElement, ? extends AnnotationValue> annotationValues = elemUtils.getElementValuesWithDefaults(annotationMirror);
for (ExecutableElement method : annotationValues.keySet())
{
    ...
    else if ("defaultBehavior".equals(method.getSimpleName().toString()))
    {

        defaultBehavior = (Behavior)( (VariableElement)annotationValues.get(method).getValue()).getConstantValue();

        // This prints "NEW" or "NULL" correctly
        processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE,annotationValues.get(method).toString());
        // This prints null incorrectly (expect "NEW" or "NULL")
        processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, defaultBehavior + "");

    }
    ...
}

编辑:处理器的更完整版本.

package annotation.processor;

import java.util.*;

import javax.annotation.processing.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import javax.tools.*;

import annotation.AnAnnotation;
import annotation.AnAnnotation.Behavior;

@SupportedAnnotationTypes("annotation.AnAnnotation")
public class AnAnnotationProcessor extends AbstractProcessor
{
    Types typeUtils;
    Elements elemUtils;

    @Override
    public void init(ProcessingEnvironment processingEnv)
    {
        super.init(processingEnv);
        typeUtils = processingEnv.getTypeUtils();
        elemUtils = processingEnv.getElementUtils();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations,
                           RoundEnvironment roundEnv)
    {
        processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE,
            "Entering AnnotationNullableClassProcessor");

        /****** Iterate over all annotaions being processed (only AnAnnotation) ******/
        for (TypeElement annotation : annotations)
        {
            /****** Iterate over all elements that are annotated with the annotation ******/
            for (Element element : roundEnv.getElementsAnnotatedWith(annotation))
            {
                /****** Iterate over all the declared annotations of the element ******/
                for (AnnotationMirror annotationMirror :  element.getAnnotationMirrors())
                {
                    final String annotationTypeName = annotationMirror.getAnnotationType().toString();

                    // Process annotations of type AnAnnotation
                    if (annotationTypeName.equals(AnAnnotation.class.getName()))
                    {
                        Map<? extends ExecutableElement, ? extends AnnotationValue> annotationValues = elemUtils.getElementValuesWithDefaults(annotationMirror);

                        /****** Iterate over the annotation's values. ******/
                        for (ExecutableElement method : accessorValues.keySet())
                        {
                            if ("defaultBehavior".equals(method.getSimpleName().toString()))
                            {
                                Behavior defaultBehavior = (Behavior)( (VariableElement)annotationValues.get(method).getValue()).getConstantValue();

                                // This prints "NEW" or "NULL" correctly
                                processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE,annotationValues.get(method).toString());
                                // This prints null incorrectly (expect "NEW" or "NULL")
                                processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, defaultBehavior + "");
                            }
                        }
                    }
                }
            }
        }

        return true;
    }
}

Answer 1

来自 getConstantValue 的文档：

“特别是，枚举常量不被视为编译时常量。要具有常量值，字段的类型必须是原始类型或字符串。”

http://java.sun.com/javase/6/docs/api/javax/lang/model/element/VariableElement.html#getConstantValue()

要获取枚举常量的值，请使用 getAnnotation API 或使用 AnnotationValueVisitor。