Java注解ElementType常量是什么意思?

2022-01-13 00:00:00 annotations java

java.lang.annotation.ElementType:

一种程序元素类型.这种枚举类型的常量提供了 Java 程序中声明元素的简单分类.这些常量与 一起使用目标元注解类型来指定使用注解类型合法的地方.

A program element type. The constants of this enumerated type provide a simple classification of the declared elements in a Java program. These constants are used with the Target meta-annotation type to specify where it is legal to use an annotation type.

有以下常量:

  • ANNOTATION_TYPE - 注释类型声明
  • CONSTRUCTOR - 构造函数声明
  • FIELD - 字段声明(包括枚举常量)
  • LOCAL_VARIABLE - 局部变量声明
  • METHOD - 方法声明
  • PACKAGE - 包声明
  • PARAMETER - 参数声明
  • TYPE - 类、接口(包括注解类型)或枚举声明
  • ANNOTATION_TYPE - Annotation type declaration
  • CONSTRUCTOR - Constructor declaration
  • FIELD - Field declaration (includes enum constants)
  • LOCAL_VARIABLE - Local variable declaration
  • METHOD - Method declaration
  • PACKAGE - Package declaration
  • PARAMETER - Parameter declaration
  • TYPE - Class, interface (including annotation type), or enum declaration

有人能解释一下它们各自是什么吗(在实际代码中它们会被注释的地方)吗?

Can someone explain what each of them are (where they'd be annotated in actual code)?

推荐答案

总结一下主要的:

@CustomTypeAnnotation
public class MyAnnotatedClass {
  @CustomFieldAnnotation
  private String foo;

  @CustomConstructorAnnotation
  public MyAnnotatedClass() {
  }

  @CustomMethodAnnotation
  public String bar(@CustomParameterAnnotation String str) {
    @CustomLocalVariableAnnotation String asdf = "asdf";
    return asdf + str;
  }
}

ANNOTATION_TYPE 是另一个注解上的注解，像这样:

ANNOTATION_TYPE is an annotation on another annotation, like this: 

@CustomAnnotationTypeAnnotation
public @interface SomeAnnotation {
  ..
}

包在包中的一个package-info.java文件中定义，像这样:

Package is defined in a package-info.java file in the package, like this:

@CustomPackageLevelAnnotation
package com.some.package;

import com.some.package.annotation.PackageLevelAnnotation;

有关 PACKAGE 注释的更多信息，请参阅此处和这里.

For more info on PACKAGE annotations see here and here.

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