Java 注释 ElementType 常量是什么意思？

What do Java annotation ElementType constants mean?

提问by Action Hymanson

java.lang.annotation.ElementType:

java.lang.annotation.ElementType：

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 Targetmeta-annotation type to specify where it is legal to use an annotation type.

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Target元注释类型一起使用，以指定在何处使用注释类型是合法的。

There are the following constants:

有以下常量：

• 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

• 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
• 包装- 包装声明
• PARAMETER- 参数声明
• TYPE- 类、接口（包括注解类型）或枚举声明

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

有人可以解释他们每个人是什么（他们会在实际代码中注释的地方）吗？

采纳答案by Javid Jamae

This summarizes the main ones:

这总结了主要的：

@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 is an annotation on another annotation, like this:

ANNOTATION_TYPE 是另一个注解上的注解，像这样：

@CustomAnnotationTypeAnnotation
public @interface SomeAnnotation {
  ..
}

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

包定义在包中的一个package-info.java文件中，如下所示：

@CustomPackageLevelAnnotation
package com.some.package;

import com.some.package.annotation.PackageLevelAnnotation;

For more info on PACKAGE annotations see hereand here.

有关 PACKAGE 注释的更多信息，请参见此处和此处。

回答by Bozho

Let's say the annotation to which you specify the ElementTypeis called YourAnnotation:

假设您指定的注释ElementType被称为YourAnnotation：

• ANNOTATION_TYPE - Annotation type declaration. Note:This goes on other annotations

  @YourAnnotation
  public @interface AnotherAnnotation {..}
  

• CONSTRUCTOR - Constructor declaration

  public class SomeClass {
      @YourAnnotation
      public SomeClass() {..}
  }
  

• FIELD - Field declaration (includes enum constants)

  @YourAnnotation
  private String someField;
  

• LOCAL_VARIABLE - Local variable declaration. Note:This can't be read at runtime, so it is used only for compile-time things, like the @SuppressWarningsannotation.

  public void someMethod() {
      @YourAnnotation int a = 0;
  }
  

• METHOD - Method declaration

  @YourAnnotation
  public void someMethod() {..}
  

• PACKAGE - Package declaration. Note:This can be used only in package-info.java.

  @YourAnnotation
  package org.yourcompany.somepackage;
  

• PARAMETER - Parameter declaration

  public void someMethod(@YourAnnotation param) {..}
  

• TYPE - Class, interface (including annotation type), or enum declaration

  @YourAnnotation
  public class SomeClass {..}
  

• ANNOTATION_TYPE - 注释类型声明。注意：这适用于其他注释

  @YourAnnotation
  public @interface AnotherAnnotation {..}
  

• CONSTRUCTOR - 构造函数声明

  public class SomeClass {
      @YourAnnotation
      public SomeClass() {..}
  }
  

• FIELD - 字段声明（包括枚举常量）

  @YourAnnotation
  private String someField;
  

• LOCAL_VARIABLE - 局部变量声明。注意：这不能在运行时读取，所以它只用于编译时的东西，比如@SuppressWarnings注解。

  public void someMethod() {
      @YourAnnotation int a = 0;
  }
  

• METHOD - 方法声明

  @YourAnnotation
  public void someMethod() {..}
  

• PACKAGE - 包声明。注意：这只能在package-info.java.

  @YourAnnotation
  package org.yourcompany.somepackage;
  

• PARAMETER - 参数声明

  public void someMethod(@YourAnnotation param) {..}
  

• TYPE - 类、接口（包括注解类型）或枚举声明

  @YourAnnotation
  public class SomeClass {..}
  

You can specify multiple ElementTypes for a given annotation. E.g.:

您可以ElementType为给定的注释指定多个s。例如：

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD})

回答by Action Hymanson

TYPE:

类型：

Annotation:

注解：

@Target({ElementType.TYPE})    // This annotation can only be applied to
public @interface Tweezable {  // class, interface, or enum declarations.
}

and an example usage:

和一个示例用法：

@Tweezable
public class Hair {
    ...
}