示例代码
常用注解 @Component
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Indexed
public @interface Component {
String value() default "";
}
@Target({ElementType.TYPE}) 注解
ElementType 这个枚举类型的常量提供了一个简单的分类：注释可能出现在Java程序中的语法位置（这些常量与元注释类型(@Target)一起指定在何处写入注释的合法位置）
/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang.annotation;
/**
* The constants of this enumerated type provide a simple classification of the
* syntactic locations where annotations may appear in a Java program. These
* constants are used in {@link Target java.lang.annotation.Target}
* meta-annotations to specify where it is legal to write annotations of a
* given type.
*
* <p>The syntactic locations where annotations may appear are split into
* <em>declaration contexts</em> , where annotations apply to declarations, and
* <em>type contexts</em> , where annotations apply to types used in
* declarations and expressions.
*
* <p>The constants {@link #ANNOTATION_TYPE} , {@link #CONSTRUCTOR} , {@link
* #FIELD} , {@link #LOCAL_VARIABLE} , {@link #METHOD} , {@link #PACKAGE} ,
* {@link #PARAMETER} , {@link #TYPE} , and {@link #TYPE_PARAMETER} correspond
* to the declaration contexts in JLS 9.6.4.1.
*
* <p>For example, an annotation whose type is meta-annotated with
* {@code @Target(ElementType.FIELD)} may only be written as a modifier for a
* field declaration.
*
* <p>The constant {@link #TYPE_USE} corresponds to the 15 type contexts in JLS
* 4.11, as well as to two declaration contexts: type declarations (including
* annotation type declarations) and type parameter declarations.
*
* <p>For example, an annotation whose type is meta-annotated with
* {@code @Target(ElementType.TYPE_USE)} may be written on the type of a field
* (or within the type of the field, if it is a nested, parameterized, or array
* type), and may also appear as a modifier for, say, a class declaration.
*
* <p>The {@code TYPE_USE} constant includes type declarations and type
* parameter declarations as a convenience for designers of type checkers which
* give semantics to annotation types. For example, if the annotation type
* {@code NonNull} is meta-annotated with
* {@code @Target(ElementType.TYPE_USE)}, then {@code @NonNull}
* {@code class C {...}} could be treated by a type checker as indicating that
* all variables of class {@code C} are non-null, while still allowing
* variables of other classes to be non-null or not non-null based on whether
* {@code @NonNull} appears at the variable's declaration.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.4.1 @Target
* @jls 4.1 The Kinds of Types and Values
*/
public enum ElementType {
/** Class, interface (including annotation type), or enum declaration */
/** 类, 接口 , 或 枚举 声明 **/
TYPE,
/** Field declaration (includes enum constants) */
/** 字段声明 **/
FIELD,
/** Method declaration */
/** 方法声明 */
METHOD,
/** Formal parameter declaration */
/** 正式的参数声明 */
PARAMETER,
/** Constructor declaration */
/** 构造函数声明 */
CONSTRUCTOR,
/** Local variable declaration */
/** 局部变量声明 */
LOCAL_VARIABLE,
/** Annotation type declaration */
/** 注释类型声明 */
ANNOTATION_TYPE,
/** Package declaration */
/** 包声明 */
PACKAGE,
/**
* Type parameter declaration
* 类型参数声明
* @since 1.8
*/
TYPE_PARAMETER,
/**
* Use of a type
* 使用的类型
* @since 1.8
*/
TYPE_USE
}
@Retention(RetentionPolicy.RUNTIME) 注解
RetentionPolicy这个枚举类型的常量描述保留注释的各种策略，它们与元注释(@Retention)一起指定注释要保留多长时间
/*
* Copyright (c) 2003, 2004, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.lang.annotation;
/**
* Annotation retention policy. The constants of this enumerated type
* describe the various policies for retaining annotations. They are used
* in conjunction with the {@link Retention} meta-annotation type to specify
* how long annotations are to be retained.
*
* @author Joshua Bloch
* @since 1.5
*/
public enum RetentionPolicy {
/**
* Annotations are to be discarded by the compiler.
* 注释只在源代码级别保留，编译时被忽略
*/
SOURCE,
/**
* Annotations are to be recorded in the class file by the compiler
* but need not be retained by the VM at run time. This is the default
* behavior.
* 注释将被编译器在类文件中记录 但在运行时不需要JVM保留。这是默认的行为.
*/
CLASS,
/**
* Annotations are to be recorded in the class file by the compiler and
* retained by the VM at run time, so they may be read reflectively.
*
* @see java.lang.reflect.AnnotatedElement
* 注释将被编译器记录在类文件中 在运行时保留VM，因此可以反读
*/
RUNTIME
}
@Documented注解
Documented注解表明这个注释是由 javadoc记录的，在默认情况下也有类似的记录工具。 如果一个类型声明被注释了文档化，它的注释成为公共API的一部分。
