java常见的注解之 @Target、@Retention、@Documented、@Component

java常见的注解之 @Target、@Retention、@Documented、@Component

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.stereotype;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {
String value() default "";
}

上面是一个spring常用的注解@Controller,下面让我们分析一下上面的注解
1.@Target({ElementType.TYPE})

@Target用来表示注解作用范围，超过这个作用范围，编译的时候就会报错。
 java.lang.annotation.ElementType
 Target通过ElementType来指定注解可使用范围的枚举集合，枚举集合如下

package java.lang.annotation;

/**
 * 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 {@link Target} meta-annotation type
 * to specify where it is legal to use an annotation type.
 *
 * @author  Joshua Bloch
 * @since 1.5
 /
public enum ElementType {
    /* Class, interface (including annotation type), or enum declaration */
    TYPE,

/** Field declaration (includes enum constants) */
    FIELD,

/** Method declaration */
    METHOD,

/** Parameter declaration */
    PARAMETER,

/** Constructor declaration */
    CONSTRUCTOR,

/** Local variable declaration */
    LOCAL_VARIABLE,

/** Annotation type declaration */
    ANNOTATION_TYPE,

/** Package declaration */
    PACKAGE
}
ElementType的用法

取值 注解使用范围
TYPE 可用于类或者接口上
FIELD 可用于域上
METHOD 可用于方法上
PARAMETER 可用于参数上
CONSTRUCTOR 可用于构造方法上
LOCAL_VARIABLE 可用于局部变量上
ANNOTATION_TYPE 可用于注解类型上（被interface修饰的类型）
PACKAGE 用于记录java文件的package信息
2.@Retention(RetentionPolicy.RUNTIME)

@Retention定义了该Annotation被保留的时间长短

@Retenrion通过RetebtionPolicy表示需要在什么级别保存该注释信息，用于描述注解的生命周期（即：被描述的注解在什么范围内有效）

RetentionPoicy取值有：

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.
    */
    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.
    */
    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
    */
    RUNTIME
    }
    用法如下：

取值 有效范围
SOURCE 在源文件中有效（即源文件保留）
CLASS 在class文件中有效（即class保留）
RUNTIMR 在运行时有效（即运行时保留）
3.@Documented

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

package java.lang.annotation;

/**

• Indicates that annotations with a type are to be documented by javadoc
• and similar tools by default. This type should be used to annotate the
• declarations of types whose annotations affect the use of annotated
• elements by their clients. If a type declaration is annotated with
• Documented, its annotations become part of the public API
• of the annotated elements.
• @author Joshua Bloch
• @since 1.5
  */
  @Documented
  @Retention(RetentionPolicy.RUNTIME)
  @Target(ElementType.ANNOTATION_TYPE)
  public @interface Documented {
  }
  4.@Component

Component泛指组件，当组件不好归类的时候，我们可以使用这个注解进行标注

package org.springframework.stereotype;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Component {
String value() default "";
}