定义

注解（Annotation）也叫元数据，它是一种代码级别的说明，它是JDK 1.5以及以后版本引入的一个特征，与类、接口、枚举是在同一个层次，它可以声明在包、类、字段、方法、局部变量、方法参数等的前面，用来对这些数据进行说明，注释

功能

编写文档：通过代码里标识元数据生成文档（javadoc生成java文档）
代码分析：配合反射获取注解和相关参数（spring 基于注解编程）
编译检查：override etc ...

Annotation内部结构

注解本质上是继承Annotation的一个接口,

元注解

元注解用于定义注解的注解，java中有如下元注解：

@Target
@Retention
@Document
@Inherited

@Target 元注解用于声明注解可以使用的作用域，如果没有使用@Target，则除了TYPE_PARAMETER，其他地方都可以使用该注解

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    /**
     * Returns an array of the kinds of elements an annotation type
     * can be applied to.
     * @return an array of the kinds of elements an annotation type
     * can be applied to
     */
    ElementType[] value();
}
public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,

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

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE
}

@Retention指定注解的声明周期，默认为CLASS

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    /**
     * Returns the retention policy.
     * @return the retention policy
     */
    RetentionPolicy value();
}
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
}

@Documented用于声明是否生成javadoc

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}

@Inherited 用于声明查询父类的注解

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}

注解属性

在注解接口的中定义的方法即为注解属性其，返回值有如下要求：

基本数据类型
String
Enum
Annotation
以上类型的数组

自定义和获取注解

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface TestAnno {
    int age();

    String name();

    //defaut
    String desc() default "test";

    TestAnno1 anno();

    String[] arrays();


}

public @interface TestAnno1 {
}


@TestAnno(age = 1, name = "a", arrays = {"a", "b"}, anno = @TestAnno1)
public class Test {
    public static void main(String[] args) {
        TestAnno anno = Test.class.getAnnotation(TestAnno.class);
        System.out.println("age: " + anno.age());
        System.out.println("name: " + anno.name());
        System.out.println("arrays: " + Arrays.toString(anno.arrays()));

    }
}

运行结果如下：