/**注解Repeatable源码*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Repeatable {
    /**
     * Indicates the <em>containing annotation type</em> for the
     * repeatable annotation type.
     * @return the containing annotation type
     */
    Class<? extends Annotation> value();
}
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package java.lang.annotation;
/**
 * The common interface extended by all annotation types.  Note that an
 * interface that manually extends this one does <i>not</i> define
 * an annotation type.  Also note that this interface does not itself
 * define an annotation type.
 *
 * More information about annotation types can be found in section 9.6 of
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * The {@link java.lang.reflect.AnnotatedElement} interface discusses
 * compatibility concerns when evolving an annotation type from being
 * non-repeatable to being repeatable.
 *
 * @author  Josh Bloch
 * @since   1.5
 */
public interface Annotation {
    /**
     * Returns true if the specified object represents an annotation
     * that is logically equivalent to this one.  In other words,
     * returns true if the specified object is an instance of the same
     * annotation type as this instance, all of whose members are equal
     * to the corresponding member of this annotation, as defined below:
     * <ul>
     *    <li>Two corresponding primitive typed members whose values are
     *    <tt>x</tt> and <tt>y</tt> are considered equal if <tt>x == y</tt>,
     *    unless their type is <tt>float</tt> or <tt>double</tt>.
     *
     *    <li>Two corresponding <tt>float</tt> members whose values
     *    are <tt>x</tt> and <tt>y</tt> are considered equal if
     *    <tt>Float.valueOf(x).equals(Float.valueOf(y))</tt>.
     *    (Unlike the <tt>==</tt> operator, NaN is considered equal
     *    to itself, and <tt>0.0f</tt> unequal to <tt>-0.0f</tt>.)
     *
     *    <li>Two corresponding <tt>double</tt> members whose values
     *    are <tt>x</tt> and <tt>y</tt> are considered equal if
     *    <tt>Double.valueOf(x).equals(Double.valueOf(y))</tt>.
     *    (Unlike the <tt>==</tt> operator, NaN is considered equal
     *    to itself, and <tt>0.0</tt> unequal to <tt>-0.0</tt>.)
     *
     *    <li>Two corresponding <tt>String</tt>, <tt>Class</tt>, enum, or
     *    annotation typed members whose values are <tt>x</tt> and <tt>y</tt>
     *    are considered equal if <tt>x.equals(y)</tt>.  (Note that this
     *    definition is recursive for annotation typed members.)
     *
     *    <li>Two corresponding array typed members <tt>x</tt> and <tt>y</tt>
     *    are considered equal if <tt>Arrays.equals(x, y)</tt>, for the
     *    appropriate overloading of {@link java.util.Arrays#equals}.
     * </ul>
     *
     * @return true if the specified object represents an annotation
     *     that is logically equivalent to this one, otherwise false
     */
    boolean equals(Object obj);
    /**
     * Returns the hash code of this annotation, as defined below:
     *
     * <p>The hash code of an annotation is the sum of the hash codes
     * of its members (including those with default values), as defined
     * below:
     *
     * The hash code of an annotation member is (127 times the hash code
     * of the member-name as computed by {@link String#hashCode()}) XOR
     * the hash code of the member-value, as defined below:
     *
     * <p>The hash code of a member-value depends on its type:
     * <ul>
     * <li>The hash code of a primitive value <tt><i>v</i></tt> is equal to
     *     <tt><i>WrapperType</i>.valueOf(<i>v</i>).hashCode()</tt>, where
     *     <tt><i>WrapperType</i></tt> is the wrapper type corresponding
     *     to the primitive type of <tt><i>v</i></tt> ({@link Byte},
     *     {@link Character}, {@link Double}, {@link Float}, {@link Integer},
     *     {@link Long}, {@link Short}, or {@link Boolean}).
     *
     * <li>The hash code of a string, enum, class, or annotation member-value
     I     <tt><i>v</i></tt> is computed as by calling
     *     <tt><i>v</i>.hashCode()</tt>.  (In the case of annotation
     *     member values, this is a recursive definition.)
     *
     * <li>The hash code of an array member-value is computed by calling
     *     the appropriate overloading of
     *     {@link java.util.Arrays#hashCode(long[]) Arrays.hashCode}
     *     on the value.  (There is one overloading for each primitive
     *     type, and one for object reference types.)
     * </ul>
     *
     * @return the hash code of this annotation
     */
    int hashCode();
    /**
     * Returns a string representation of this annotation.  The details
     * of the representation are implementation-dependent, but the following
     * may be regarded as typical:
     * <pre>
     *   &#064;com.acme.util.Name(first=Alfred, middle=E., last=Neuman)
     * </pre>
     *
     * @return a string representation of this annotation
     */
    String toString();
    /**
     * Returns the annotation type of this annotation.
     * @return the annotation type of this annotation
     */
    Class<? extends Annotation> annotationType();
}
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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,   /* Annotation信息仅存在于编译器处理期间，编译器处理完之后就没有该Annotation信息了  */
    /**
     * 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,   /* 编译器将Annotation存储于类对应的.class文件中。默认行为  */
    /**
     * 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    /* 编译器将Annotation存储于class文件中，并且可由JVM读入 */
}
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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 */
    TYPE,     /* 类、接口（包括注释类型）或枚举声明  */
    /** Field declaration (includes enum constants) */
    FIELD,       /* 字段声明（包括枚举常量）  */
    /** Method declaration */
    METHOD,     /* 方法声明  */
    /** Formal parameter declaration */
    PARAMETER,   /* 参数声明  */
    /** Constructor declaration */
    CONSTRUCTOR,   /* 构造方法声明  */
    /** Local variable declaration */
    LOCAL_VARIABLE,   /* 局部变量声明  */
    /** Annotation type declaration */
    ANNOTATION_TYPE,    /* 注释类型声明  */
    /** Package declaration */
    PACKAGE,     /* 包声明  */
    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,  /*类型参数声明*/
    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE  /*使用类型*/
}
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//就x像定义一个接口一样，只不过它多了一个@
public @interface MyTestAnnotation {
}
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public @interface MyTestAnnotation {
    //定义了两个成员变量
    String username();
    int age();
}
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public @interface MyTestAnnotation {
    //定义了两个成员变量
    String username() default "江夏";;
    int age() default 20;
}
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//注解拥有什么属性，在修饰的时候就要给出相对应的值，
@MyTestAnnotation (name="江夏",age = 20)
public class User{
}
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public @interface MyTestAnnotation {
    String value();
}
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@MyTestAnnotation("江夏")
public void User() {
}
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/**是否存在对应 Annotation 对象*/
  public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass) {
        return GenericDeclaration.super.isAnnotationPresent(annotationClass);
    }
 /**获取 Annotation 对象*/
    public <A extends Annotation> A getAnnotation(Class<A> annotationClass) {
        Objects.requireNonNull(annotationClass);
        return (A) annotationData().annotations.get(annotationClass);
    }
 /**获取所有 Annotation 对象数组*/   
 public Annotation[] getAnnotations() {
        return AnnotationParser.toArray(annotationData().annotations);
    }    
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package com.jiang.AnnotationPackage;
import java.lang.annotation.*;
@Documented
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyTestAnnotation {
    String name() default "江夏";
    int age() default 18;
}
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package com.jiang.AnnotationPackage;
/**
 * 声明一个类，使用自定义注解
 */
@MyTestAnnotation(name = "江夏",age = 20)
public class AnnotationUser {
}
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package com.jiang.AnnotationPackage;
public class AnnotationDemo {
    public static void main(String[] args) {
        /**
         * 获取类注解属性
         */
        Class<AnnotationUser> userClass = AnnotationUser.class;
        /**是否存在对应 Annotation 对象*/
        boolean annotationPresent = userClass.isAnnotationPresent(MyTestAnnotation.class);
        if(annotationPresent){
            /**获取 Annotation 对象*/
            MyTestAnnotation myTestAnnotation = userClass.getAnnotation(MyTestAnnotation.class);
            System.out.println("姓名是："+myTestAnnotation.name());
            System.out.println("年龄是："+myTestAnnotation.age());
        }
    }
}
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//1、反射出该类的方法
  Class classA = AnnotationDemo2.class;
  Method method = classA .getMethod("say", String.class, int.class);
  //2、通过该方法得到注解上的具体信息
  MyTestAnnotation annotation = method.getAnnotation(MyTestAnnotation.class);
  String name = annotation.username();
  int age = annotation.age();
  //将注解上的信息注入到方法上
  Object o = classA.newInstance();
  method.invoke(o, name, age);
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package com.jiang.AnnotationPackage;
import java.lang.annotation.*;
/**
 * 声明一个自定义注解
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyTestAnnotation2 {
    int result() default 50;
}
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package com.jiang.AnnotationPackage;
import java.lang.reflect.Method;
/**
 * 自定义注解在方法上的使用
 */
public class MyTestAnnotationDemo2 {
    /**
     * @param number 猜数的大小
     */
    @MyTestAnnotation2(result = 85)
    public static void guess(int number){
        System.out.println(processGuess(number));
    }
    private static String processGuess(int number){
        try {
            Method guessnumber = MyTestAnnotationDemo2.class.getDeclaredMethod("guess",int.class);
            boolean annotationPresent = guessnumber.isAnnotationPresent(MyTestAnnotation2.class);
            if(annotationPresent){
                MyTestAnnotation2 annotation2 = guessnumber.getAnnotation(MyTestAnnotation2.class);
                if(number>annotation2.result()){
                    return "猜的数字大于指定数字";
                }else if (number==annotation2.result()){
                    return "猜的数字等于指定数字";
                }else{
                    return "猜的数字小于指定数字";
                }
            }
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        return "猜测程序有误";
    }
    public static void main(String[] args) {
        guess(85);
        //guess(84);
        //guess(86);
    }
}
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