【Java】如何自定义注解

一、问题
在实体类中，如何通过注解校验必输字段，字段长度，是否必输等过滤条件？

二、分析

1.自定义注解（annotation）
像@Autowired自动织入注解等，都是通过注解，让编译器在编译过程中，自动做一些解析操作，我们可以通过这一机制，让编译器自动为我们做一些动作；首先，我们来看一些如何自定义注解：

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

/**
 * 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
 */
@Retention(RetentionPolicy.RUNTIME)
/** Field declaration (includes enum constants) */
@Target({ElementType.FIELD})
public @interface DemoAnnotation {
    //字段长度
    int length() default 0;

    //是否必输
    boolean isNotNull() default true;

    //文件名称
    String fieldName() default "";


}

这个类，是一个最基本的注解类，解析如下：
1、实体类是通过class来定义的，接口是用过interface来定义的，而注解是通过@interface来定义的；你可以把标枪当成一种特殊的接口；
2、@Retention表示了作用时间，滞留周期，通过官方的注解，我们可以知道：class文件中的注解会被编译器记录，并且当虚拟机运行时一直被保存（言下之意即知道虚拟机结束运行），它会通过反射的方式读取；

@Retention(RetentionPolicy.RUNTIME)

3、@Target({ElementType.FIELD})表名了作用范围，这里我们如果做字段的校验，选枚举类中的FIELD即可；除此之外，我们还可以选ElementType 中枚举的其他类型：

/*
 * 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.
 *
 * 
The syntactic locations where annotations may appear are split into
 * declaration contexts , where annotations apply to declarations, and
 * type contexts , where annotations apply to types used in
 * declarations and expressions.
 *
 * 
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.
 *
 * 
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.
 *
 * 
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.
 *
 * 
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.
 *
 * 
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,

    /** 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
}

3、注解中的成员变量只允许public和abstract修饰，因为注解的特性就是通用的，公共的，所以不允许private等这类的小范围的标签；

4、定义成员变量使用

类型+方法名+（）+defvalue +默认值

的方式来定义方法；