级联是一个数据库实体的概念，级联不是必须的，级联的好处是获取关联数据十分方便，但是级联过多会增加系统的复杂度，同时降低系统的性能，所以当级联的层级超过3层时，就不要考虑使用级联，这样会导致多个对象的关联，导致系统耦合度增加，难以维护。
MyBatis的级联分为3种：
鉴别器（discriminator）：它是一个根据某些条件决定采用具体实现类级联的方案。一对一（association）：比如身份证和人就是一种一对一的级联关系。一对多（collection）：比如班主任和学生就是一种一对多的级联关系。注意！MyBatis中没有多对多级联关系，因为多对多级联比较复杂，使用困难，而且可以通过两个一对多级联进行替换，所以MyBatis不支持多对多级联。
下面主要讲解常用的一对一级联与一对多级联。
JavaBean
package com.ronin.blog.entity; import lombok.Data; import java.util.Date; import java.util.List; @Data public class Article { private Integer articleId; private Integer articleUserId; private String articleTitle; private String articleSummary; private Integer articleViewCount; private Integer articleCommentCount; private Integer articleLikeCount; private Integer articleIsComment; private Integer articleStatus; private Date articleUpdateTime; private Date articleCreateTime; private String articleHtml; private String articleContent; //一对一关系 private User user; }配置文件
<resultMap id="BaseResultMap" type="com.ronin.blog.entity.Article" > <id column="article_id" property="articleId" jdbcType="INTEGER" /> <result column="article_user_id" property="articleUserId" jdbcType="INTEGER" /> <result column="article_title" property="articleTitle" jdbcType="VARCHAR" /> <result column="article_summary" property="articleSummary" jdbcType="VARCHAR" /> <result column="article_view_count" property="articleViewCount" jdbcType="INTEGER" /> <result column="article_comment_count" property="articleCommentCount" jdbcType="INTEGER" /> <result column="article_like_count" property="articleLikeCount" jdbcType="INTEGER" /> <result column="article_is_comment" property="articleIsComment" jdbcType="INTEGER" /> <result column="article_status" property="articleStatus" jdbcType="INTEGER" /> <result column="article_update_time" property="articleUpdateTime" jdbcType="TIMESTAMP" /> <result column="article_create_time" property="articleCreateTime" jdbcType="TIMESTAMP" /> <result column="article_html" property="articleHtml" jdbcType="LONGVARCHAR" /> <result column="article_content" property="articleContent" jdbcType="LONGVARCHAR" /> <!-- 一对一级联 --> <association property="user" javaType="User"> <id column="user_id" property="userId"/> <result column="user_name" property="userName"/> </association> </resultMap>JavaBean
package com.ronin.blog.entity; import lombok.Data; import java.util.Date; import java.util.List; @Data public class Article { private Integer articleId; private Integer articleUserId; private String articleTitle; private String articleSummary; private Integer articleViewCount; private Integer articleCommentCount; private Integer articleLikeCount; private Integer articleIsComment; private Integer articleStatus; private Date articleUpdateTime; private Date articleCreateTime; private String articleHtml; private String articleContent; //一对多关系 private List<Tag> tagList; }配置文件
<resultMap id="BaseResultMap" type="com.ronin.blog.entity.Article" > <id column="article_id" property="articleId" jdbcType="INTEGER" /> <result column="article_user_id" property="articleUserId" jdbcType="INTEGER" /> <result column="article_title" property="articleTitle" jdbcType="VARCHAR" /> <result column="article_summary" property="articleSummary" jdbcType="VARCHAR" /> <result column="article_view_count" property="articleViewCount" jdbcType="INTEGER" /> <result column="article_comment_count" property="articleCommentCount" jdbcType="INTEGER" /> <result column="article_like_count" property="articleLikeCount" jdbcType="INTEGER" /> <result column="article_is_comment" property="articleIsComment" jdbcType="INTEGER" /> <result column="article_status" property="articleStatus" jdbcType="INTEGER" /> <result column="article_update_time" property="articleUpdateTime" jdbcType="TIMESTAMP" /> <result column="article_create_time" property="articleCreateTime" jdbcType="TIMESTAMP" /> <result column="article_html" property="articleHtml" jdbcType="LONGVARCHAR" /> <result column="article_content" property="articleContent" jdbcType="LONGVARCHAR" /> <!-- 一对多级联 --> <collection property="tagList" javaType="list" ofType="Tag"> <!-- 映射主键 --> <id column="tag_id" property="tagId"/> <!-- 映射普通属性 --> <result column="tag_name" property="tagName"/> <result column="tag_description" property="tagDescription"/> </collection> </resultMap>