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MyBatis浅尝笔记
MyBatis应属于一种轻量级的java持久层技术，它通过简单的SQL xml或注解，将数据库数据映射到接口与POJO。最近项目要用到mybatis，所以学习之后在这里做个总结，文中的示例以xml配置为主，mybatis也支持注解的方式。
测试数据
先给出demo所使用的表结构，以典型的用户（1）-文章（n）的关系表做demo数据
1 # 2 # mysql数据库：数据库名 :dblog 3 # 4 5 DROP TABLE IF EXISTS m_category; 6 CREATE TABLE m_category ( 7 id int(11) NOT NULL AUTO_INCREMENT, 8 name varchar(64) NOT NULL COMMENT ‘分类名称‘, 9 parent_id INT NOT NULL ,10 level INT NOT NULL DEFAULT 0,11 path VARCHAR(64) NOT NULL COMMENT ‘栏目路径，rootId-xxId-xxId‘,12 PRIMARY KEY (id)13 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;14 15 DROP TABLE IF EXISTS m_post;16 CREATE TABLE m_post (17 id int(11) NOT NULL AUTO_INCREMENT,18 category_id INT NOT NULL ,19 user_id INT NOT NULL ,20 title varchar(64) NOT NULL COMMENT ‘标题‘,21 content text COMMENT ‘正文‘,22 created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,23 updated_at timestamp NOT NULL DEFAULT ‘0000-00-00 00:00:00‘,24 PRIMARY KEY (id)25 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;26 27 DROP TABLE IF EXISTS m_user;28 CREATE TABLE m_user (29 id int(11) NOT NULL AUTO_INCREMENT,30 username varchar(64) NOT NULL,31 password varchar(255) NOT NULL,32 salt VARCHAR(32) NOT NULL ,33 avatar varchar(64) DEFAULT NULL,34 type enum(‘customer‘,‘admin‘,‘root‘) NOT NULL DEFAULT ‘customer‘,35 remember_token varchar(128) DEFAULT NULL,36 PRIMARY KEY (id)37 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;38 39 INSERT INTO m_user(id,username, password, salt,type)40 VALUE (1,‘lvyahui‘,‘XXXXXXX‘,‘abcs‘,‘admin‘);41 42 DROP TABLE IF EXISTS m_post_comment;43 CREATE TABLE m_post_comment(44 id int(11) AUTO_INCREMENT PRIMARY KEY ,45 post_id INT NOT NULL ,46 user_id INT NOT NULL ,47 content VARCHAR(512) NOT NULL DEFAULT ‘‘,48 created_at TIMESTAMP NOT NULL DEFAULT current_timestamp,49 updated_at TIMESTAMP NOT NULL DEFAULT ‘0000-00-00 00:00:00‘50 ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
对应的实体类
Post
1 package org.lyh.java.mybatis.model; 2 3 import java.sql.Timestamp; 4 5 /** 6 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 7 * @since 2017/1/1 14:00 8 */ 9 @SuppressWarnings("unused")10 public class Post extends BaseModel {11 12 private String title;13 private String content;14 private Timestamp createdAt;15 private Timestamp updatedAt;16 17 private Integer userId;18 private Integer categoryId;19 20 private User user;21 private Category category;22 23 public String getTitle() {24 return title;25 }26 27 public void setTitle(String title) {28 this.title = title;29 }30 31 public String getContent() {32 return content;33 }34 35 public void setContent(String content) {36 this.content = content;37 }38 39 public Timestamp getCreatedAt() {40 return createdAt;41 }42 43 public void setCreatedAt(Timestamp createdAt) {44 this.createdAt = createdAt;45 }46 47 public Timestamp getUpdatedAt() {48 return updatedAt;49 }50 51 public void setUpdatedAt(Timestamp updatedAt) {52 this.updatedAt = updatedAt;53 }54 55 public Integer getUserId() {56 return userId;57 }58 59 public void setUserId(Integer userId) {60 this.userId = userId;61 }62 63 public Integer getCategoryId() {64 return categoryId;65 }66 67 public void setCategoryId(Integer categoryId) {68 this.categoryId = categoryId;69 }70 71 public User getUser() {72 return user;73 }74 75 public void setUser(User user) {76 this.user = user;77 }78 79 public Category getCategory() {80 return category;81 }82 83 public void setCategory(Category category) {84 this.category = category;85 }86 87 88 @Override89 public String toString() {90 return "Post{" +91 "title=‘" + title + ‘\‘‘ +92 ", content=‘" + content + ‘\‘‘ +93 ", createdAt=" + createdAt +94 ", updatedAt=" + updatedAt +95 ", userId=" + userId +96 ", categoryId=" + categoryId +97 ‘}‘;98 }99 }
User
1 package org.lyh.java.mybatis.model; 2 3 import org.lyh.java.mybatis.type.UserType; 4 5 import java.util.List; 6 7 /** 8 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 9 * @since 2017/1/12 22:4410 */11 @SuppressWarnings("unused")12 public class User extends BaseModel {13 14 15 private String username;16 private String password;17 private String salt;18 private String avatar;19 private UserType type;20 private String rememberToken;21 22 private List<Post> posts ;23 private List<PostComment> postComments;24 25 public String getUsername() {26 return username;27 }28 29 public void setUsername(String username) {30 this.username = username;31 }32 33 public String getPassword() {34 return password;35 }36 37 public void setPassword(String password) {38 this.password = password;39 }40 41 public String getSalt() {42 return salt;43 }44 45 public void setSalt(String salt) {46 this.salt = salt;47 }48 49 public String getAvatar() {50 return avatar;51 }52 53 public void setAvatar(String avatar) {54 this.avatar = avatar;55 }56 57 public UserType getType() {58 return type;59 }60 61 public void setType(UserType type) {62 this.type = type;63 }64 65 public String getRememberToken() {66 return rememberToken;67 }68 69 public void setRememberToken(String rememberToken) {70 this.rememberToken = rememberToken;71 }72 73 public List<Post> getPosts() {74 return posts;75 }76 77 public void setPosts(List<Post> posts) {78 this.posts = posts;79 }80 81 public List<PostComment> getPostComments() {82 return postComments;83 }84 85 public void setPostComments(List<PostComment> postComments) {86 this.postComments = postComments;87 }88 }
一些辅助类
查询条件Condition
1 package org.lyh.java.mybatis.bean; 2 3 /** 4 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 5 * @since 2016/12/12 13:27 6 */ 7 @SuppressWarnings("unused") 8 public class Condition { 9 10 private String key;11 private String opt = "=";12 private Object value;13 14 public Condition(String key, String opt, Object value) {15 this.key = key;16 this.opt = opt;17 this.value =http://www.mamicode.com/ value;18 }19 20 public Condition(String key, Object value){21 this(key,"=",value);22 }23 24 public String getKey() {25 return key;26 }27 28 public void setKey(String key) {29 this.key = key;30 }31 32 public String getOpt() {33 return opt;34 }35 36 public void setOpt(String opt) {37 this.opt = opt;38 }39 40 public Object getValue() {41 return value;42 }43 44 public void setValue(Object value) {45 this.value =http://www.mamicode.com/ value;46 }47 }
分页工具类PageData
1 package org.lyh.java.mybatis.bean; 2 3 4 import org.lyh.java.mybatis.model.BaseModel; 5 6 import java.util.List; 7 8 /** 9 * 10 * Created by lvyahui on 2015/7/12. 11 */ 12 @SuppressWarnings("unused") 13 public class PageData<T extends BaseModel> { 14 15 /** 16 * 前端做分页，所以这里limit设置的非常大，相当于不分页 17 */ 18 public static final int DEFAULT_SIZE = 1000; 19 20 private List<T> datas; 21 22 private int currentPage = 1; 23 24 private int totalPage; 25 26 private int totalItem; 27 28 29 private int maxBtnCount = 10; 30 31 private int pageSize = DEFAULT_SIZE; 32 33 private int start = 1; 34 private int end; 35 36 /** 37 * 总项目数 38 */ 39 public int getTotalItem() { 40 return totalItem; 41 } 42 43 public void setTotalItem(int totalItem) { 44 this.totalItem = totalItem; 45 paging(); 46 } 47 48 private void paging() { 49 totalPage = totalItem / pageSize + 1; 50 if(totalPage > maxBtnCount){ 51 if(currentPage <= (maxBtnCount-1)/2){ 52 // 靠近首页 53 start = 1; 54 }else if(totalPage-currentPage < (maxBtnCount-1)/2){ 55 // 靠近尾页 56 start = totalPage - maxBtnCount - 1; 57 }else{ 58 start = currentPage - (maxBtnCount-1)/2; 59 } 60 end = maxBtnCount-1 + start > totalPage ? totalPage : maxBtnCount - 1 + start; 61 }else{ 62 end = totalPage; 63 } 64 // System.out.println("start:"+start+",end:"+end); 65 } 66 67 /** 68 * 总页数 69 */ 70 public int getTotalPage() { 71 return totalPage; 72 } 73 74 /** 75 * 当前页 76 */ 77 public int getCurrentPage() { 78 return currentPage; 79 } 80 81 public void setCurrentPage(int currentPage) { 82 this.currentPage = currentPage; 83 } 84 85 /** 86 * 页面数据 87 */ 88 public List<T> getDatas() { 89 return datas; 90 } 91 92 public void setDatas(List<T> datas) { 93 this.datas = datas; 94 } 95 96 /** 97 * 每页大小，可放多少个项，默认为10 98 */ 99 100 101 public int getPageSize() {102 return pageSize;103 }104 105 public void setPageSize(int pageSize) {106 this.pageSize = pageSize;107 }108 109 /**110 * @return 最大分页按钮数，默认值为10111 */112 public int getMaxBtnCount() {113 return maxBtnCount;114 }115 116 public void setMaxBtnCount(int maxBtnCount) {117 this.maxBtnCount = maxBtnCount;118 }119 120 /**121 * @return 第一个按钮的页号122 */123 public int getStart() {124 return start;125 }126 127 /**128 * @return 最后一个按钮上的页号129 */130 public int getEnd() {131 return end;132 }133 134 public void setEnd(int end) {135 this.end = end;136 }137 138 public void setStart(int start) {139 this.start = start;140 }141 142 143 private String listUrl;144 145 public String getListUrl() {146 return listUrl;147 }148 149 public void setListUrl(String listUrl) {150 this.listUrl = listUrl;151 }152 153 @Override154 public String toString() {155 return "PageData{" +156 "datas_size=" + datas.size() +157 ", currentPage=" + currentPage +158 ", totalPage=" + totalPage +159 ", totalItem=" + totalItem +160 ", maxBtnCount=" + maxBtnCount +161 ", pageSize=" + pageSize +162 ", start=" + start +163 ", end=" + end +164 ‘}‘;165 }166 167 }
基础Model与注解
1 package org.lyh.java.mybatis.annotation; 2 3 import java.lang.annotation.ElementType; 4 import java.lang.annotation.Retention; 5 import java.lang.annotation.RetentionPolicy; 6 import java.lang.annotation.Target; 7 8 /** 9 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 10 * @since 2017/1/16 10:44 11 */ 12 @Target(value =http://www.mamicode.com/ { ElementType.FIELD }) 13 @Retention(RetentionPolicy.RUNTIME) 14 public @interface JsonField { 15 String value() default ""; 16 } 17 18 19 package org.lyh.java.mybatis.annotation; 20 21 import java.lang.annotation.Retention; 22 import java.lang.annotation.RetentionPolicy; 23 24 /** 25 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 26 * @since 2017/1/15 15:18 27 */ 28 @Retention(RetentionPolicy.RUNTIME) 29 public @interface NonTableFiled { 30 31 } 32 33 34 package org.lyh.java.mybatis.model; 35 36 import org.lyh.java.mybatis.annotation.JsonField; 37 import org.lyh.java.mybatis.annotation.NonTableFiled; 38 39 import java.lang.reflect.Field; 40 import java.util.ArrayList; 41 import java.util.HashMap; 42 import java.util.List; 43 import java.util.Map; 44 45 /** 46 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com) 47 * @since 2017/1/12 22:40 48 */ 49 @SuppressWarnings("unused") 50 public class BaseModel { 51 52 public Map<String,Object> jsonValues ; 53 54 protected Integer id; 55 public Integer getId() { 56 return id; 57 } 58 59 public void setId(Integer id) { 60 this.id = id; 61 } 62 63 public Map<String,String> getFieldMap(){ 64 Map<String,String> fieldMap = new HashMap<String,String>(); 65 Field[] fields = this.getClass().getDeclaredFields(); 66 for (Field field : fields){ 67 if(field.getAnnotation(NonTableFiled.class) == null){ 68 fieldMap.put( 69 // table field -- snake 70 field.getName().replaceAll("([A-Za-z])([A-Z])","$1_$2").toLowerCase(), 71 // bean field -- hump 72 field.getName() 73 ); 74 } 75 } 76 return fieldMap; 77 } 78 79 public List<Field> getJsonFields(){ 80 Field fields[] = this.getClass().getDeclaredFields(); 81 List<Field> jsonFields = new ArrayList<Field>(); 82 for(Field field : fields){ 83 JsonField jsonField = field.getAnnotation(JsonField.class); 84 if(jsonField == null){ 85 continue; 86 } 87 jsonFields.add(field); 88 } 89 return jsonFields; 90 } 91 92 public Map<String,Object> getJsonValues(){ 93 if(jsonValues != null){ 94 return jsonValues; 95 } 96 jsonValues = new HashMap<String, Object>(); 97 List<Field> fields = getJsonFields(); 98 for (Field field : fields){ 99 field.setAccessible(true);100 JsonField jsonField = field.getAnnotation(JsonField.class);101 try {102 jsonValues.put(jsonField.value(),field.get(this));103 } catch (IllegalAccessException e) {104 //105 } finally {106 field.setAccessible(false);107 }108 }109 return jsonValues;110 }111 }
其中BaseModel方法，getFieldMap用来获取数据库字段名称->模型属性名称的映射关系，约定数据库中使用"_"分割单词的蛇形字符串，而属性名使用首字母小写的驼峰字符串，例如数据库字段created_at对应属性createdAt。个人认为编程时约定很重要，有了约定很多通用方法才好写。
一、单表查询
不涉及关系查询的情况还是比较简单的，并且有除去字段名与表名不一致外，有高度的可重用性。笔者在学习mybatis时，试图借助注解、泛型、反射等方法编写出一个通用的DAO类的集合，但因为xml或者注解无法继承包含等原因，一直没有完成一个很好的方案。单表查询示例以m_post表为示例。先来看看基础PostMapper与Xml ResultMap
PostMapper 接口
1 package org.lyh.java.mybatis.mapper; 2 3 import org.apache.ibatis.annotations.Param; 4 import org.lyh.java.mybatis.bean.Condition; 5 import org.lyh.java.mybatis.model.Post; 6 7 import java.util.List; 8 9 /**10 * @author lvyahui (lvyahui8@gmail.com,lvyahui8@126.com)11 * @since 2017/1/1 13:5912 */13 public interface PostMapper {14 //String table = "m_post";15 Post get(Integer id);16 int insert(Post post);17 int updateByPrimaryKey(Post post);18 int updateByPrimaryKeySelective(@Param("post") Post post);19 int deleteByPrimaryKey(Integer id);20 int batchInsert(@Param("posts") List<Post> posts);21 22 int countSizeWithCondition(@Param("conditions") List<Condition> conditions);23 List<Post> getPageDataByCondition(@Param("conditions") List<Condition> conditions,24 @Param("offset") Integer offset,25 @Param("size") Integer size,26 @Param("orderProp") String orderProp,27 @Param("desc") boolean desc);28 }
1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE mapper 3 PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 4 "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 5 <mapper namespace="org.lyh.java.mybatis.mapper.PostMapper"> 6 7 <resultMap id="BaseResultMap" type="org.lyh.java.mybatis.model.Post" > 8 <id column="id" property="id" jdbcType="INTEGER" /> 9 <result column="user_id" property="userId" jdbcType="INTEGER"/>10 <result column="category_id" property="categoryId" jdbcType="INTEGER"/>11 <result column="title" property="title" jdbcType="VARCHAR" />12 <result column="content" property="content" jdbcType="VARCHAR" />13 <result column="created_at" property="createdAt" jdbcType="TIMESTAMP" />14 <result column="updated_at" property="updatedAt" jdbcType="TIMESTAMP"/>15 16 <result column="post_id" property="id" jdbcType="INTEGER"/>17 <result column="post_user_id" property="userId" jdbcType="INTEGER"/>18 <result column="post_category_id" property="categoryId" jdbcType="INTEGER"/>19 <result column="post_title" property="title" jdbcType="VARCHAR" />20 <result column="post_content" property="content" jdbcType="VARCHAR" />21 <result column="post_created_at" property="createdAt" jdbcType="TIMESTAMP" />22 <result column="post_updated_at" property="updatedAt" jdbcType="TIMESTAMP"/>23 </resultMap>24 25 <resultMap id="BaseResultWithUserMap" type="org.lyh.java.mybatis.model.Post">26 <association property="user" column="user_id" javaType="org.lyh.java.mybatis.model.User"27 resultMap="org.lyh.java.mybatis.mapper.UserMapper.BaseResultMap"28 />29 </resultMap>30 31 <!-- SQL配置在下面一一给出 -->32 33 </mapper>
这里除定义了原字段名到模型属性的映射外，还定义了以"post_"前缀开头的字段名到模型属性的映射，这样做是为了后面做关系查询时要用到，是为了防止其余关系表中存在同名字段时，使用as 别名不冲突。
查询
查询使用select标签
按主键查询单条记录
1 <select id="get" resultMap="BaseResultMap">2 select * from m_post where id = #{id}3 </select>
按条件查询多条记录，这里按条件查询记录条数、查询记录只需要将count(1)换成*。
1 <select id="countSizeWithCondition" resultType="int"> 2 SELECT count(1) FROM m_post 3 <if test="conditions != null"> 4 WHERE 5 <foreach item="item" collection="conditions" 6 open="" separator="AND" close=""> 7 ${item.key} ${item.opt} #{item.value} 8 </foreach> 9 </if>10 </select>
按条件查询记录并分页。看网上是有大量的mybatis的分页插件，这里是自己写的分页方法。
1 <select id="getPageDataByCondition" resultMap="BaseResultMap"> 2 SELECT * FROM m_post 3 <if test="conditions != null and conditions.size() > 0"> 4 WHERE 5 <foreach item="item" collection="conditions" 6 open="" separator="AND" close=""> 7 ${item.key} ${item.opt} #{item.value} 8 </foreach> 9 </if>10 <if test="orderProp != null">11 ORDER BY ${orderProp}12 <if test="desc">13 DESC14 </if>15 </if>16 LIMIT #{offset},#{size}17 </select>
当SQL映射需要多个参数时，需要在Mapper对应的方法参数上注解上参数名称，否则只能按mybatis约定的名称或索引来访问变量，比如List会映射到list或者paramter1等等。
更新
更新使用update标签。
指定更新字段更新记录
1 <update id="updateByPrimaryKey" parameterType="org.lyh.java.mybatis.model.Post"> 2 UPDATE m_post SET 3 user_id = #{userId}, 4 category_id = #{categoryId}, 5 title = #{title}, 6 content = #{content}, 7 created_at = #{createdAt}, 8 updated_at = #{updatedAt} 9 WHERE id = #{id}10 </update>
判断属性值更新非null值字段
1 <update id="updateByPrimaryKeySelective" parameterType="org.lyh.java.mybatis.model.Post"> 2 UPDATE m_post 3 SET 4 <foreach collection="post.fieldMap" item="value" index="key" separator=","> 5 <if test="post[value] != null"> 6 ${key} = #{post.${value}} 7 </if> 8 </foreach> 9 WHERE id = #{post.id}10 </update>
注意这里，在foreach中#{post.${value}}基于ongl的语法，由内向外求值，并且，在mybatis中，$与#存在区别，$ 在动态 SQL 解析阶段将会进行变量值string形式替换，# 解析为一个 JDBC 预编译语句（prepared statement）的参数标记符，所以上面xml中的写法是可行。当然还可以在where字句中继续迭代出查询条件。
删除
硬删除
1 <delete id="deleteByPrimaryKey" >2 DELETE FROM m_post WHERE id = #{id}3 </delete>
插入与批量插入
单条插入支持返回auto_increament类型的主键id值
1 <insert id="insert" useGeneratedKeys="true" keyProperty="id" keyColumn="id">2 INSERT INTO m_post (category_id,user_id,title,content)3 VALUE (#{categoryId},#{userId},#{title},#{content})4 <selectKey keyProperty="id" resultType="int" order="AFTER">5 SELECT LAST_INSERT_ID();6 </selectKey>7 </insert>
批量插入，在批量插入时，加了if判断，如果传递的是个空集合，则执行一条select 0语句，insert的返回值为-1，如果执行成功（posts非空），返回值为插入成功的记录条数。
1 <insert id="batchInsert" parameterType="java.util.List"> 2 <if test="posts.size > 0"> 3 INSERT INTO m_post 4 (category_id,user_id, 5 title,content, 6 created_at,updated_at) 7 VALUES 8 <foreach collection="posts" item="post" index="index" separator=","> 9 (#{post.categoryId},#{post.userId},10 #{post.title},#{post.content},11 #{post.createdAt},#{post.updatedAt})12 </foreach>13 </if>14 <if test="posts.size == 0">15 select 0;16 </if>17 </insert>
二、关联查询
resultMap中除了result标签指定字段映射外，还支持以association（1）与collection（n）来映射关系模型的查询结果。
一对一
双向绑定的话，只需要在两端以association配置映射即可。
1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE mapper 3 PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 4 "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 5 <mapper namespace="org.lyh.java.mybatis.mapper.PostMapper"> 6 7 <resultMap id="BaseResultMap" type="org.lyh.java.mybatis.model.Post" > 8 <id column="id" property="id" jdbcType="INTEGER" /> 9 <result column="user_id" property="userId" jdbcType="INTEGER"/>10 <result column="category_id" property="categoryId" jdbcType="INTEGER"/>11 <result column="title" property="title" jdbcType="VARCHAR" />12 <result column="content" property="content" jdbcType="VARCHAR" />13 <result column="created_at" property="createdAt" jdbcType="TIMESTAMP" />14 <result column="updated_at" property="updatedAt" jdbcType="TIMESTAMP"/>15 16 <result column="post_id" property="id" jdbcType="INTEGER"/>17 <result column="post_user_id" property="userId" jdbcType="INTEGER"/>18 <result column="post_category_id" property="categoryId" jdbcType="INTEGER"/>19 <result column="post_title" property="title" jdbcType="VARCHAR" />20 <result column="post_content" property="content" jdbcType="VARCHAR" />21 <result column="post_created_at" property="createdAt" jdbcType="TIMESTAMP" />22 <result column="post_updated_at" property="updatedAt" jdbcType="TIMESTAMP"/>23 </resultMap>24 25 <resultMap id="BaseResultWithUserMap" type="org.lyh.java.mybatis.model.Post">26 <association property="user" column="user_id" javaType="org.lyh.java.mybatis.model.User"27 resultMap="org.lyh.java.mybatis.mapper.UserMapper.BaseResultMap"28 />29 </resultMap>30 </mapper>
主要这里，在association标签中，并没有通过字标签result来映射结果，而是直接通过resultMap属性来映射结果，注意英文UserMapper.BaseResultMap与PostMapper.BaseResultMap并不处在同一个命名空间，所以要写上命名空间。
一对多
一对多以在1端配置collection映射，并在n端配置association映射实现，其中collection配置如下
1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE mapper 3 PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 4 "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 5 <mapper namespace="org.lyh.java.mybatis.mapper.UserMapper"> 6 7 <resultMap id="BaseResultMap" type="org.lyh.java.mybatis.model.User" > 8 <!--<id column="id" property="id" jdbcType="INTEGER" />--> 9 <result column="username" property="username" jdbcType="VARCHAR"/>10 <result column="password" property="password" jdbcType="VARCHAR"/>11 <result column="salt" property="salt" jdbcType="VARCHAR" />12 <result column="avatar" property="avatar" jdbcType="VARCHAR" />13 <result column="type" property="type" typeHandler="org.lyh.java.mybatis.type.UserTypeHandler"/>14 <result column="remember_token" property="rememberToken" jdbcType="VARCHAR"/>15 16 <result column="user_id" property="id" jdbcType="INTEGER"/>17 <result column="user_username" property="username" jdbcType="VARCHAR"/>18 <result column="user_password" property="password" jdbcType="VARCHAR"/>19 <result column="user_salt" property="salt" jdbcType="VARCHAR" />20 <result column="user_avatar" property="avatar" jdbcType="VARCHAR" />21 <result column="user_type" property="type" typeHandler="org.lyh.java.mybatis.type.UserTypeHandler"/>22 <result column="user_remember_token" property="rememberToken" jdbcType="VARCHAR"/>23 </resultMap>24 25 <resultMap id="BaseResultWithPostsMap" type="org.lyh.java.mybatis.model.User" extends="BaseResultMap">26 <collection property="posts" ofType="org.lyh.java.mybatis.model.Post"27 resultMap="org.lyh.java.mybatis.mapper.PostMapper.BaseResultMap"28 column="user_id"29 />30 </resultMap>31 32 <resultMap id="BaseResultSelectPostsMap" type="org.lyh.java.mybatis.model.User" >33 <collection property="posts" ofType="org.lyh.java.mybatis.model.Post"34 select="org.lyh.java.mybatis.mapper.PostMapper.getByUserId"35 column="user_id"36 />37 </resultMap>38 </mapper>
对应的SQL映射可以是关联查询或者先查询主表记录、再查询副表记录。注意如果字段可以确保不会有歧义，则可以直接写字段名，如果有歧义，则应该分别as一个别名，并且是已经在resultMap中配置好了的别名。
1 <select id="getWithPosts" resultMap="BaseResultWithPostsMap"> 2 SELECT 3 user.id AS user_id, 4 username, 5 password, 6 salt, 7 avatar, 8 type, 9 remember_token,10 11 post.id AS post_id,12 category_id,13 title,14 content,15 created_at,16 updated_at17 FROM m_user user18 LEFT OUTER JOIN m_post post ON user.id = post.user_id19 WHERE user.id = #{id}20 </select>
拦截器
Mybatis为每次查询维护了一个拦截器链，通过调用InterceptorChain#pluginAll结合Plugin.wrap方法将待拦截对象转成代理对象，当调用待拦截对象的待拦截方法时，被转发到代理对象执行，而这个代理对象就是mybatis定义大插件或者说拦截器。拦截器通过定义在类上注解Signature说明拦截的class与method定义拦截，并通过配置注册插件。
下面在执行sql语句时拦截打印SQL及执行耗时的拦截器代码。
1 package org.lyh.java.mybatis.interceptor; 2 3 4 import org.apache.ibatis.executor.Executor; 5 import org.apache.ibatis.mapping.BoundSql; 6 import org.apache.ibatis.mapping.MappedStatement; 7 import org.apache.ibatis.mapping.ParameterMapping; 8 import org.apache.ibatis.plugin.*; 9 import org.apache.ibatis.reflection.MetaObject; 10 import org.apache.ibatis.session.Configuration; 11 import org.apache.ibatis.session.ResultHandler; 12 import org.apache.ibatis.session.RowBounds; 13 import org.apache.ibatis.type.TypeHandlerRegistry; 14 15 import java.text.DateFormat; 16 import java.util.Date; 17 import java.util.List; 18 import java.util.Locale; 19 import java.util.Properties; 20 21 /** 22 * @author samlv 23 */ 24 @Intercepts({ 25 @Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class}), 26 @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}) 27 }) 28 public class SQLMonitorPlugin implements Interceptor { 29 30 public Object intercept(Invocation invocation) throws Throwable { 31 MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0]; 32 Object parameter = null; 33 if (invocation.getArgs().length > 1) { 34 parameter = invocation.getArgs()[1]; 35 } 36 String sqlId = mappedStatement.getId(); 37 BoundSql boundSql = mappedStatement.getBoundSql(parameter); 38 Configuration configuration = mappedStatement.getConfiguration(); 39 Object returnValue; 40 long start = System.currentTimeMillis(); 41 returnValue =http://www.mamicode.com/ invocation.proceed(); 42 long end = System.currentTimeMillis(); 43 long time = (end - start); 44 if (time > 1) { 45 String sql = getSql(configuration, boundSql, sqlId, time); 46 System.err.println(sql); 47 } 48 return returnValue; 49 } 50 51 public static String getSql(Configuration configuration, BoundSql boundSql, String sqlId, long time) { 52 String sql = showSql(configuration, boundSql); 53 return sqlId + " : " + sql + " : " + time + "ms"; 54 } 55 56 private static String getParameterValue(Object obj) { 57 String value; 58 if (obj instanceof String) { 59 valuehttp://www.mamicode.com/= "http://www.mamicode.com/‘" + obj.toString() + "‘"; 60 } else if (obj instanceof Date) { 61 DateFormat formatter = DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT, Locale.CHINA); 62 valuehttp://www.mamicode.com/= "http://www.mamicode.com/‘" + formatter.format(new Date()) + "‘"; 63 } else { 64 if (obj != null) { 65 value =http://www.mamicode.com/ obj.toString(); 66 } else { 67 valuehttp://www.mamicode.com/= ""; 68 } 69 70 } 71 return value; 72 } 73 74 public static String showSql(Configuration configuration, BoundSql boundSql) { 75 Object parameterObject = boundSql.getParameterObject(); 76 List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); 77 String sql = boundSql.getSql().replaceAll("[\\s]+", " "); 78 if (parameterMappings.size() > 0 && parameterObject != null) { 79 TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry(); 80 if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) { 81 sql = sql.replaceFirst("\\?", getParameterValue(parameterObject)); 82 83 } else { 84 MetaObject metaObject = configuration.newMetaObject(parameterObject); 85 for (ParameterMapping parameterMapping : parameterMappings) { 86 String propertyName = parameterMapping.getProperty(); 87 if (metaObject.hasGetter(propertyName)) { 88 Object obj = metaObject.getValue(propertyName); 89 sql = sql.replaceFirst("\\?", getParameterValue(obj)); 90 } else if (boundSql.hasAdditionalParameter(propertyName)) { 91 Object obj = boundSql.getAdditionalParameter(propertyName); 92 sql = sql.replaceFirst("\\?", getParameterValue(obj)); 93 } 94 } 95 } 96 } 97 return sql; 98 } 99 100 101 public Object plugin(Object o) {102 return Plugin.wrap(o, this);103 }104 105 public void setProperties(Properties properties) {106 107 }108 }
注册插件，xml方式，这里没有单独为mybatis创建配置文件，而是直接在spring配置文件中定义插件，效果是一样的。
1 <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean"> 2 <property name="dataSource" ref="dataSource"/> 3 <!--<property name="configLocation" value="http://www.mamicode.com/classpath:mybatis-config.xml"/>--> 4 <property name="plugins"> 5 <array> 6 <bean class="org.lyh.java.mybatis.interceptor.SQLMonitorPlugin"/> 7 </array> 8 </property> 9 <!-- 自动扫描mapping.xml文件 -->10 <property name="mapperLocations" value="classpath:org/lyh/java/mybatis/mapper/*.xml"/>11 </bean>
三、源码浅析
Mapper代理对象获取
首先看调用栈
Mybatis通过调用SqlSession.getMapper方法，传递mapperInterface(PostMapper.class)为参数，最后以sqlSession，mapperInterface，methodCache为参数构造得到代理对象MapperProxy。最后对mapperInterface（PostMapper）的方法调用，都转发到代理对象执行invoke方法。
调用mapper接口的方法，将调用mapperProxy.invoke方法。在invoke方法中，会封装一个MapperMethod对象，这是被调用的mapper方法的进一步封装。
1 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 2 if (Object.class.equals(method.getDeclaringClass())) { 3 try { 4 return method.invoke(this, args); 5 } catch (Throwable t) { 6 throw ExceptionUtil.unwrapThrowable(t); 7 } 8 } 9 final MapperMethod mapperMethod = cachedMapperMethod(method);10 return mapperMethod.execute(sqlSession, args);11 }
拦截器执行
sqlSession#selectOne调用被代理到org.mybatis.spring.SqlSessionTemplate.SqlSessionInterceptor#invoke方法上
有四个地方调用了拦截器链的pluginAll方法，pluginAll实际是将待执行对象代理到代理对象上，也就是Plugin对象，demo程序中就是SQLMonitorPlugin。下面列表顺序也代表了被拦截的顺序
- org.apache.ibatis.session.Configuration#newExecutor
- org.apache.ibatis.session.Configuration#newParameterHandler
- org.apache.ibatis.session.Configuration#newResultSetHandler
- org.apache.ibatis.session.Configuration#newStatementHandler
执行查询
在MapperProxy中调用org.apache.ibatis.binding.MapperMethod#execute方法，可以看到该方法默认时调用selectOne查询方法，在做多表（一对多）连接查询时，要保证主表与副表id不要一致，配置的resultMap不要相同，否则mybatis会认为主表查询结果返回了多条记录，从而抛出org.apache.ibatis.exceptions.TooManyResultsException异常。convertArgsToSqlCommandParam转换Mapper接口被调方法的参数为基础包装类、集合类等等。
1 public Object execute(SqlSession sqlSession, Object[] args) { 2 // ... 3 Object result; 4 switch (command.getType()) { 5 case SELECT: 6 if (method.returnsVoid() && method.hasResultHandler()) { 7 executeWithResultHandler(sqlSession, args); 8 result = null; 9 } else if (method.returnsMany()) {10 result = executeForMany(sqlSession, args);11 } else if (method.returnsMap()) {12 result = executeForMap(sqlSession, args);13 } else if (method.returnsCursor()) {14 result = executeForCursor(sqlSession, args);15 } else {16 Object param = method.convertArgsToSqlCommandParam(args);17 result = sqlSession.selectOne(command.getName(), param);18 }19 break;20 }21 // ...22 return result;23 }
执行步骤如下：
- sqlSession实际是SQLSessionTemplate类的对象，调用其selectOne方法，最终调用的是代理方法SqlSessionInterceptor#invoke，在该方法中，获取到一个sqlSession（实际是DefaultSqlSession），
- 调用DefaultSqlSession#selectOne方法进行查询。DefaultSqlSession中封装了所有的对数据库的CRUD操作接口。
- 在DefaultSqlSession#selectList方法中获取了一个特殊的对象MappedStatement，这个对象是对mapper xml中sql、参数及resultMap的封装。
- 以MappedStatement、查询参数、分页参数、返回结果处理类（这里是null）为参数调用CachingExecutor#query方法
- 前面说到，因为Executor已经被代理到SQLMonitorPlugin对象，所以第一个拦截器被执行
- 在拦截器中，才再次调用CachingExecutor#query方法，在该方法中生成SQL，由SQL及查询参数得到查询缓存的Key
- 最后再缓存不存在的情况下，会调用到BaseExecutor#queryFromDatabase方法
- 最后调用SimpleExecutor#doQuery方法得到查询，在该方法中，会调用创建各种Handler（如StatementHandler），如果有对应拦截器，Handler就对被代理到拦截器
- 最后执行了查询之后，调用DefaultResultSetHandler#handleResultSets按照mappedStatement.getResultMaps()解析查询结果
具体步骤可以以测试代码debug一次
示例代码位置
https://github.com/lvyahui8/java-all/tree/master/mybatis-all
另外可参考阅读笔者之前写的
基于原始JDBC+方式写的通用DAO类
http://www.cnblogs.com/lvyahui/p/4009961.html
与
通用数库查询
http://www.cnblogs.com/lvyahui/p/5626466.html
笔者一直希望能将一些简单基础的CRUD操作一键化，工程化，省去一些简单且重复的劳动。
MyBatis浅尝笔记
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