总结一下工作中用到的Mybatis业务逻辑

1.简单说明。

　　MyBatis 可以使用简单的XML 或注解用于配置和原始映射，将接口和 Java 的 POJO（ Plain Old Java Objects，普通的Java 对象）映射成数据库中的记录。

　　每一个MyBatis项目中都以一个对象sqlSessionFactory为核心，它可以通过sqlSessionFactoryBuilder来获得，它产生了一个个的sqlSession。

2.关于项目中的使用做详尽说明。

　　什么是Mapper对象？根据Mybatis的官方手册，应用程序除了要初始并启动Mybatis之外，还需要定义一些接口，接口里定义访问数据库的方法，存放接口的包路径下需要放置同名的XML配置文件。每个mapper元素对应一个mapper配置文件。

3.Mybatis中参数传递的四种方式讲解

　　http://edu.51cto.com/index.php?do=lesson&id=69884

4.使用mybatis generator自动生成映射文件详解

　　http://www.cnblogs.com/mikelij/p/3841716.html  跟这篇一模一样。

5.项目实例分析

 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 <mapper namespace="org.iMybatis.abc.dao.UserDao">
     <cache type="PERPETUAL" eviction="LRU" flushInterval="60000"
         size="512" readOnly="true" />
     <resultMap id="userResultMap" type="UserDto">
         <id property="userid" column="userid" />
         <result property="username" column="username" />
         <result property="password" column="password" />
     </resultMap>
     <sql id="userColumns"> userid,username,password </sql>
     <select id="queryUsers" parameterType="UserDto" resultType="UserDto"
         useCache="false">
         select <include refid="userColumns" />
         from t_user t where t.username = #{username}
     </select>
     <insert id="insertUser" parameterType="UserDto"
         useGeneratedKeys="true" keyProperty="userid">
         insert into t_user (userid,username,password)
         values (#{userid},#{username},#{password})
     </insert>
     <update id="updateUser" parameterType="UserDto">
         update t_user set
         username= #{username},
         password = #{password},
         where userid = #{userid}
     </update>
     <delete id="deleteUser" parameterType="UserDto">
         delete from t_user where userid = #{userid}
     </delete>
 </mapper>

一段热乎乎的mapper的 XML 的配置文件

　　看上面代码，Mapper元素只有一个属性namespace，它有两个作用：一是用于区分不同的mapper（在不同的mapper文件里，子元素的id可以相同，mybatis通过namespace和子元素的id联合区分），二是与接口关联（应用程序通过接口访问mybatis时，mybatis通过接口的完整名称查找对应的mapper配置，因此namespace的命名务必小心一定要某接口同名）。此外，mapper配置文件还有几个顶级子元素（它们须按照顺序定义）：cache -配置本定命名空间的缓存。resultMap –结果映射，用来描述如何从数据库结果集映射到你想要的对象。接下来详解一下resultMap。

<resultMap id="userResultMap" type="User">
    <id property=" userid " column="userid" />
    <result property="username" column="username" />
    <result property="password" column="password" />
</resultMap> 

resultMap 配置

　　resultMap提供了从数据库表列名到java对象属性的映射管理，示例只是提供了最简单的情况。在mapper配置文件中可以配置多个resultMap，不同的resultMap用id加以区分。type属性标记java类型（别名）。子元素中的property指带java中的属性，column指带数据库表的列名。

　　Example类，用于构造复杂的筛选条件，详细分析如下：

上面的sql语句，创建了表'order_detail',以及各个字段。

package redcliff.cobara.entity;

import java.math.BigDecimal;

public class CobaraOrderDetail {
    private Long id;
    private Integer cityId;
    private Integer orderId;
    private String name;
    private Byte type;
    private BigDecimal cost;
    private Integer shardx;
    private Integer shardy;
    private Integer shardz;

    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }

    public Integer getCityId() {
        return cityId;
    }
    public void setCityId(Integer cityId) {
        this.cityId = cityId;
    }

    public Integer getOrderId() {
        return orderId;
    }
    public void setOrderId(Integer orderId) {
        this.orderId = orderId;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name == null ? null : name.trim();
    }

    public Byte getType() {
        return type;
    }
    public void setType(Byte type) {
        this.type = type;
    }

    public BigDecimal getCost() {
        return cost;
    }
    public void setCost(BigDecimal cost) {
        this.cost = cost;
    }

    public Integer getShardx() {
        return shardx;
    }
    public void setShardx(Integer shardx) {
        this.shardx = shardx;
    }

    public Integer getShardy() {
        return shardy;
    }
    public void setShardy(Integer shardy) {
        this.shardy = shardy;
    }

    public Integer getShardz() {
        return shardz;
    }
    public void setShardz(Integer shardz) {
        this.shardz = shardz;
    }
}

redcliff.cobara.entity.CobaraOrderDetail.java

上面的entitiy包下的CobaraOrderDetail.java文件定义了实体类，是数据库的各个字段在Java web项目中一模一样的写照。当然了它是mybatis的generator生成器自动生成的。注意到，结构public class CobaraOrderDetail;里面的变量定义为private类型，方法定义为public类型。

package redcliff.cobara.entity;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

public class CobaraOrderDetailExample {

    protected String orderByClause;
    protected boolean distinct;
    protected List<Criteria> oredCriteria;

    public CobaraOrderDetailExample() {
        oredCriteria = new ArrayList<Criteria>();
    }

    public void setOrderByClause(String orderByClause) {
        this.orderByClause = orderByClause;
    }

    public String getOrderByClause() {
        return orderByClause;
    }

    public void setDistinct(boolean distinct) {
        this.distinct = distinct;
    }

    public boolean isDistinct() {
        return distinct;
    }

    public List<Criteria> getOredCriteria() {
        return oredCriteria;
    }

    public void or(Criteria criteria) {
        oredCriteria.add(criteria);
    }

    public Criteria or() {
        Criteria criteria = createCriteriaInternal();
        oredCriteria.add(criteria);
        return criteria;
    }

    public Criteria createCriteria() {
        Criteria criteria = createCriteriaInternal();
        if (oredCriteria.size() == 0) {
            oredCriteria.add(criteria);
        }
        return criteria;
    }

    protected Criteria createCriteriaInternal() {
        Criteria criteria = new Criteria();
        return criteria;
    }

    public void clear() {
        oredCriteria.clear();
        orderByClause = null;
        distinct = false;
    }

    protected abstract static class GeneratedCriteria {
        protected List<Criterion> criteria;
        protected GeneratedCriteria() {
            super();
            criteria = new ArrayList<Criterion>();
        }

        public boolean isValid() {
            return criteria.size() > 0;
        }

        public List<Criterion> getAllCriteria() {
            return criteria;
        }

        public List<Criterion> getCriteria() {
            return criteria;
        }

        protected void addCriterion(String condition) {
            if (condition == null) {
                throw new RuntimeException("Value for condition cannot be null");
            }
            criteria.add(new Criterion(condition));
        }

        protected void addCriterion(String condition, Object value, String property) {
            if (value == null) {
                throw new RuntimeException("Value for " + property + " cannot be null");
            }
            criteria.add(new Criterion(condition, value));
        }

        protected void addCriterion(String condition, Object value1, Object value2, String property) {
            if (value1 == null || value2 == null) {
                throw new RuntimeException("Between values for " + property + " cannot be null");
            }
            criteria.add(new Criterion(condition, value1, value2));
        }

        public Criteria andIdIsNull() {
            addCriterion("id is null");
            return (Criteria) this;
        }

        public Criteria andIdIsNotNull() {
            addCriterion("id is not null");
            return (Criteria) this;
        }

        public Criteria andIdEqualTo(Long value) {
            addCriterion("id =", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotEqualTo(Long value) {
            addCriterion("id <>", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdGreaterThan(Long value) {
            addCriterion("id >", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdGreaterThanOrEqualTo(Long value) {
            addCriterion("id >=", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdLessThan(Long value) {
            addCriterion("id <", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdLessThanOrEqualTo(Long value) {
            addCriterion("id <=", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdIn(List<Long> values) {
            addCriterion("id in", values, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotIn(List<Long> values) {
            addCriterion("id not in", values, "id");
            return (Criteria) this;
        }

        public Criteria andIdBetween(Long value1, Long value2) {
            addCriterion("id between", value1, value2, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotBetween(Long value1, Long value2) {
            addCriterion("id not between", value1, value2, "id");
            return (Criteria) this;
        }

        public Criteria andCityIdIsNull() {
            addCriterion("city_id is null");
            return (Criteria) this;
        }

        public Criteria andCityIdIsNotNull() {
            addCriterion("city_id is not null");
            return (Criteria) this;
        }

        public Criteria andCityIdEqualTo(Integer value) {
            addCriterion("city_id =", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdNotEqualTo(Integer value) {
            addCriterion("city_id <>", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdGreaterThan(Integer value) {
            addCriterion("city_id >", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdGreaterThanOrEqualTo(Integer value) {
            addCriterion("city_id >=", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdLessThan(Integer value) {
            addCriterion("city_id <", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdLessThanOrEqualTo(Integer value) {
            addCriterion("city_id <=", value, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdIn(List<Integer> values) {
            addCriterion("city_id in", values, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdNotIn(List<Integer> values) {
            addCriterion("city_id not in", values, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdBetween(Integer value1, Integer value2) {
            addCriterion("city_id between", value1, value2, "cityId");
            return (Criteria) this;
        }

        public Criteria andCityIdNotBetween(Integer value1, Integer value2) {
            addCriterion("city_id not between", value1, value2, "cityId");
            return (Criteria) this;
        }

        public Criteria andOrderIdIsNull() {
            addCriterion("order_id is null");
            return (Criteria) this;
        }

        public Criteria andOrderIdIsNotNull() {
            addCriterion("order_id is not null");
            return (Criteria) this;
        }

        public Criteria andOrderIdEqualTo(Integer value) {
            addCriterion("order_id =", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdNotEqualTo(Integer value) {
            addCriterion("order_id <>", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdGreaterThan(Integer value) {
            addCriterion("order_id >", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdGreaterThanOrEqualTo(Integer value) {
            addCriterion("order_id >=", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdLessThan(Integer value) {
            addCriterion("order_id <", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdLessThanOrEqualTo(Integer value) {
            addCriterion("order_id <=", value, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdIn(List<Integer> values) {
            addCriterion("order_id in", values, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdNotIn(List<Integer> values) {
            addCriterion("order_id not in", values, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdBetween(Integer value1, Integer value2) {
            addCriterion("order_id between", value1, value2, "orderId");
            return (Criteria) this;
        }

        public Criteria andOrderIdNotBetween(Integer value1, Integer value2) {
            addCriterion("order_id not between", value1, value2, "orderId");
            return (Criteria) this;
        }

        public Criteria andNameIsNull() {
            addCriterion("name is null");
            return (Criteria) this;
        }

        public Criteria andNameIsNotNull() {
            addCriterion("name is not null");
            return (Criteria) this;
        }

        public Criteria andNameEqualTo(String value) {
            addCriterion("name =", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameNotEqualTo(String value) {
            addCriterion("name <>", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameGreaterThan(String value) {
            addCriterion("name >", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameGreaterThanOrEqualTo(String value) {
            addCriterion("name >=", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameLessThan(String value) {
            addCriterion("name <", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameLessThanOrEqualTo(String value) {
            addCriterion("name <=", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameLike(String value) {
            addCriterion("name like", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameNotLike(String value) {
            addCriterion("name not like", value, "name");
            return (Criteria) this;
        }

        public Criteria andNameIn(List<String> values) {
            addCriterion("name in", values, "name");
            return (Criteria) this;
        }

        public Criteria andNameNotIn(List<String> values) {
            addCriterion("name not in", values, "name");
            return (Criteria) this;
        }

        public Criteria andNameBetween(String value1, String value2) {
            addCriterion("name between", value1, value2, "name");
            return (Criteria) this;
        }

        public Criteria andNameNotBetween(String value1, String value2) {
            addCriterion("name not between", value1, value2, "name");
            return (Criteria) this;
        }

        public Criteria andTypeIsNull() {
            addCriterion("type is null");
            return (Criteria) this;
        }

        public Criteria andTypeIsNotNull() {
            addCriterion("type is not null");
            return (Criteria) this;
        }

        public Criteria andTypeEqualTo(Byte value) {
            addCriterion("type =", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeNotEqualTo(Byte value) {
            addCriterion("type <>", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeGreaterThan(Byte value) {
            addCriterion("type >", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeGreaterThanOrEqualTo(Byte value) {
            addCriterion("type >=", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeLessThan(Byte value) {
            addCriterion("type <", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeLessThanOrEqualTo(Byte value) {
            addCriterion("type <=", value, "type");
            return (Criteria) this;
        }

        public Criteria andTypeIn(List<Byte> values) {
            addCriterion("type in", values, "type");
            return (Criteria) this;
        }

        public Criteria andTypeNotIn(List<Byte> values) {
            addCriterion("type not in", values, "type");
            return (Criteria) this;
        }

        public Criteria andTypeBetween(Byte value1, Byte value2) {
            addCriterion("type between", value1, value2, "type");
            return (Criteria) this;
        }

        public Criteria andTypeNotBetween(Byte value1, Byte value2) {
            addCriterion("type not between", value1, value2, "type");
            return (Criteria) this;
        }

        public Criteria andCostIsNull() {
            addCriterion("cost is null");
            return (Criteria) this;
        }

        public Criteria andCostIsNotNull() {
            addCriterion("cost is not null");
            return (Criteria) this;
        }

        public Criteria andCostEqualTo(BigDecimal value) {
            addCriterion("cost =", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostNotEqualTo(BigDecimal value) {
            addCriterion("cost <>", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostGreaterThan(BigDecimal value) {
            addCriterion("cost >", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostGreaterThanOrEqualTo(BigDecimal value) {
            addCriterion("cost >=", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostLessThan(BigDecimal value) {
            addCriterion("cost <", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostLessThanOrEqualTo(BigDecimal value) {
            addCriterion("cost <=", value, "cost");
            return (Criteria) this;
        }

        public Criteria andCostIn(List<BigDecimal> values) {
            addCriterion("cost in", values, "cost");
            return (Criteria) this;
        }

        public Criteria andCostNotIn(List<BigDecimal> values) {
            addCriterion("cost not in", values, "cost");
            return (Criteria) this;
        }

        public Criteria andCostBetween(BigDecimal value1, BigDecimal value2) {
            addCriterion("cost between", value1, value2, "cost");
            return (Criteria) this;
        }

        public Criteria andCostNotBetween(BigDecimal value1, BigDecimal value2) {
            addCriterion("cost not between", value1, value2, "cost");
            return (Criteria) this;
        }

        public Criteria andShardxIsNull() {
            addCriterion("shardx is null");
            return (Criteria) this;
        }

        public Criteria andShardxIsNotNull() {
            addCriterion("shardx is not null");
            return (Criteria) this;
        }

        public Criteria andShardxEqualTo(Integer value) {
            addCriterion("shardx =", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxNotEqualTo(Integer value) {
            addCriterion("shardx <>", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxGreaterThan(Integer value) {
            addCriterion("shardx >", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxGreaterThanOrEqualTo(Integer value) {
            addCriterion("shardx >=", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxLessThan(Integer value) {
            addCriterion("shardx <", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxLessThanOrEqualTo(Integer value) {
            addCriterion("shardx <=", value, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxIn(List<Integer> values) {
            addCriterion("shardx in", values, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxNotIn(List<Integer> values) {
            addCriterion("shardx not in", values, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxBetween(Integer value1, Integer value2) {
            addCriterion("shardx between", value1, value2, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardxNotBetween(Integer value1, Integer value2) {
            addCriterion("shardx not between", value1, value2, "shardx");
            return (Criteria) this;
        }

        public Criteria andShardyIsNull() {
            addCriterion("shardy is null");
            return (Criteria) this;
        }

        public Criteria andShardyIsNotNull() {
            addCriterion("shardy is not null");
            return (Criteria) this;
        }

        public Criteria andShardyEqualTo(Integer value) {
            addCriterion("shardy =", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyNotEqualTo(Integer value) {
            addCriterion("shardy <>", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyGreaterThan(Integer value) {
            addCriterion("shardy >", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyGreaterThanOrEqualTo(Integer value) {
            addCriterion("shardy >=", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyLessThan(Integer value) {
            addCriterion("shardy <", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyLessThanOrEqualTo(Integer value) {
            addCriterion("shardy <=", value, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyIn(List<Integer> values) {
            addCriterion("shardy in", values, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyNotIn(List<Integer> values) {
            addCriterion("shardy not in", values, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyBetween(Integer value1, Integer value2) {
            addCriterion("shardy between", value1, value2, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardyNotBetween(Integer value1, Integer value2) {
            addCriterion("shardy not between", value1, value2, "shardy");
            return (Criteria) this;
        }

        public Criteria andShardzIsNull() {
            addCriterion("shardz is null");
            return (Criteria) this;
        }

        public Criteria andShardzIsNotNull() {
            addCriterion("shardz is not null");
            return (Criteria) this;
        }

        public Criteria andShardzEqualTo(Integer value) {
            addCriterion("shardz =", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzNotEqualTo(Integer value) {
            addCriterion("shardz <>", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzGreaterThan(Integer value) {
            addCriterion("shardz >", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzGreaterThanOrEqualTo(Integer value) {
            addCriterion("shardz >=", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzLessThan(Integer value) {
            addCriterion("shardz <", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzLessThanOrEqualTo(Integer value) {
            addCriterion("shardz <=", value, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzIn(List<Integer> values) {
            addCriterion("shardz in", values, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzNotIn(List<Integer> values) {
            addCriterion("shardz not in", values, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzBetween(Integer value1, Integer value2) {
            addCriterion("shardz between", value1, value2, "shardz");
            return (Criteria) this;
        }

        public Criteria andShardzNotBetween(Integer value1, Integer value2) {
            addCriterion("shardz not between", value1, value2, "shardz");
            return (Criteria) this;
        }
    }

    public static class Criteria extends GeneratedCriteria {

        protected Criteria() {
            super();
        }
    }

    public static class Criterion {

        private String condition;
        private Object value;
        private Object secondValue;
        private boolean noValue;
        private boolean singleValue;
        private boolean betweenValue;
        private boolean listValue;
        private String typeHandler;

        public String getCondition() {
            return condition;
        }

        public Object getValue() {
            return value;
        }

        public Object getSecondValue() {
            return secondValue;
        }

        public boolean isNoValue() {
            return noValue;
        }

        public boolean isSingleValue() {
            return singleValue;
        }

        public boolean isBetweenValue() {
            return betweenValue;
        }

        public boolean isListValue() {
            return listValue;
        }

        public String getTypeHandler() {
            return typeHandler;
        }

        protected Criterion(String condition) {
            super();
            this.condition = condition;
            this.typeHandler = null;
            this.noValue = true;
        }

        protected Criterion(String condition, Object value, String typeHandler) {
            super();
            this.condition = condition;
            this.value = value;
            this.typeHandler = typeHandler;
            if (value instanceof List<?>) {
                this.listValue = true;
            } else {
                this.singleValue = true;
            }
        }

        protected Criterion(String condition, Object value) {
            this(condition, value, null);
        }

        protected Criterion(String condition, Object value, Object secondValue, String typeHandler) {
            super();
            this.condition = condition;
            this.value = value;
            this.secondValue = secondValue;
            this.typeHandler = typeHandler;
            this.betweenValue = true;
        }

        protected Criterion(String condition, Object value, Object secondValue) {
            this(condition, value, secondValue, null);
        }
    }
}

redcliff.cobara.entity.CobaraOrderDetailExample.java

上面的entity包下的CobaraOrderDetailExample.java文件同样也是用于映射的。只不过它用于构造复杂的筛选条件。注意到，里面的变量定义为protected类型，方法定义为public类型;结构public class CobaraOrderDetailExample。两个Java类独立存在，不相互依存。

package redcliff.cobara.mapper;

import com.dianwoba.redcliff.cobara.entity.CobaraOrderDetail;
import com.dianwoba.redcliff.cobara.entity.CobaraOrderDetailExample;
import java.util.List;
import org.apache.ibatis.annotations.Param;

public interface CobaraOrderDetailMapper {

    int countByExample(CobaraOrderDetailExample example);

    int deleteByExample(CobaraOrderDetailExample example);

    int deleteByPrimaryKey(Long id);

    int insert(CobaraOrderDetail record);

    int insertSelective(CobaraOrderDetail record);

    List<CobaraOrderDetail> selectByExample(CobaraOrderDetailExample example);

    CobaraOrderDetail selectByPrimaryKey(Long id);

    int updateByExampleSelective(@Param("record") CobaraOrderDetail record, @Param("example") CobaraOrderDetailExample example);

    int updateByExample(@Param("record") CobaraOrderDetail record, @Param("example") CobaraOrderDetailExample example);

    int updateByPrimaryKeySelective(CobaraOrderDetail record);

    int updateByPrimaryKey(CobaraOrderDetail record);
}

redcliff.cobara.mapper.CobaraOrderDetailMapper.java

上面的mapper包下的CobaraOrderDetailMapper.java类中，定义了基于类CobaraOrderDetailExample的CRUD方法。从结构public interface CobarOrderDetailExample上来看，当然是没有实现的。

package redcliff.cobara.mapper;

public interface CobaraOrderDetailMapperExt extends CobaraOrderDetailMapper {

}

redcliff.cobara.mapper.CobaraOrderDetailMapperExt.java

上面的mapper包下的CobaraOrderDetailMapperExt.java类中，是一个空的接口。从结构上来看public interface CobaraOrderDetailMapperExt  extends CobaraOrderDetailMapper。从名字上来看Ext是英文单词external的简写，'external'的本意是外部的，外面的简写。可见此类是用于扩展的。

redcliff.cobara.mapper.CobaraOrderDetailMapper.xml

上面的redcliff.cobara.mapper.CobaraOrderDetailMapperExt.xml映射文件定义了基本的CRUD四个方法。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="redcliff.cobara.mapper.CobaraOrderDetailMapperExt" >

</mapper>

redcliff.cobara.mapper.CobaraOrderDetailMapperExt.xml

上面的redcliff.cobara.mapper.CobaraOrderDetailMapperExt.xml映射文件，扩展了CRUD操作，因为从mapper标签的唯一属性namespace两个xml是完全一样的。

以上的这些代码，用一句话描述就是：“entity.java建立了与数据库的映射，mapper.java中定义了CRUD的方法，mapper.xml中实现了方法”，那么他们是怎么联系起来的呢？mapper.java中的方法接收的参数是entityExample(半成品的sql语句)。mapper标签的namespace指示到了mapper.java类，意思是我对你的方法进行了实现。

附录，补充

学习资料参考地址