新建了一张user表，含有id,username,password

通过逆向工程生成四个文件：User.java,UserExample.java,UserMapper.java,UserMapper.xml

User.java

public class User {
    private Integer id;

    private String username;

    private String password;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username == null ? null : username.trim();
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }
}

User类的作用是将Id,username,password作为私有属性封装起来。

UserExample.java

public class UserExample {
    protected String orderByClause;//排序条件

    protected boolean distinct;//是否进行排序

    protected List<Criteria> oredCriteria;//旧标准集合

    public UserExample() {//初始化一个旧标准集合
        oredCriteria = new ArrayList<Criteria>();
    }

    public void setOrderByClause(String orderByClause) {//设置排序方式
        this.orderByClause = orderByClause;
    }

    public String getOrderByClause() {//获取排序方式
        return orderByClause;
    }

    public void setDistinct(boolean distinct) {//设置是否排序
        this.distinct = distinct;
    }

    public boolean isDistinct() {//获取是否排序
        return distinct;
    }

    public List<Criteria> getOredCriteria() {//获取标准集合
        return oredCriteria;
    }

    public void or(Criteria criteria) {//旧标准集合加新标准集合
        oredCriteria.add(criteria);
    }

    public Criteria or() {//初始化一个新标准集合并且和旧标准集合相加
        Criteria criteria = createCriteriaInternal();
        oredCriteria.add(criteria);
        return criteria;
    }

    public Criteria createCriteria() {//初始化一个新标准集合并且旧标准集合长度为0时和新集合相加
        Criteria criteria = createCriteriaInternal();
        if (oredCriteria.size() == 0) {
            oredCriteria.add(criteria);
        }
        return criteria;
    }

    protected Criteria createCriteriaInternal() {//初始化一个新标准集合
        Criteria criteria = new Criteria();
        return criteria;
    }

    public void clear() {//清除旧标准集合
        oredCriteria.clear();
        orderByClause = null;
        distinct = false;
    }

    protected abstract static class GeneratedCriteria {
        protected List<Criterion> criteria;//新标准集合

        protected GeneratedCriteria() {//初始化一个新标准集合
            super();
            criteria = new ArrayList<Criterion>();
        }

        public boolean isValid() {//判断新标准集合长度是否不为0
            return criteria.size() > 0;
        }

        public List<Criterion> getAllCriteria() {//获取标准集合
            return criteria;
        }

        public List<Criterion> getCriteria() {//获取标准集合
            return criteria;
        }

        protected void addCriterion(String condition) {//设置约束
            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) {//设置某属性的约束
                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) {//设置某属性的范围
            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() {//判断id是否为空
            addCriterion("id is null");
            return (Criteria) this;
        }

        public Criteria andIdIsNotNull() {//判断id是否不为空
            addCriterion("id is not null");
            return (Criteria) this;
        }

        public Criteria andIdEqualTo(Integer value) {//判断id的值是否与某数相等
            addCriterion("id =", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotEqualTo(Integer value) {//判断id的值是否与某数不相等
            addCriterion("id <>", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdGreaterThan(Integer value) {//判断id是否大于某值
            addCriterion("id >", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdGreaterThanOrEqualTo(Integer value) {//判断id是否大于等于某值
            addCriterion("id >=", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdLessThan(Integer value) {//判断id是否小于某值
            addCriterion("id <", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdLessThanOrEqualTo(Integer value) {//判断id是否小于等于某值
            addCriterion("id <=", value, "id");
            return (Criteria) this;
        }

        public Criteria andIdIn(List<Integer> values) {//判断id是否在某范围内
            addCriterion("id in", values, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotIn(List<Integer> values) {//判断id是否不在某范围内
            addCriterion("id not in", values, "id");
            return (Criteria) this;
        }

        public Criteria andIdBetween(Integer value1, Integer value2) {//判断id在某范围内
            addCriterion("id between", value1, value2, "id");
            return (Criteria) this;
        }

        public Criteria andIdNotBetween(Integer value1, Integer value2) {//判断id不在某范围内
            addCriterion("id not between", value1, value2, "id");
            return (Criteria) this;
        }

        public Criteria andUsernameIsNull() {
            addCriterion("username is null");
            return (Criteria) this;
        }

        public Criteria andUsernameIsNotNull() {
            addCriterion("username is not null");
            return (Criteria) this;
        }

        public Criteria andUsernameEqualTo(String value) {
            addCriterion("username =", value, "username");
            return (Criteria) this;
        }

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

        public Criteria andUsernameGreaterThan(String value) {
            addCriterion("username >", value, "username");
            return (Criteria) this;
        }

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

        public Criteria andUsernameLessThan(String value) {
            addCriterion("username <", value, "username");
            return (Criteria) this;
        }

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

        public Criteria andUsernameLike(String value) {
            addCriterion("username like", value, "username");
            return (Criteria) this;
        }

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

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

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

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

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

        public Criteria andPasswordIsNull() {
            addCriterion("password is null");
            return (Criteria) this;
        }

        public Criteria andPasswordIsNotNull() {
            addCriterion("password is not null");
            return (Criteria) this;
        }

        public Criteria andPasswordEqualTo(String value) {
            addCriterion("password =", value, "password");
            return (Criteria) this;
        }

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

        public Criteria andPasswordGreaterThan(String value) {
            addCriterion("password >", value, "password");
            return (Criteria) this;
        }

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

        public Criteria andPasswordLessThan(String value) {
            addCriterion("password <", value, "password");
            return (Criteria) this;
        }

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

        public Criteria andPasswordLike(String value) {
            addCriterion("password like", value, "password");
            return (Criteria) this;
        }

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

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

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

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

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

    public static class Criteria extends GeneratedCriteria {

        protected Criteria() {
            super();
        }
    }

    public static class Criterion {
        private String condition;//约束

        private Object value;//值

        private Object secondValue;//第二值

        private boolean noValue;//是否没有值

        private boolean singleValue;//是否单一值

        private boolean betweenValue;//是否范围值

        private boolean listValue;//是否列值

        private String typeHandler;//Data值转化为String值

        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);
        }
    }
}

以id为基础注解的每个函数的作用，criterion内的都为取值，判断和构造函数，所以不做注解

UserMapper.xml

public interface UserMapper {
    int countByExample(UserExample example);//统计example的数目

    int deleteByExample(UserExample example);//根据example删除对象

    int deleteByPrimaryKey(Integer id);//根据主键删除对象

    int insert(User record);//插入User对象(没有传入的数值就设置为NULL)

    int insertSelective(User record);//插入User对像(没有传入的参数就不保存到数据库)

    List<User> selectByExample(UserExample example);//根据条件筛选对象集合

    User selectByPrimaryKey(Integer id);//根据主键查询对象

    int updateByExampleSelective(@Param("record") User record, @Param("example") UserExample example);//根据example更新User对象(没有传入的参数就不保存到数据库)

    int updateByExample(@Param("record") User record, @Param("example") UserExample example);//根据example更新User对象(没有传入的数值就设置为NULL)

    int updateByPrimaryKeySelective(User record);根据主键更新User对象(没有传入的参数就不修改数据库中的数据)

    int updateByPrimaryKey(User record);根据主键更新User对象(没有传入的参数就设置为NULL)
}

UserMapper.xml提供实现方法

UserMapper.xml

<?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="com.dorm.mapper.UserMapper" >
  <resultMap id="BaseResultMap" type="com.dorm.pojo.User" >
    <id column="id" property="id" jdbcType="INTEGER" />
    <result column="username" property="username" jdbcType="VARCHAR" />
    <result column="password" property="password" jdbcType="VARCHAR" />
  </resultMap>
  <sql id="Example_Where_Clause" >
    <where >
      <foreach collection="oredCriteria" item="criteria" separator="or" >
        <if test="criteria.valid" >
          <trim prefix="(" suffix=")" prefixOverrides="and" >
            <foreach collection="criteria.criteria" item="criterion" >
              <choose >
                <when test="criterion.noValue" >
                  and ${criterion.condition}
                </when>
                <when test="criterion.singleValue" >
                  and ${criterion.condition} #{criterion.value}
                </when>
                <when test="criterion.betweenValue" >
                  and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
                </when>
                <when test="criterion.listValue" >
                  and ${criterion.condition}
                  <foreach collection="criterion.value" item="listItem" open="(" close=")" separator="," >
                    #{listItem}
                  </foreach>
                </when>
              </choose>
            </foreach>
          </trim>
        </if>
      </foreach>
    </where>
  </sql>
  <sql id="Update_By_Example_Where_Clause" >
    <where >
      <foreach collection="example.oredCriteria" item="criteria" separator="or" >
        <if test="criteria.valid" >
          <trim prefix="(" suffix=")" prefixOverrides="and" >
            <foreach collection="criteria.criteria" item="criterion" >
              <choose >
                <when test="criterion.noValue" >
                  and ${criterion.condition}
                </when>
                <when test="criterion.singleValue" >
                  and ${criterion.condition} #{criterion.value}
                </when>
                <when test="criterion.betweenValue" >
                  and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
                </when>
                <when test="criterion.listValue" >
                  and ${criterion.condition}
                  <foreach collection="criterion.value" item="listItem" open="(" close=")" separator="," >
                    #{listItem}
                  </foreach>
                </when>
              </choose>
            </foreach>
          </trim>
        </if>
      </foreach>
    </where>
  </sql>
  <sql id="Base_Column_List" >
    id, username, password
  </sql>
  <select id="selectByExample" resultMap="BaseResultMap" parameterType="com.dorm.pojo.UserExample" >
    select
    <if test="distinct" >
      distinct
    </if>
    <include refid="Base_Column_List" />
    from user
    <if test="_parameter != null" >
      <include refid="Example_Where_Clause" />
    </if>
    <if test="orderByClause != null" >
      order by ${orderByClause}
    </if>
  </select>
  <select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.Integer" >
    select 
    <include refid="Base_Column_List" />
    from user
    where id = #{id,jdbcType=INTEGER}
  </select>
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer" >
    delete from user
    where id = #{id,jdbcType=INTEGER}
  </delete>
  <delete id="deleteByExample" parameterType="com.dorm.pojo.UserExample" >
    delete from user
    <if test="_parameter != null" >
      <include refid="Example_Where_Clause" />
    </if>
  </delete>
  <insert id="insert" parameterType="com.dorm.pojo.User" >
    insert into user (id, username, password
      )
    values (#{id,jdbcType=INTEGER}, #{username,jdbcType=VARCHAR}, #{password,jdbcType=VARCHAR}
      )
  </insert>
  <insert id="insertSelective" parameterType="com.dorm.pojo.User" >
    insert into user
    <trim prefix="(" suffix=")" suffixOverrides="," >
      <if test="id != null" >
        id,
      </if>
      <if test="username != null" >
        username,
      </if>
      <if test="password != null" >
        password,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides="," >
      <if test="id != null" >
        #{id,jdbcType=INTEGER},
      </if>
      <if test="username != null" >
        #{username,jdbcType=VARCHAR},
      </if>
      <if test="password != null" >
        #{password,jdbcType=VARCHAR},
      </if>
    </trim>
  </insert>
  <select id="countByExample" parameterType="com.dorm.pojo.UserExample" resultType="java.lang.Integer" >
    select count(*) from user
    <if test="_parameter != null" >
      <include refid="Example_Where_Clause" />
    </if>
  </select>
  <update id="updateByExampleSelective" parameterType="map" >
    update user
    <set >
      <if test="record.id != null" >
        id = #{record.id,jdbcType=INTEGER},
      </if>
      <if test="record.username != null" >
        username = #{record.username,jdbcType=VARCHAR},
      </if>
      <if test="record.password != null" >
        password = #{record.password,jdbcType=VARCHAR},
      </if>
    </set>
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
  </update>
  <update id="updateByExample" parameterType="map" >
    update user
    set id = #{record.id,jdbcType=INTEGER},
      username = #{record.username,jdbcType=VARCHAR},
      password = #{record.password,jdbcType=VARCHAR}
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
  </update>
  <update id="updateByPrimaryKeySelective" parameterType="com.dorm.pojo.User" >
    update user
    <set >
      <if test="username != null" >
        username = #{username,jdbcType=VARCHAR},
      </if>
      <if test="password != null" >
        password = #{password,jdbcType=VARCHAR},
      </if>
    </set>
    where id = #{id,jdbcType=INTEGER}
  </update>
  <update id="updateByPrimaryKey" parameterType="com.dorm.pojo.User" >
    update user
    set username = #{username,jdbcType=VARCHAR},
      password = #{password,jdbcType=VARCHAR}
    where id = #{id,jdbcType=INTEGER}
  </update>
</mapper>

UserMapper.java的实现