代码生成器原理和使用、QBC、Mybattis扩展

QBC(Query By Criteria) API提供了检索对象的另一种方式，它主要由Criteria接口、Criterion接口和Expresson类组成，它支持在运行时动态生成查询语句。
Hibernate中共提供了三种检索方式：HQL(Hibernate Query Language)、QBC、QBE(Query By Example)。

QBC检索步骤：
　　1、调用Session的createCriteria()方法创建一个Criteria对象。
　　2、设定查询条件。Restrictions类提供了一系列用于设定查询条件的静态方法，
　　这些静态方法都返回Criterion实例，每个Criterion实例代表一个查询条件。
　　Criteria的add()方法用于加入查询条件。
　　3、调用Criteria的list()方法执行查询语句。该方法返回List类型的查询结果，在
　　List集合中存放了符合查询条件的持久化对象。

代码生成器：


从后往前进行分析：

零、以tb_brand表为例：

CREATE TABLE `tb_brand` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL COMMENT '品牌名称',
  `first_char` varchar(1) DEFAULT NULL COMMENT '品牌首字母',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=23 DEFAULT CHARSET=utf8;

一、dao层的映射配置为文件　　TbBrandMapper.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.pinyougou.mapper.TbBrandMapper" >
  
  <!-- 数据库中查询的字段和pojo类属性的映射 -->
  <resultMap id="BaseResultMap" type="com.pinyougou.pojo.TbBrand" >
    <id column="id" property="id" jdbcType="BIGINT" />
    <result column="name" property="name" jdbcType="VARCHAR" />
    <result column="first_char" property="firstChar" jdbcType="VARCHAR" />
  </resultMap>
  
  <!-- 添加的 WHERE 子句 -->
  <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>
  
  <!-- 该表中含有的各个列 -->
  <sql id="Base_Column_List" >
    id, name, first_char
  </sql>
  
  
  <!-- SELECT DISTINCT FROM tb_brand WHERE 。。。ORDER BY 。。 -->
  <select id="selectByExample" resultMap="BaseResultMap" parameterType="com.pinyougou.pojo.TbBrandExample" >
    select
    <if test="distinct" >
      distinct
    </if>
    <include refid="Base_Column_List" />
    from tb_brand
    <if test="_parameter != null" >        <!-- _parameter? Mybatis的内置参数 代表传入的TbBrandExample对象。也就是把_parameter看作了TbBrandExample的别名-->
      <include refid="Example_Where_Clause" />        <!-- Example_Where_Clause子句 -->
    </if>
    <if test="orderByClause != null" >    <!-- orderByClause是TbBrandExample类中的一个属性。 -->
      order by ${orderByClause}
    </if>
  </select>
  
  
  <!-- SELECT id, name, first_char FROM tb_brand WHERE id =。。 -->
  <select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.Long" >
    select 
    <include refid="Base_Column_List" />
    from tb_brand
    where id = #{id,jdbcType=BIGINT}      <!-- #{}表示一个占位符。因为parameterType是基本数据类型，所以这个变量名可以随意写。但最好还是写字段名 -->
  </select>
  
  
  <!-- DELETE FROM tb_brand WHERE id=。。。-->
  <delete id="deleteByPrimaryKey" parameterType="java.lang.Long" >
    delete from tb_brand
    where id = #{id,jdbcType=BIGINT}
  </delete>
  
  
  <!-- DELETE FROM tb_brand WHERE 。。。使用的是TbBrandExample对象-->
  <delete id="deleteByExample" parameterType="com.pinyougou.pojo.TbBrandExample" >
    delete from tb_brand
    <if test="_parameter != null" >
      <include refid="Example_Where_Clause" />
    </if>
  </delete>
  
  
  <!-- INSERT INTO tb_brand (id, name, firstChar) VALUES (1 ,"payn" ,null) -->
  <insert id="insert" parameterType="com.pinyougou.pojo.TbBrand" >
    insert into tb_brand (id, name, first_char
      )
    values (#{id,jdbcType=BIGINT}, #{name,jdbcType=VARCHAR}, #{firstChar,jdbcType=VARCHAR}
      )
  </insert>
  
  
  <!-- INSERT INTO tb_brand (id, name。。) VALUES (1 ,"payn" 。。。)   使用的TBrand对象-->
  <insert id="insertSelective" parameterType="com.pinyougou.pojo.TbBrand" > 
    insert into tb_brand
    <trim prefix="(" suffix=")" suffixOverrides="," >  <!-- prefix前缀 suffix后缀 suffixOverrides=","  去掉最后一个“,”-->
      <if test="id != null" >
        id,
      </if>
      <if test="name != null" >
        name,
      </if>
      <if test="firstChar != null" >
        first_char,
      </if>
    </trim>
    <trim prefix="values (" suffix=")" suffixOverrides="," >
      <if test="id != null" >
        #{id,jdbcType=BIGINT},
      </if>
      <if test="name != null" >
        #{name,jdbcType=VARCHAR},
      </if>
      <if test="firstChar != null" >
        #{firstChar,jdbcType=VARCHAR},
      </if>
    </trim>
  </insert>
  
  
  <!-- SELECT count(*) FROM tb_brand WHERE 。。。 使用的是TbBrandExample对象-->
  <select id="countByExample" parameterType="com.pinyougou.pojo.TbBrandExample" resultType="java.lang.Integer" >
    select count(*) from tb_brand
    <if test="_parameter != null" >
      <include refid="Example_Where_Clause" />
    </if>
  </select>
  
  
  <!-- UPDATE tb_brand set id=。。 ,name=。。 ,first_char=。。 WHERE 。。。 使用的是TbBrand对象和TbBrandExample对象-->
  <update id="updateByExampleSelective" parameterType="map" >
    update tb_brand
    <set >
      <if test="record.id != null" >
        id = #{record.id,jdbcType=BIGINT},
      </if>
      <if test="record.name != null" >
        name = #{record.name,jdbcType=VARCHAR},
      </if>
      <if test="record.firstChar != null" >
        first_char = #{record.firstChar,jdbcType=VARCHAR},
      </if>
    </set>
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
  </update>
  
  
  <!-- UPDATE tb_brand SET id=xx,name=xx,first_char=xx WHERE。。。 使用的是TbBrand对象(全字段的更新)和TbBrandExample对象  -->
  <!-- parameterType表示传入的参数的类型  map是什么意思？？？
  因为方法： int updateByExample(@Param("record") TbBrand record, @Param("example") TbBrandExample example); 
  使用@param注释的多个参数值也会组装成一个map数据结构,和直接传递map进来没有区别。-->
  <update id="updateByExample" parameterType="map" >     
    update tb_brand
    set id = #{record.id,jdbcType=BIGINT},
      name = #{record.name,jdbcType=VARCHAR},
      first_char = #{record.firstChar,jdbcType=VARCHAR}
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
  </update>
  
  
  <!-- UPDATE tb_brand SET name=。。,first_char=。。WHERE id=xx 使用的是TbBrand对象。更新的是选中的字段。没选中的不会被设置为null。-->
  <update id="updateByPrimaryKeySelective" parameterType="com.pinyougou.pojo.TbBrand" >
    update tb_brand
    <set >
      <if test="name != null" >
        name = #{name,jdbcType=VARCHAR},
      </if>
      <if test="firstChar != null" >
        first_char = #{firstChar,jdbcType=VARCHAR},
      </if>
    </set>
    where id = #{id,jdbcType=BIGINT}
  </update>
  
  
  <!-- UPDATE tb_brand SET name=xx,first_char=xx WHERE id=xx 使用的是TbBrand对象。更新的是全属性。可能会被更新为null。 -->
  <update id="updateByPrimaryKey" parameterType="com.pinyougou.pojo.TbBrand" >
    update tb_brand
    set name = #{name,jdbcType=VARCHAR},
      first_char = #{firstChar,jdbcType=VARCHAR}
    where id = #{id,jdbcType=BIGINT}
  </update>
  

  <!-- SELECT id,name as text FROM tb_brand;  -->
  <select id="selectOptionList" resultType="java.util.Map">
      SELECT id,name as text FROM tb_brand;
  </select>
  
</mapper>

二、mapper 接口　　TbBrandMapper.java

public interface TbBrandMapper {
    int countByExample(TbBrandExample example);

    int deleteByExample(TbBrandExample example);

    int deleteByPrimaryKey(Long id);

    int insert(TbBrand record);

    int insertSelective(TbBrand record);

    List<TbBrand> selectByExample(TbBrandExample example);

    TbBrand selectByPrimaryKey(Long id);

    int updateByExampleSelective(@Param("record") TbBrand record, @Param("example") TbBrandExample example);

    int updateByExample(@Param("record") TbBrand record, @Param("example") TbBrandExample example);

    int updateByPrimaryKeySelective(TbBrand record);

    int updateByPrimaryKey(TbBrand record);
    
    List<Map> selectOptionList();
}

三、单表简单查询用到的实体类　　TbBrand.java  TbBrandExample.java

public class TbBrand implements Serializable{
    private Long id;

    private String name;

    private String firstChar;

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getFirstChar() {
        return firstChar;
    }

    public void setFirstChar(String firstChar) {
        this.firstChar = firstChar == null ? null : firstChar.trim();
    }
}

public class TbBrandExample {
    protected String orderByClause;

    protected boolean distinct;

    protected List<Criteria> oredCriteria;

    public TbBrandExample() {
        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 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 andFirstCharIsNull() {
            addCriterion("first_char is null");
            return (Criteria) this;
        }

        public Criteria andFirstCharIsNotNull() {
            addCriterion("first_char is not null");
            return (Criteria) this;
        }

        public Criteria andFirstCharEqualTo(String value) {
            addCriterion("first_char =", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharNotEqualTo(String value) {
            addCriterion("first_char <>", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharGreaterThan(String value) {
            addCriterion("first_char >", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharGreaterThanOrEqualTo(String value) {
            addCriterion("first_char >=", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharLessThan(String value) {
            addCriterion("first_char <", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharLessThanOrEqualTo(String value) {
            addCriterion("first_char <=", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharLike(String value) {
            addCriterion("first_char like", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharNotLike(String value) {
            addCriterion("first_char not like", value, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharIn(List<String> values) {
            addCriterion("first_char in", values, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharNotIn(List<String> values) {
            addCriterion("first_char not in", values, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharBetween(String value1, String value2) {
            addCriterion("first_char between", value1, value2, "firstChar");
            return (Criteria) this;
        }

        public Criteria andFirstCharNotBetween(String value1, String value2) {
            addCriterion("first_char not between", value1, value2, "firstChar");
            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);
        }
    }
}

Mybatis扩展一：<trim>标签
trim标记是一个格式化的标记，可以完成set或者是where标记的功能，如下代码：

select * from user 
<trim prefix="WHERE" prefixoverride="AND |OR">
　　<if test="name != null and name.length()>0"> AND name=#{name}</if>
　　<if test="gender != null and gender.length()>0"> AND gender=#{gender}</if>
</trim>

假如说name和gender的值都不为null的话打印的SQL为：select * from user where ||||| name = 'xx' and gender = 'xx'
在红色标记的地方是不存在第一个and的，上面两个属性的意思如下：
　　prefix：前缀　　　　　　
　　prefixoverride：去掉第一个and或者是or

update user
<trim prefix="set" suffix=" where id = #{id}" suffixoverride=",">
　　<if test="name != null and name.length()>0"> name=#{name} , </if>
　　<if test="gender != null and gender.length()>0"> gender=#{gender} ,  </if>
</trim>

假如说name和gender的值都不为null的话打印的SQL为：update user set name='xx' , gender='xx' ||||| where id='x'
在红色标记的地方不存在逗号，而且自动加了一个set前缀和where后缀，上面三个属性的意义如下，其中prefix意义如上：
　　suffixoverride：去掉最后一个逗号（也可以是其他的标记，就像是上面前缀中的and一样）
　　suffix：后缀

Mybatis扩展二： @Param  parameterType="map"解析

<update id="updateByExample" parameterType="map" >
  update tb_user
  set id = #{user.id,jdbcType=INTEGER},
  ...
  <if test="_parameter != null" >
    <include refid="Update_By_Example_Where_Clause" />
  </if>
</update>

这里的parameterType=“map”
在mapper接口中：存在如下方法
　　int updateByExample(@Param("user") User user, @Param("example") UserExample example);

附注：
　　@Param注解的作用是给参数命名,参数命名后就能根据名字得到参数值,正确的将参数传入sql语句中 。如下：

public interface Mapper { 
@Select("select s_id id,s_name name,class_id classid from student where  s_name= #{aaaa} and class_id = #{bbbb}") 
    public Student select(@Param("aaaa") String name,@Param("bbbb")int class_id);  
@Insert...... 
} 

注意：
传入map类型,直接通过#{keyname}就可以引用到键对应的值。使用@param注释的多个参数值也会组装成一个map数据结构,和直接传递map进来没有区别。
不管传递进来的map是否为空，仍然可以使用_parameter。

Mybatis扩展三：内置参数_parameter和_dataBaseId
当mybatis的核心配置文件中配置了databaseIdProvider：

<databaseIdProvider type="DB_VENDOR">
    <property name="MySQL" value="mysql"/>
    <!--//多个数据库提供商配置...-->
</databaseIdProvider>

此时在mybatis中的内置参数 _databaseId 中保存了用户指定的对应的数据库厂商标识。

<select id="selectUsrs" databaseId="mysql" resultType="com.heiketu.pojo.Users">
    <if test="_databaseId == 'mysql'">
        select * from usrs where id = 2
    </if>
</select>

mybatis的另一个内置参数_parameter 保存了对应传入的对象。

<insert id="insertData" parameterType="com.heiketu.pojo.Users">
    insert into usrs values(
      null,
      <if test="_parameter != null">
      #{_parameter.name},
      </if>
      #{_parameter.age},
      #{_parameter.address},
      #{_parameter.companyId}
    )
</insert>

此时_parameter参数就保存了com.heiketu.pojo.Users这个对象。所以可以通用通过 ONGL 表达式从_parameter参数中获取带Users的对应属性值。(也就是将_parameter看做Users的别名)。