ResultHandler的用法

ResultHandler，顾名思义，对返回的结果进行处理，最终得到自己想要的数据格式或类型。也就是说，可以自定义返回类型。下面通过一个例子讲解它的使用方法：

创建Goods实体类：

public class Goods implements Serializable{

    /**

     *

     */

    private static final long serialVersionUID = 1L;

    private String id;

    private String name;

    private String remark;

    private String detail;

    private BigDecimal price;

    public String getId() {

        return id;

    }

    public void setId(String id) {

        this.id = id;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    public String getRemark() {

        return remark;

    }

    public void setRemark(String remark) {

        this.remark = remark;

    }

    public String getDetail() {

        return detail;

    }

    public void setDetail(String detail) {

        this.detail = detail;

    }

    public BigDecimal getPrice() {

        return price;

    }

    public void setPrice(BigDecimal price) {

        this.price = price;

    }

}

创建Mapper接口：

public interface GoodsDao {

    public void selectGoods(ResultHandler resultHandler);

}

创建映射文件：

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE mapper

PUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN"

"http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">

<mapper namespace="com.yht.mybatisTest.dao.GoodsDao">

    <select id="selectGoods" resultType="map">

        select price,name from goods

    </select>

</mapper>

mybatis配置文件：

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE configuration

  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"

  "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>

    <properties resource="db.properties">

        <property name="age" value="26" />

    </properties>

    <!-- settings是 MyBatis 中极为重要的调整设置，它们会改变 MyBatis 的运行时行为。 -->

    <settings>

        <!-- <setting name="localCacheScope" value="STATEMENT"/> -->

        <setting name="logImpl" value="STDOUT_LOGGING" />

        <setting name="cacheEnabled" value="true"/>

    </settings>

    <typeAliases>

        <typeAlias alias="goods"  type="com.yht.mybatisTest.entity.Goods" />

    </typeAliases>

    <environments default="development">

        <environment id="development">

            <transactionManager type="JDBC" />

            <dataSource type="POOLED">

                <property name="driver" value="${driver}" />

                <property name="url" value="${url}" />

                <property name="username" value="${username}" />

                <property name="password" value="${password}" />

            </dataSource>

        </environment>

    </environments>

    <!-- 映射文件 -->

    <mappers>

        <!-- 通过resource指定Mapper文件 -->

        <mapper resource="com/yht/mybatisTest/dao/goods.xml" />

    </mappers>

</configuration>

根据以上这些代码，我们知道，selectGoods方法的查询结果是List<Map>,假如现在有这么一个需求：想统计价格低于50的书籍有哪些，价格高于50的书籍有哪些，该怎么处理呢？这个时候就用到了ResultHandler，我们创建一个SelectGoodsResultHandler：

/**

 * @author chenyk

 * @date 2018年9月11日

 * 假如price<50的书为低价书，price>=50的书为高价书

 */

public class SelectGoodsResultHandler implements ResultHandler{

    private Map<String, List<String>> resultMap = new HashMap<String, List<String>>();

    @SuppressWarnings("unchecked")

    public void handleResult(ResultContext context) {

        if(resultMap.get("lowPrice") == null && resultMap.get("highPrice") == null){

            List<String> lowList = new ArrayList<String>();

            List<String> highList = new ArrayList<String>();

            resultMap.put("lowPrice", lowList);

            resultMap.put("highPrice", highList);

        }

        @SuppressWarnings("unchecked")

        Map<String,Object> resultObject = (Map<String, Object>)context.getResultObject();

        BigDecimal price = (BigDecimal) resultObject.get("price");

        String name = (String) resultObject.get("name");

        if(price.intValue() < 50){

            ((List<String>)resultMap.get("lowPrice")).add(name);

        }else{

            ((List<String>)resultMap.get("highPrice")).add(name);

        }

    }

    public Map<String, List<String>> getResults(){

        return resultMap;

    }

}

然后写一个测试demo运行：

public class GoodsDaoTest {

    private static SqlSessionFactory sqlSessionFactory = null;

    @Test

    public void selectGoodsTest(){

        SqlSession sqlSession = getSqlSessionFactory().openSession(true);

        GoodsDao goodsMapper = sqlSession.getMapper(GoodsDao.class);

        SelectGoodsResultHandler resultHandler = new SelectGoodsResultHandler();

        goodsMapper.selectGoods(resultHandler);

        System.out.println(resultHandler.getResults().toString());;

    }

    public static SqlSessionFactory getSqlSessionFactory() {

        String resource = "spring-ibatis.xml";

        if(sqlSessionFactory == null){

            try {

                sqlSessionFactory = new SqlSessionFactoryBuilder().build(Resources

                        .getResourceAsReader(resource));

            } catch (IOException e) {

                // TODO Auto-generated catch block

                e.printStackTrace();

            }

        }

        return sqlSessionFactory;

    }

}

打印的结果：{highPrice=[c++从入门到放弃], lowPrice=[java从入门到放弃, php从入门到放弃, golang从入门到放弃, c从入门到放弃, python从入门到放弃]}

对于源码部分，在SQL的执行过程这篇文章中将进行分析。

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