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。