mybatis ResultHandler 示例，,有些sql查询会返回一些

mybatis ResultHandler 示例，,有些sql查询会返回一些

有些sql查询会返回一些复杂类型，这些复杂类型没有办法简单的通过xml或者注解配置来实现，这种时候我们需要实现mybatis的ResultHandler接口，来做自定义的对象属性映射。

ResultHandler例子：

Mybatis在调用ResultHandler的handleResult(ResultContext)方法之后生成bean，首先我们定义一个复杂对象。

public interface GrandFatherMapper {    public static class GrandFatherWithGrandChildren {        public GrandFather grandFather;        public Child child;    }    public void selectComplex(ResultHandler handler);}

然后我们创建一个mapper：

<mapper namespace="ca.qc.ircm.examples.resulthandler.GrandFatherMapper">    <resultMap id="ComplexMap" type="ca.qc.ircm.examples.resulthandler.GrandFatherMapper$GrandFatherWithGrandChildren">          <association property="grandFather" javaType="GrandFather">            <id property="id" column="GrandFather_id"/>            <result property="name" column="GrandFather_name"/>          </association>          <association property="child" javaType="Child">            <id property="id" column="Child_id"/>            <result property="name" column="Child_name"/>          </association>    </resultMap>    <select id="selectComplex" resultMap="ComplexMap">          SELECT GrandFather.id AS GrandFather_id, GrandFather.name AS GrandFather_name, Child.id AS Child_id, Child.name AS Child_name          FROM GrandFather          JOIN Father ON GrandFather.id = Father.grand_father_id          JOIN Child ON Father.id = Child.father_id    </select></mapper>

然后我们在selectComplex方法中使用自定义的handler，并得到处理之后的结果：

    public Map<GrandFather, List<Child>> selectGrandFathersWithChildren() {        SqlSession sqlSession = sqlSessionFactory.openSession();        try {            class MyResultHandler implements ResultHandler {                Map<GrandFather, List<Child>> grandFatherWithChildren = new HashMap<GrandFather, List<Child>>();                @Override                public void handleResult(ResultContext context) {                    final GrandFatherWithGrandChildren complex = (GrandFatherWithGrandChildren)context.getResultObject();                    if (!grandFatherWithChildren.containsKey(complex.grandFather)) {                        grandFatherWithChildren.put(complex.grandFather, new ArrayList<Child>());                    }                    grandFatherWithChildren.get(complex.grandFather).add(complex.child);                }            };            MyResultHandler handler = new MyResultHandler();            GrandFatherMapper grandFatherMapper = sqlSession.getMapper(GrandFatherMapper.class);            grandFatherMapper.selectComplex(handler);            return handler.grandFatherWithChildren;        } finally {            sqlSession.close();        }    }