mybatis 联表查询的问题

   <select id="findPage" parameterType="RequestBean" resultType="Sap">
        select a.id, a.conname, a.conmax,
        a.server, a.client, a.username, a.password, a.language, a.sysno, a.appid,
        b.name as appName, b.id as appId
        from haf_sap as a join haf_app as b on a.appid = b.id
        <where>
            <include refid="where_clause"/>
        </where>
    </select><resultMap id="sapMap" type="Sap">
        <id property="id" column="id"/>
        <result property="conname" column="conname"/>
        <result property="conmax" column="conmax"/>
        <result property="server" column="server"/>        <result property="client" column="client"/>
        <result property="username" column="username"/>
        <result property="password" column="password"/>        <result property="language" column="language"/>
        <result property="sysno" column="sysno"/>
        <result property="appId" column="appid"/>
        <association property="app" column="appid" javaType="App">
            <id property="id" column="appId" />
            <result property="name" column="appName" />
            <!--
            <result property="code" column="code" />
            <result property="des" column="des" />-->
        </association> </resultMap>现在的问题是，组合成的sql语句执行是有结果的（这个语句我是通过log4jdbc打印出来的），但findPage返回的对象里面<result property="conname" column="conname"/>
        <result property="conmax" column="conmax"/>
        <result property="server" column="server"/>        <result property="client" column="client"/>
        <result property="username" column="username"/>
        <result property="password" column="password"/>        <result property="language" column="language"/>
        <result property="sysno" column="sysno"/>
        <result property="appId" column="appid"/>这些都有对应的值，只是<association property="app" column="appid" javaType="App">
            <id property="id" column="appId" />
            <result property="name" column="appName" />
            <!--
            <result property="code" column="code" />
            <result property="des" column="des" />-->
        </association>
对应的对象app是null
请问如何解决，，谢谢！