有三个对象类，分别对应三个表。
Project类中有个属性是客户对象属性

public class Project {
	private int id;

	/** 项目名/标签名 */
	private String state;

	/** 客户对象*/
	private Custom custom;
	.
	.省略其他字段
	.省略get set,必须写不然映射不进来
}

Custom 类中有个属性是客户类型对象属性

public class Custom  {
	/** id */
	private int id;
	
	/** 客户类型对象*/
	private CustomType customType;
	.
	.省略其他字段
	.省略get set,必须写不然映射不进来
}

public class CustomType extends BaseEntity {
	private static final long serialVersionUID = 1L;

	/**  */
	private Long id;

	/** 客户类型编号 */
	private Long customTypeCode;

	/** 客户类型 */
	private String customTypeName;
	.
	.省略其他字段
	.省略get set,必须写不然映射不进来
	}

	<resultMap type="Project" id="ProjectResult">
		<result property="id" column="proid" />
		<result property="state" column="state" />
		<association property="custom" column="custom_id"
			javaType="Custom" resultMap="CustomResult" />
	</resultMap>

	<resultMap type="Custom" id="CustomResult">
		<result property="id" column="id" />
		<result property="customTypeId" column="custom_type_id" />
		<association property="customType" column="custom_type_id"
			javaType="CustomType" resultMap="CustomTypeResult" />
	</resultMap>

	<resultMap type="CustomType" id="CustomTypeResult">
		<result property="id" column="tyid" />
		<result property="customTypeCode" column="custom_type_code" />
		<result property="customTypeName" column="custom_type_name" />
	</resultMap>

	<sql id="selectProjectVo">
		select a.id proid, a.state,
		a.custom_id,
		cus.*,ty.id tyid,ty.custom_type_code,ty.custom_type_name
		from
		t_p_project a left
		join
		t_p_custom
		cus on a.custom_id=cus.id
		left join t_p_custom_type ty on cus.custom_type_id=ty.id
	</sql>

	<select id="selectProjectList" resultMap="ProjectResult">
		<include refid="selectProjectVo" />
	</select>

如果查询的字段中有相同的名字，最好查询的时候取别名区分