mybatis获取对象及其关联对象的两种方式

场景如下：有一个Book类，一个Category类，Book类有一个对Category的引用，要求获取Book对象的同时取出Category对象

Book.java类代码如下

public class Book implements Serializable {

/**
*
*/
private static final long serialVersionUID = 8634505584095346474L;

private String id;
private String name;
private Date publishDate;
private String author;
private Float price;
private Category category;
private Long quantity;
// setter and getter method is omited!
}

 Category.java类代码如下

public class Category implements Serializable {

/**
*
*/
private static final long serialVersionUID = 8964146416205979692L;

private String id;
private String name;
private String spell;
private String comments;
//setter and getter method is omited!
}

 对应获取Book对象的两种方式，通过配置BookMapper.xml中实现

方式一（通过配置映射实现）：

<resultMap id="bookMap" type="Book">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="publishDate" column="publishDate"/>
<result property="author" column="author"/>
<result property="price" column="price"/>
<result property="quantity" column="quantity"/>
<association property="category" javaType="Category">
<id property="id" column="category_id" javaType="string"/>
<id property="name" column="category_name"/>
<id property="spell" column="category_spell"/>
<id property="comments" column="category_comments"/>
</association>
</resultMap>

<select id="getBook" parameterType="string" resultMap="bookMap">
select b.id,
b.name,
b.publishDate,
b.author,
b.price,
b.category,
b.quantity,
c.id as category_id,
c.name as category_name,
c.spell as category_spell,
c.comments as category_comments
from t_book b left outer join t_bookcategory c on b.category=c.id where b.id=#{id}
</select>

 方式二(通过OGNL实现)：

<select id="getBook" parameterType="string" resultType="Book">
select b.id,
b.name,
b.publishDate,
b.author,
b.price,
b.quantity,
c.id as "category.id",
c.name as "category.name",
c.spell as "category.spell",
c.comments as "category.comments"
from t_book b left outer join t_bookcategory c on b.category=c.id where b.id=#{id}
</select>