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

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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

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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)：

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<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>

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