Why I cannot get the value of associated data through ${book.type.sortName} in jsp?

I'm a beginner in Java web.I am using spring4 mvc and mybatis for the book manage system.

There are two tables t_book and t_type in database bms in mysql.

This is the Book entity:

public class Book {

    private Long bookNum;

    private String bookName;

    private String writer;

    private String callNumber;

    private BigDecimal price;

    private String pubCompany;

    private Date pubDate;

    private Long totalNum;

    private Long currentNum;

    private String brief;

    private Type type; // the associated class

    // getter and setter

}

This is the Type entity:

public class Type {

    private Long sortId;

    private String sortName;

    // getter and setter
}

But in the bookDetail.jsp,the page can show the value of ${book.bookName},${book.writer},${book.pubDate} and so on,but cannot show the value of ${book.type.sortName}.

This is a part of bookDetail.jsp:

<table class="table table-bordered table-striped">
                    <tr>
                        <th>Book ID</th>
                        <td>${book.bookNum}</td>
                    </tr>
                    <tr>
                        <th>Book name</th>
                        <td>${book.bookName}</td>
                    </tr>
                    <tr>
                        <th>Book writer</th>
                        <td>${book.writer}</td>
                    </tr>
                    <tr>
                        <th>callNumber</th>
                        <td>${book.callNumber}</td>
                    </tr>
                    <tr>
                        <th>sort</th>
                        <td>${book.type.sortName}</td>
                    </tr>
                    <tr>
                        <th>publish Date</th>
                        <td><fmt:formatDate value="${book.pubDate}" pattern="yyyy-MM-dd"/></td>
                    </tr>
                </table>

Why I can't get the value of associated data through ${book.type.sortName} in .jsp? How can I get it?

Please help! Thanks in advance!