Using Hibernate for Java - 2

In the previous article we saw how to use Hibernate as a middleware layer between the class structure of our Java program and the representation of instances of these classes in a database.


The previous article illustrated the mapping between a Java class and a database table, namelly the class Book and the table with the same name.

Let us now see how to map the class relationships, of type 1-to-N and N-to-N, to the database.

Let's start by adding to our model, which only had one class, two new classes:
     - BookCopy (A Book contains the information of a book, and is related to several BookCopy, which correspond to the book copies a library has of that book) --> 1-N relationship.
     - Author (A Book has several Author and vice-versa) --> relação N-N relationship.

In the Java project, developed in the previous post, let's start by adding class BookCopy and the respective Hibernate mapping file BookCopy.hbm.xml.

Class BookCopy:

Note that, in BookCopy, we must declare an instance of Book, which corresponds to the attribute, with cardinality 1, imported by the relation between Book and BookCopy.

File BookCopy.hbm.xml:

In class Book, we need to add an instance of Set<BookCopy>, corresponding to the attribute, of cardinality 0..*, imported by Book from the relation between Book and BookCopy:

File Book.hbm.xml after modification:

Note that, in the Book's mapping file, we added:

<set name="bookcopies" table="BOOKCOPY" inverse="true" lazy="true" fetch="select">
    <key column="BOOKID" not-null="true"/>
    <one-to-many class="javahibernateexample.beans.BookCopy" />
</set>

corresponding to the bookcopies associated to a given Book.

And, in BookCopy's mapping file, we put:

<many-to-one name="book" class="javahibernateexample.beans.Book" fetch="select">
    <column name="BOOKID" not-null="true" />
</many-to-one>

corresponding to the book associated to a given BookCopy.


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Let us now add the information relative to the authors.
For that, let's create class Author:

And the respetive hibernate's mapping file Author.hbm.xml:

 Class Book needs, now, to reference the book's authors, through an attribute of type Set<Author>:

In the end, the file Book.hbm.xml will look like this:

Note the attribute authors, in Book:

<set name="authors" table="BOOKAUTHOR" cascade="all">
    <key column="BOOKID" />
    <many-to-many column="AUTHORID"  class="javahibernateexample.beans.Author" />
</set>

and the corresponding attribute books in Author:

<set name="books" table="BOOKAUTHOR" cascade="all">
    <key column="AUTHORID" />
    <many-to-many column="BOOKID"  class="javahibernateexample.beans.Book" />
</set>

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It's still missing the reference to the new resources in file hibernate.cfg.xml, which we shall now add:

We can now use the created infrastructure with some test cases:

Some test results:

Finally, we may also modify, in the hibernate configuration file, property hibernate.show_sql to false, which will hide all the SQL output generated by the hibernate:

Testes output with show_sql set to false:

I hope you were able to follow this example and that you are ready to use hibernate in your future projects.