hibernate entity to json

Question

i use Hibernate 4 and Spring 3.

i have two entity.

Book entity

@Entity
@Table(name = "book")
public class Book implements Serializable {

    public Book() {
    }

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue( strategy = GenerationType.IDENTITY)
    private int id;

    @ManyToOne()
    @JoinColumn( name = "author_id" )
    private Author author;

    private String name;
    private int pages;

    @Version
    @Column( name = "VERSION")
    private int version;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Author getAuthor() {
        return author;
    }

    public void setAuthor(Author author) {
        this.author = author;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPages() {
        return pages;
    }

    public void setPages(int pages) {
        this.pages = pages;
    }

    public int getVersion() {
        return version;
    }

    public void setVersion(int version) {
        this.version = version;
    }


}

and Author entity

@Entity
@Table(name = "author")
public class Author implements Serializable {

    public Author() {
    }

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue( strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

    @OneToMany( mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Book> books = new HashSet<Book>();


    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    public Set<Book> getBooks() {
        return books;
    }
    public void setBooks(Set<Book> books) {
        this.books = books;
    }

    public void addBook(Book book) {
        book.setAuthor(this);
        getBooks().add(book);
    }

    public void removeBook(Book book) {
        getBooks().remove(book);        
    }

}

and JSON depend in pom.xml

<dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.1.2</version>
        </dependency>


        <dependency>
            <groupId>com.fasterxml.jackson.datatype</groupId>
            <artifactId>jackson-datatype-hibernate4</artifactId>
            <version>2.1.2</version>
        </dependency>

My Root-context is here -

    <!-- Root Context: defines shared resources visible to all other web components -->
    <context:annotation-config/>

    <context:component-scan base-package="org.jar.libs.dao" />
    <context:component-scan base-package="org.jar.libs.service" />

    <tx:annotation-driven transaction-manager="transactionManager" />

     <bean id="jspViewResolver"
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
        <property name="prefix" value="/WEB-INF/views/"/>
        <property name="suffix" value=".jsp"/>
    </bean>

    <bean id="dataSource"
        class="org.springframework.jdbc.datasource.DriverManagerDataSource"
        p:driverClassName="com.mysql.jdbc.Driver" p:url="jdbc:mysql://localhost:3306/hibernate"
        p:username="root" p:password="root" />

    <bean id="sessionFactory"
        class="org.springframework.orm.hibernate4.LocalSessionFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="annotatedClasses">
            <list>
                <value>org.jar.libs.domain.Book</value>
                <value>org.jar.libs.domain.Author</value>
            </list>
        </property>
        <property name="hibernateProperties">
            <props>
                <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
                <prop key="hibernate.show_sql">true</prop>
                <prop key="hibernate.hbm2ddl.auto">update</prop>
            </props>
        </property>
    </bean>

    <bean id="transactionManager"
        class="org.springframework.orm.hibernate4.HibernateTransactionManager">
        <property name="sessionFactory" ref="sessionFactory" />
    </bean>

</beans>

...servlet-context.xml

<!-- Enables the Spring MVC @Controller programming model -->
    <annotation-driven />

    <!-- Handles HTTP GET requests for /resources/** by efficiently serving up static resources in the ${webappRoot}/resources directory -->
    <resources mapping="/resources/**" location="/resources/" />

    <!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory -->
    <beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <beans:property name="prefix" value="/WEB-INF/views/" />
        <beans:property name="suffix" value=".jsp" />
    </beans:bean>

    <context:component-scan base-package="org.jar.libs.controller" />

Controller.

@Controller
@RequestMapping (value = "books/rest")
public class BookController {

    @Autowired
    private BookService bookService;

    // logger
    private static final Logger logger = LoggerFactory.getLogger(BookController.class);


    @SuppressWarnings("unchecked")
    @RequestMapping( method = RequestMethod.GET )
    public @ResponseBody List<Book> getBook() {


        List<Book> res = bookService.findAll();
        return res;


    }

}

findAll in my DAO :

public List<Book> findAll() {
        Session session = sessionFactory.getCurrentSession();
        List<Book> result = (List<Book>) session.createQuery("select c from Book c").list();
        return result;
    }

in debug i see that method return 2 records, but Spring can not convert result to JSON and return 406 HTTP error. What's wrong?

I attach image what i see in debug. - http://tinypic.com/view.php?pic=35kvi9i&s=6

Answer 1

Generally, when you call getter methods of entity classes(which returns relation object) out of transaction, then you get LazyInitializationExceptions.

That's what might be happening in your case if you are converting entity class objects(retrieved from query) to json out of transaction.

I had same issue, I converted my entity object retrieved by hibernate to json in controller. As controller was out of transaction(Transaction at service layer), while converting to json, getter methods of entity class objects are called and I got LazyInitializationException. Which obstructed object conversion to json, and response was not returned.

My solution, Try this :

@SuppressWarnings("unchecked")
@RequestMapping( method = RequestMethod.GET )
public @ResponseBody List<Book> getBook() {
    List<Book> res = bookService.findAll();
    for(Book book : res) {
       book.getAuthor().setBooks(null);
    }
    return res;
}

Answer 2

As others have suggested, I would really not advise you to try to JSON serialize (or actually perform any serialization) of hibernate entities.
You must remember that the fetched entities are actually "proxified" objects (Hibernate uses ASM, CGLIB and other "dynamic proxiy" frameworks).
As a result for example, collections get replaced with [PersistenceBags] which may be initialized "lazily" , and cause you hibernate exceptions 1.

But the problems do not stop there, you may see issues when trying to serialize an Hibernate custom type
I know this might sound you like writing "boillerplate" code but you might end up coding DTOs - data transfer objects which will take the entity returned from your DAL, and transform them to an object that can be serialized.

You can use a framework like dozer in order to ease development of serialization between an entity to a DTO.

Answer 3

Try using these two Jackson artifacts instead

<dependency>
<groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-mapper-asl</artifactId>
<version>1.9.9</version>
</dependency>

<dependency>
<groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-core-asl</artifactId>
<version>1.9.9</version>
</dependency>

Also on your controller try by changing it to -

 @SuppressWarnings("unchecked")
@RequestMapping( method = RequestMethod.GET,  produces = MediaType.APPLICATION_JSON_VALUE )
public @ResponseBody List<Book> getBook() {

Lastly, make sure your view is making a json request.