Java JAXB: XML to Java object conversion

Question

Testdata.xml

<Users>
    <User>
        <Name>Ammu</Name>
        <Books>
            <Book>book1</Book>
            <Book>book2</Book>
            <Book>book3</Book>
        </Books>
    </User>
    <User>
        <Name>Unni</Name>
        <Books>
            <Book>book1</Book>
            <Book>book2</Book>
            <Book>book4</Book>
        </Books>
    </User>
</Users>

A call to an external service returns data in the above XML format, I am trying to design to convert this structure to a POJO so as to convert the XML to POJO using JAXB

Problem: Current output: books is returned as empty. Why?

com.example.Users@45db05b2[
  users=[com.example.User@2e530cf2[
  name=Ammu
  books=[]
], com.example.User@4e76fba0[
  name=Unni
  books=[]
]]
]

Book.java

import javax.xml.bind.annotation.XmlElement;
import org.apache.commons.lang3.builder.ReflectionToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

public class Book {
    private String name;

    @XmlElement(name = "Book")
    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return ReflectionToStringBuilder.toString(this,
                ToStringStyle.MULTI_LINE_STYLE);
    }
}

User.java

import java.util.ArrayList;
import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

import org.apache.commons.lang3.builder.ReflectionToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

@XmlRootElement(name = "User")
public class User {

    private String name;

    private List<Book> books = new ArrayList<Book>();

    @XmlElement(name = "Book", type = Book.class)
    public List<Book> getBooks() {
        return books;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }

    @XmlElement(name="Name")
    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return ReflectionToStringBuilder.toString(this,
                ToStringStyle.MULTI_LINE_STYLE);
    }
}

Users.java

@XmlAccessorType(XmlAccessType.FIELD)
@XmlRootElement(name = "Users")
public class Users {

    @XmlElement(name = "User", type = User.class)
    private List<User> users = new ArrayList<User>();

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

    @Override
    public String toString() {
        return ReflectionToStringBuilder.toString(this,
                ToStringStyle.MULTI_LINE_STYLE);
    }

}

Update:

After adding @XmlElementWrapper in User.java, there is great progress.

@XmlElementWrapper(name="Books")
    @XmlElement(name = "Book")
    public List<Book> getBooks() {
        return books;
    }

The current output is,

com.example.Users@39b8d6f7[
  users=[com.example.User@16290fbc[
  name=Ammu
  books=[com.example.Book@144aa0ce[
  name=<null>
], com.example.Book@2f833eca[
  name=<null>
], com.example.Book@518f5824[
  name=<null>
]]
], com.example.User@61c80b01[
  name=Unni
  books=[com.example.Book@19e3cd51[
  name=<null>
], com.example.Book@3abc8e1e[
  name=<null>
], com.example.Book@311671b2[
  name=<null>
]]
]]

But The values for <book> are null

Update2: Final

As per Blaise Doughan's update, tried adding @XmlValue, and that's it...

No need of any other annotation on Book.java, [and no need of Books.java , which I once thought would be required to declare just a list of Book] . cool

public class Book {
    private String name;

    @XmlValue
    public String getName() {
        return name;
    }

Output:

com.example.Users@3882764b[
  users=[com.example.User@7d2452e8[
  name=Ammu
  books=[com.example.Book@6860991f[
  name=book1
], com.example.Book@1de4f7c2[
  name=book2
], com.example.Book@2345f0e3[
  name=book3
]]
], com.example.User@5bbf3d87[
  name=Unni
  books=[com.example.Book@44c9d92c[
  name=book1
], com.example.Book@1fd0fafc[
  name=book2
], com.example.Book@510dc6b5[
  name=book4
]]
]]
]

This was slightly different usecase compared to those available as examples elsewhere. Thanks all esp Blaise Doughan.

Test kickoff

public static void main(String[] args) {
        try {
            File file = new File("C:\\temp\\testdata.xml");
            JAXBContext jaxbContext = JAXBContext.newInstance(Users.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            Users users = (Users) jaxbUnmarshaller.unmarshal(file);
            System.out.println(users);
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

In User.java should not the annotation be @XmlElement(name = "Books", type = Book.class) (Books with an s) — jny
– jny, Commented Aug 19, 2014 at 19:34

bdoughan · Accepted Answer · 2014-08-19 20:03:01Z

3

You need to leverage the @XmlElementWrapper annotation for your use case:

@XmlElementWrapper(name="Books")
@XmlElement(name = "Book")
public List<Book> getBooks() {
    return books;
}

With the @XmlElementWrapper it will write the XML as:

<User>
    <Books>
        <Book>...</Book>
        <Book>...</Book>
    </Books>
</User>

without it, the expected XML is:

<User>
    <Book>...</Book>
    <Book>...</Book>
</User>

UPDATE

On the Book class you should annotate the name property with @XmlValue.

public class Book {
    private String name;

    @XmlValue
    public String getName() {
        return name;
    }

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

}

edited Aug 19, 2014 at 20:03

answered Aug 19, 2014 at 19:44

bdoughan

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3 Comments

spiderman Over a year ago

Thank you Blaise Doughan, @XmlElementWrapper was definitely the missing one +1. Now program is trying to pull the three <book> inside the books. but values are null. Something wrong in Book.java? I will post the current o/p in the question section as an update

bdoughan Over a year ago

@prash - See my update about using @XmlValue on the name property in the Book class.

spiderman Over a year ago

That's it. Thanks a lot Blaise Doughan.

Bruno Franco · Accepted Answer · 2014-08-19 19:35:45Z

1

When using an array, for in you example of Users, you are mapping using @XmlRootElement(name = "Users"), but for Books there is no @XmlElement(name = "Books", type = Book.class), you are just mapping for "book".

Try putting:

@XmlElement(name = "Books", type = Book.class)

Instead of:

@XmlElement(name = "Book", type = Book.class)

answered Aug 19, 2014 at 19:35

Bruno Franco

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1 Comment

spiderman Over a year ago

I tried this, now the o/p structure slightly differs , ie it pulls 1 Book with empty name.

com.example.Users@4e76fba0[   users=[com.example.User@6581ed9e[   name=Ammu   books=[com.example.Book@e949f69[   name=<null> ]] ], com.example.User@35175422[   name=Unni   books=[com.example.Book@3f6dadf9[   name=<null> ]] ]] ]

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