JAXB读取XML文档

Question

I'm trying to read an XML document and decode it in to Java Beans. I have the reading part settled but I run in to an issue. I'm basically trying to decode all the child nodes of the XML document, root being "catalog". How do I do this using the XMLDecoder?

XMLDecoder:

private static Book jaxbXMLToObject() {
    try {
        JAXBContext context = JAXBContext.newInstance(Book.class);
        Unmarshaller un = context.createUnmarshaller();
        Book book = (Book) un.unmarshal(new File("PATH"));
        return book;
    } catch (JAXBException e) {
        e.printStackTrace();
    }
    return null;
}

I'm trying to read the following document

    <?xml version="1.0"?>
    <catalog>
       <book id="1">
          <author>Isaac Asimov</author>
          <title>Foundation</title>
          <genre>Science Ficition</genre>
          <price>164</price>
          <publish_date>1951-08-21</publish_date>
          <description>Foundation is the first novel in Isaac Asimovs Foundation Trilogy (later expanded into The Foundation Series). Foundation is a cycle of five interrelated short stories, first published as a single book by Gnome Press in 1951. Collectively they tell the story of the Foundation, an institute to preserve the best of galactic civilization after the collapse of the Galactic Empire.</description>
       </book>
   </catalog>

And Parse it in to a Book Object

@XmlRootElement(name = "book")
@XmlType(propOrder = {"id", "price", "title", "author", "genre", "description"})
public class Book {
    private int id;
    private int price;
    private String title;
    private String author;
    private String genre;
    private String description;
    private Date publish_date;

    public Book() {

    }

...... I get the error: jjavax.xml.bind.UnmarshalException: unexpected element (uri:"", local:"catalog"). Expected elements are <{}book> jjavax.xml.bind.UnmarshalException: unexpected element (uri:"", local:"catalog"). Expected elements are <{}book>

How do I only access the child nodes using JAXB?

UPDATE

Catalog Class:

@XmlRootElement(name = "catalog")

    public class Catalog {
        @XmlElement(name = "book")
        List<Book> books;

        public List<Book> getBooks() {
            return books;
        }

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

Book class:

@XmlAccessorType(XmlAccessType.FIELD)
public class Book {
    @XmlAttribute
    int id;
    private int price;
    private String title;
    private String author;
    private String genre;
    private String description;
    private Date publish_date;

    public Book() {

    }

    public Book(int id, int price, String title, String genre, String description, Date publicationDate) {
        this.id = id;
        this.price = price;
        this.title = title;
        this.genre = genre;
        this.description = description;
        this.publish_date = publicationDate;
    }

    public int getId() {
        return id;
    }

    public int getPrice() {
        return price;
    }

    public String getTitle() {
        return title;
    }

    public String getGenre() {
        return genre;
    }

    public String getDescription() {
        return description;
    }

    public Date getPublicationDate() {
        return publish_date;
    }

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

    public void setPrice(int price) {
        this.price = price;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setGenre(String genre) {
        this.genre = genre;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public void setPublish_date(String publish_date) {
        this.publish_date = new Date();
    }

    public String getAuthor() {
        return author;
    }

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

    public Date getPublish_date() {
        return publish_date;
    }

    public String toJSON() {
        ObjectMapper mapper = new ObjectMapper();

        try {
            return mapper.writeValueAsString(this);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    public String toString() {
        return "Book{" +
                "id=" + id +
                ", price=" + price +
                ", title='" + title + '\'' +
                ", genre='" + genre + '\'' +
                ", description='" + description + '\'' +
                ", publicationDate=" + publish_date +
                '}';
    }
}

DAO:

public class BooksDAO {

    public BooksDAO() {
    }

    public List<Book> getBooks() {
        Catalog catalog = jaxbXMLToObject();
        return catalog.getBooks();
    }

    private static Catalog jaxbXMLToObject() {
        try {
            return JAXB.unmarshal(new File("PATH"), Catalog.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

Answer 1

As already pointed out by JB Nizet you definitely need an enclosing Catalog object. The following is the bare minimum to be able to unmarshal the provided XML document using JAXB and extract the book from it:

public class ReadXMLUsingJAXB {

    static class Catalog {
        @XmlElement(name = "book")
        List<Book> books;
    }

    @XmlAccessorType(XmlAccessType.FIELD)
    static class Book {
        @XmlAttribute
        int id;
        String author;
        String title;
        String genre;
        int price;
        Date publish_date;
        String description;
    }

    private static Book firstBookFromXML() {
        Catalog catalog = JAXB.unmarshal(new File("PATH"), Catalog.class);
        return catalog.books.get(0);
    }

    public static void main(String[] args) {
        Book book = firstBookFromXML();
        System.out.println(book.id + ", " + book.author + ", " + book.title 
                + ", " + book.genre + ", " + book.price 
                + ", " + book.publish_date + ", " + book.description);
    }

}

Some things are worth mentioning here:

1. The @XmlAccessorType -Annotation is not necessary with Catalog as there is only one field which is annotated with @XmlElement .
2. When chosing FIELD as access type all fields are taken into account regardless of their visibility unless annotated with @XmlTransient .
3. The book ID is an attribute in the document, so it must be declared as such using @XmlAttribute .
4. @XmlElement on Catalog.books was necessary to reflect the name of the book-Elements. JAXB defaults to the field (or property) name which would be book s instead and thus not match the elements.

As said before the demonstration code is the bare minimum and should be changed to fit your needs (ie field visibility, proper constructor, getters, equals, hashCode, toString etc.)