2.Xml与多个对象的映射（聚合或组合）及注意事项

　在我们的实际应用中，Xml中的结构往往不止这么简单，一般都会有2，3层。也就是说如果映射成对象就是聚合（组合）的情况 。

就用我们上一章的例子继续来讲，简单我们的Book的author现在不止是一个String类型的名子，他是一个对象Author，并包含作者的相关个人信息。那我们怎么做列？

直接看代码

package com.jaxb.first;

import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

@XmlRootElement(name = "book")
// If you want you can define the order in which the fields are written
// Optional
@XmlType(propOrder = { "name", "author", "publisher", "isbn" })
public class Book {

    private String name;
    private Author author;
    private String publisher;
    private String isbn;

    // If you like the variable name, e.g. "name", you can easily change this
    // name for your XML-Output:
    public String getName() {
        return name;
    }

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


    public Author getAuthor() {
        return author;
    }

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

    public String getPublisher() {
        return publisher;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    public String getIsbn() {
        return isbn;
    }

    public void setIsbn(String isbn) {
        this.isbn = isbn;
    }

}

package com.jaxb.first;

import javax.xml.bind.annotation.XmlAttribute;

public class Author {


    private String name;
    private int age;

    @XmlAttribute
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }



}

package com.jaxb.first;

import java.util.ArrayList;

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

@XmlRootElement(namespace="abc")
public class Bookstore {

    // XmLElementWrapper generates a wrapper element around XML representation
    @XmlElementWrapper(name = "bookList")
    // XmlElement sets the name of the entities
    @XmlElement(name = "book")
    private ArrayList<Book> bookList;
    private String name;
    private String location;

    public void setBookList(ArrayList<Book> bookList) {
        this.bookList = bookList;
    }

    public ArrayList<Book> getBooksList() {
        return bookList;
    }

    public String getName() {
        return name;
    }

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

    public String getLocation() {
        return location;
    }

    public void setLocation(String location) {
        this.location = location;
    }
}

 

package com.jaxb.first;

import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.ArrayList;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

import com.sun.xml.bind.marshaller.NamespacePrefixMapper;

public class BookMain {

    private static final String BOOKSTORE_XML = "./bookstore-jaxb.xml";

    public static void main(String[] args) throws JAXBException, IOException {

        ArrayList<Book> bookList = new ArrayList<Book>();

        // create books
        Book book1 = new Book();
        book1.setIsbn("978-0060554736");
        book1.setName("The Game");
        Author a = new Author();
        a.setAge(28);
        a.setName("Gosling");
        book1.setAuthor(a);
        book1.setPublisher("Harpercollins");
        bookList.add(book1);

        Book book2 = new Book();
        book2.setIsbn("978-3832180577");
        book2.setName("Feuchtgebiete");
        Author a2 = new Author();
        a2.setAge(32);
        a2.setName("James Green");
        book2.setAuthor(a2);
        book2.setPublisher("Dumont Buchverlag");
        bookList.add(book2);

        // create bookstore, assigning book
        Bookstore bookstore = new Bookstore();
        bookstore.setName("Fraport Bookstore");
        bookstore.setLocation("Frankfurt Airport");
        bookstore.setBookList(bookList);

        // create JAXB context and instantiate marshaller
        JAXBContext context = JAXBContext.newInstance(Bookstore.class);
        Marshaller m = context.createMarshaller();
        NamespacePrefixMapper mapper = new PreferredMapper();
        m.setProperty("com.sun.xml.bind.namespacePrefixMapper", mapper);

        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
        m.marshal(bookstore, System.out);

        Writer w = null;
        try {
            w = new FileWriter(BOOKSTORE_XML);
            m.marshal(bookstore, w);
        } finally {
            try {
                w.close();
            } catch (Exception e) {
            }
        }

        // get variables from our xml file, created before
        System.out.println();
        System.out.println("Output from our XML File: ");
        Unmarshaller um = context.createUnmarshaller();
        Bookstore bookstore2 = (Bookstore) um.unmarshal(new FileReader(
                BOOKSTORE_XML));

        for (int i = 0; i < bookstore2.getBooksList().toArray().length; i++) {
            System.out.println("Book " + (i + 1) + ": "
                    + bookstore2.getBooksList().get(i).getName() + " from "
                    + bookstore2.getBooksList().get(i).getAuthor());
        }

    }

    public static class PreferredMapper extends NamespacePrefixMapper {
        @Override
        public String getPreferredPrefix(String namespaceUri,
                String suggestion, boolean requirePrefix) {
            return "pre";
        }


    }
}

看下输出结果：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<pre:bookstore xmlns:pre="abc">
    <bookList>
        <book>
            <name>The Game</name>
            <author name="Gosling">
                <age>28</age>
            </author>
            <publisher>Harpercollins</publisher>
            <isbn>978-0060554736</isbn>
        </book>
        <book>
            <name>Feuchtgebiete</name>
            <author name="James Green">
                <age>32</age>
            </author>
            <publisher>Dumont Buchverlag</publisher>
            <isbn>978-3832180577</isbn>
        </book>
    </bookList>
    <location>Frankfurt Airport</location>
    <name>Fraport Bookstore</name>
</pre:bookstore>

Output from our XML File:
Book 1: The Game from com.jaxb.first.Author@1774b9b
Book 2: Feuchtgebiete from com.jaxb.first.Author@104c575

OK 是我们想要的格式吧。 那么事情就解决了

   值 得注意的是：如果你要对属性做注解，必须将注解写在属性的get方法上， 就如我们Author类中的 @XmlAttribute那样，否则运行的时候他会提示：the same element name xxx..