JAXB--@XmlElementWrapper注解和泛型一起使用

当java对象的某个属性使用泛型时，普通对象都没问题，但是遇到HashSet这种集合类封装的元素时，就会出现元素内容序列化不出来的问题，详见如下：

一、示例：

第一步：定义java对象

package step3;

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

@XmlRootElement
@XmlAccessorType(value = XmlAccessType.PROPERTY)
public class Customer<T> {
    String name;
    int age;
    int id;
    T t;

    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;
    }

    public int getId() {
        return id;
    }

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


    @Override
    public String toString() {
        return "Customer [id=" + id + ",name=" + name + ",age=" + age + ",t=" + t + "]";
    }

    public T getT() {
        return t;
    }

    public void setT(T t) {
        this.t = t;
    }
}

package step3;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;

@XmlAccessorType(value = XmlAccessType.PROPERTY)
public class Book {

    private String id;
    private String name;
    private float price;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "Book [id=" + id + ",name=" + name + ",price=" + price + "]";
    }
}

第二步：编组（JAXBContext.newInstance(Customer.class,HashSet.class);方法添加了

HashSet的class对象，以提供给JAXBContext使用。）

package step3;
import java.io.File;
import java.util.HashSet;

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

//Marshaller
public class Object2XmlDemo {
    public static void main(String[] args) {

        Customer<HashSet<Book>> customer = new Customer<HashSet<Book>>();
        customer.setId(100);
        customer.setName("suo");
        customer.setAge(29);

        Book book = new Book();
        book.setId("1");
        book.setName("哈里波特");
        book.setPrice(100);

        Book book2 = new Book();
        book2.setId("2");
        book2.setName("苹果");
        book2.setPrice(50);

        HashSet<Book> bookSet = new HashSet<Book>();
        bookSet.add(book);
        bookSet.add(book2);

        customer.setT(bookSet);

        try {
            File file = new File("C:\file1.xml");
            JAXBContext jaxbContext = JAXBContext.newInstance(Customer.class,
                    HashSet.class);
            Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
            // output pretty printed
            jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            jaxbMarshaller.marshal(customer, file);
            jaxbMarshaller.marshal(customer, System.out);
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}

得到的xml：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <age>29</age>
    <id>100</id>
    <name>suo</name>
    <t xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="hashSet"/>
</customer>

注：

1.泛型使用集合元素替代时，泛型属性所对应的xml没有序列化出来。

2.若JAXBContext.newInstance(Customer.class,HashSet.class);不添加HashSet

的class对象，则报错：

[javax.xml.bind.JAXBException: class java.util.HashSet nor any of its super class is known to this context.]

解决办法：

第一步：新增HashSet的包装类

Book类和Customer类相关代码均不需要改变，新增一个HashSet的包装类，定义如下：

package step4;

import java.util.HashSet;

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

public class BookSet {

    private HashSet<Book> bookSet = new HashSet<Book>();

    //仅包含get方法，未包含set方法
    @XmlElementWrapper(name = "bookSet")//该注解非必须，仅是标注集合元素
    @XmlElement(name="book")
    public HashSet<Book> getBookSet() {
        return bookSet;
    }

    public void addBook(Book book){
        bookSet.add(book);
    }

}

注：

1.BookSet类内部使用HashSet实现.

2.BookSet类在get方法上添加了@XmlElementWrapper(name = "bookSet")注解。

第二步：编组

package step4;

import java.io.File;

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

//Marshaller
public class Object2XmlDemo {
    public static void main(String[] args) {

        Customer<BookSet> customer = new Customer<BookSet>();
        customer.setId(100);
        customer.setName("suo");
        customer.setAge(29);

        Book book = new Book();
        book.setId("1");
        book.setName("哈里波特");
        book.setPrice(100);

        Book book2 = new Book();
        book2.setId("2");
        book2.setName("苹果");
        book2.setPrice(50);

        BookSet bookSet = new BookSet();
        bookSet.addBook(book);
        bookSet.addBook(book2);

        customer.setT(bookSet);

        try {
            File file = new File("C:\file1.xml");
            JAXBContext jaxbContext = JAXBContext.newInstance(Customer.class,
                    BookSet.class);
            Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
            // output pretty printed
            jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            jaxbMarshaller.marshal(customer, file);
            jaxbMarshaller.marshal(customer, System.out);
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}

注：

1.定义Customer对象时，使用包装类，即：

Customer<BookSet> customer = new Customer<BookSet>();

2.JAXBContext调用newInstance()方法时，传入BookSet的class对象，告知BookSet的类型，即：JAXBContext.newInstance(Customer.class,BookSet.class);

得到的xml：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <age>29</age>
    <id>100</id>
    <name>suo</name>
    <t xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="bookSet">
        <bookSet>
            <book>
                <id>2</id>
                <name>苹果</name>
                <price>50.0</price>
            </book>
            <book>
                <id>1</id>
                <name>哈里波特</name>
                <price>100.0</price>
            </book>
        </bookSet>
    </t>
</customer>