JAXB--@XmlElementWrapper注解和泛型一起使用（三）

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;}@Overridepublic 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;}@Overridepublic 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;//Marshallerpublic 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 printedjaxbMarshaller.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;//Marshallerpublic 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 printedjaxbMarshaller.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>