JAXB 解组 XML 类强制转换异常 [英] JAXB Unmarshalling XML Class Cast Exception

问题描述

我正在使用这个 JAXB Collection Generics 来解组我的字符串 xml 并返回 List 类型.这是我使用的方法.

I'm using this JAXB Collection Generics to unmarshall my string xml and return the List type. Here's the methods I used.

public static <T> List<T> unmarshalCollection(Class<T> cl, String s)
        throws JAXBException {
    return unmarshalCollection(cl, new StringReader(s));
}

public static <T> List<T> unmarshalCollection(Class<T> cl, Reader r)
        throws JAXBException {
    return unmarshalCollection(cl, new StreamSource(r));
}


private static <T> List<T> unmarshalCollection(Class<T> cl, Source s)
        throws JAXBException {
    JAXBContext ctx = JAXBContext.newInstance(JAXBCollection.class, cl);
    Unmarshaller u = ctx.createUnmarshaller();
    JAXBCollection<T> collection = u.unmarshal(s, JAXBCollection.class).getValue();
    return collection.getItems();
}

getter 和 setter 示例:

Example getters and setters:

   @XmlRootElement(name = "person")
class Person{

    private String firstName;
    private String lastName;
    private String address;


    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "Person [firstName ="+firstName+" , lastName = "+lastName+" , address = "+address+"]"; 
    }

}

主类:

public static void main(String[] args) throws JAXBException {
        String xml = "<?xml version="1.0" encoding="UTF-8"?><person><firstName>Foo</firstName><lastName>Bar</lastName><address>U.S</address></person>";
        List<Person> p = unmarshalCollection(Person.class,xml);
        for(Person person : p ){
            System.out.println(person);
        }
    }

抛出异常

Exception in thread "main" java.lang.ClassCastException: org.apache.xerces.dom.ElementNSImpl cannot be cast to  com.Person
at com.JAXBUtil.main(JAXBUtil.java:62)

我做错了什么?有什么想法吗?谢谢.

What did I do wrong? Any ideas?Thanks.

推荐答案

您的列表不是您期望的 Person 对象列表.由于 java 的泛型类型擦除，在您尝试在循环中强制转换为 person 之前，您不会看到错误.

Your list is not a list of Person objects as you expect. Due to java's generic type erasure, you're not seeing the error until you try to cast to person in the loop.

试试:

List<Person> p = unmarshalCollection(Person.class,xml);
for(Object person : p ){
   System.out.println(person.getClass());
}

参见 http://en.wikipedia.org/wiki/Generics_in_Java#Problems_with_type_erasure

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