問題描述

我正在使用這個 JAXB Collection Generics 來解組我的字符串 xml 并返回 List 類型.這是我使用的方法.

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+"]"; 
    }

}

主類:

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);
        }
    }

拋出異常

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)

我做錯了什么?有什么想法嗎?謝謝.

What did I do wrong? Any ideas?Thanks.

推薦答案

您的列表不是您期望的 Person 對象列表.由于 java 的泛型類型擦除，在您嘗試在循環中強制轉換為 person 之前，您不會看到錯誤.

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.

試試:

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

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

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