java - Unmarshaller.unmarshal() 的二元形式

Question

我在各种堆栈溢出帖子和博客条目中看到了以下语法:

JAXBElement<SomeClass> sc = unmarshaller.unmarshal(is, SomeClass.class);

那么当我尝试使用此语法时，为什么 Eclipse 会给出编译错误呢？为什么这个语法不在 api 中，您可以阅读 at this link ？

这是编译错误:

The method unmarshal(Node, Class<T>) in the type Unmarshaller  
is not applicable for the arguments (FileInputStream, Class<SomeClass>)  

这是一个使用上述语法的完整方法:

public void unmarshal() throws FileNotFoundException{
    Unmarshaller unmarshaller;
    try {
        JAXBContext ctx = JAXBContext.newInstance(ObjectFactory.class);
        unmarshaller = ctx.createUnmarshaller();
        FileInputStream is = new FileInputStream("path/to/some.xml");
        JAXBElement<SomeClass> sc = unmarshaller.unmarshal(is, SomeClass.class);//error here
        System.out.println("title is: "+sc.getValue().getTitle());
    } catch (JAXBException e) {e.printStackTrace();}
}

示例中给出了此语法，其中开发人员需要解码不包含已定义根元素的 xml。 Sayantam 的回答就是一个例子 to this question .

Answer 1

该错误是由于错误的类型参数引起的，FileInputStream 而不是 Node。

此方法更适合解码一段 xml。当您想要解析整个文件时，请使用 unsmarshal(File) 方法。

public void unmarshal() throws FileNotFoundException{
    Unmarshaller unmarshaller;
    try {
        JAXBContext ctx = JAXBContext.newInstance(ObjectFactory.class);
        unmarshaller = ctx.createUnmarshaller();
        FileInputStream is = new FileInputStream("path/to/some.xml");
        SomeClass sc = (SomeClass) unmarshaller.unmarshal(is);
        System.out.println("title is: "+sc.getValue().getTitle());
    } catch (JAXBException e) {e.printStackTrace();}
}

如果您不想进行类型转换，请尝试以下操作:

public void unmarshal() throws FileNotFoundException{
    Unmarshaller unmarshaller;
    try {
        JAXBContext ctx = JAXBContext.newInstance(ObjectFactory.class);
        unmarshaller = ctx.createUnmarshaller();

        XMLInputFactory xmlInputFactory = XMLInputFactory.newFactory();
        StreamSource streamSource = new StreamSource("path/to/some.xml");
        XMLStreamReader xmlStreamReader = xmlInputFactory.createXMLStreamReader(streamSource);
        JAXBElement<SomeClass> sc = unmarshaller.unmarshal(xmlStreamReader, SomeClass.class);//error here
        System.out.println("title is: "+sc.getValue().getTitle());
    } catch (JAXBException e) {e.printStackTrace();}
}