java - JAXB 将 XML 编码为 OutputStream 与 StringWriter

Question

如果有人回答了这个问题，我深表歉意，但我一直在使用的搜索词(即 JAXB @XmlAttribute condensed 或 JAXB XML marshal to字符串不同的结果)没有得出任何结果。

我正在使用 JAXB 取消/编码用 @XmlElement 和 @XmlAttribute 注释注释的对象。我有一个格式化程序类，它提供了两种方法——一种包装编码方法并接受要编码的对象和一个 OutputStream，另一种只接受对象并将 XML 输出作为字符串返回。不幸的是，这些方法不会为相同的对象提供相同的输出。编码到文件时，内部用 @XmlAttribute 标记的简单对象字段打印为:

<element value="VALUE"></element>

当编码为字符串时，它们是:

<element value="VALUE"/>

对于这两种情况，我都更喜欢第二种格式，但我很好奇如何控制差异，并且无论如何都会满足于它们相同。我什至创建了一个静态编码器，两种方法都使用它来消除不同的实例值。格式化代码如下:

/** Marker interface for classes which are listed in jaxb.index */
public interface Marshalable {}

---

/** Local exception class */
public class XMLMarshalException extends BaseException {}

---

/** Class which un/marshals objects to XML */
public class XmlFormatter {
    private static Marshaller marshaller = null;
    private static Unmarshaller unmarshaller = null;

    static {
        try {
            JAXBContext context = JAXBContext.newInstance("path.to.package");
            marshaller = context.createMarshaller();
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            marshaller.setProperty(Marshaller.JAXB_ENCODING, "UTF-8");

            unmarshaller = context.createUnmarshaller();
        } catch (JAXBException e) {
            throw new RuntimeException("There was a problem creating a JAXBContext object for formatting the object to XML.");
        }
    }

    public void marshal(Marshalable obj, OutputStream os) throws XMLMarshalException {
        try {
            marshaller.marshal(obj, os);
        } catch (JAXBException jaxbe) {
            throw new XMLMarshalException(jaxbe);
        }
    }

    public String marshalToString(Marshalable obj) throws XMLMarshalException {
        try {
            StringWriter sw = new StringWriter();
            return marshaller.marshal(obj, sw);
        } catch (JAXBException jaxbe) {
            throw new XMLMarshalException(jaxbe);
        }
    }
}

---

/** Example data */
@XmlType
@XmlAccessorType(XmlAccessType.FIELD)
public class Data {

    @XmlAttribute(name = value)
    private String internalString;
}

---

/** Example POJO */
@XmlType
@XmlRootElement(namespace = "project/schema")
@XmlAccessorType(XmlAccessType.FIELD)
public class Container implements Marshalable {

    @XmlElement(required = false, nillable = true)
    private int number;

    @XmlElement(required = false, nillable = true)
    private String word;

    @XmlElement(required = false, nillable = true)
    private Data data;
}

调用marshal(container, new FileOutputStream("output.xml"))和marshalToString(container)的结果如下:

输出到文件

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>  
<ns2:container xmlns:ns2="project/schema">  
    <number>1</number>  
    <word>stackoverflow</word>  
    <data value="This is internal"></data>  
</ns2:container>

和

输出为字符串

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>  
<ns2:container xmlns:ns2="project/schema">  
    <number>1</number>  
    <word>stackoverflow</word>  
    <data value="This is internal"/>  
</ns2:container>

Answer 1

看起来这可能是 JAXB 中的一个“错误”。查看源代码，对 marshal() 的调用根据输出/编写器类型参数创建不同的编写器:

public void marshal(Object obj, OutputStream out, NamespaceContext inscopeNamespace) throws JAXBException {
    write(obj, createWriter(out), new StAXPostInitAction(inscopeNamespace,serializer));
}

public void marshal(Object obj, XMLStreamWriter writer) throws JAXBException {
    write(obj, XMLStreamWriterOutput.create(writer,context), new StAXPostInitAction(writer,serializer));
}

编写者的实现在处理“空元素”方面有所不同。以上代码来自:

jaxb-ri\runtime\src\com\sun\xml\bind\v2\runtime\MarshallerImpl.java.

您正在创建的两个作者是:

jaxb-ri\runtime\src\com\sun\xml\bind\v2\runtime\output\UTF8XmlOutput.java

jaxb-ri\runtime\src\com\sun\xml\bind\v2\runtime\output\XMLStreamWriterOutput.java