Java JAXB 以不同的方式封送 XML 到 OutputStream 和 StringWriter

JAXB marshals XML differently to OutputStream vs. StringWriter

提问by Andy

I apologize if this has been answered, but the search terms I have been using (i.e. JAXB @XmlAttribute condensedor JAXB XML marshal to String different results) aren't coming up with anything.

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 String different results）没有提出任何内容。

I am using JAXB to un/marshal objects annotated with @XmlElementand @XmlAttributeannotations. I have a formatter class which provides two methods -- one wraps the marshal method and accepts the object to marshal and an OutputStream, the other just accepts the object and returns the XML output as a String. Unfortunately, these methods do not provide the same output for the same objects. When marshaling to a file, simple object fields internally marked with @XmlAttributeare printed as:

我正在使用 JAXB 来取消/封送用@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 marshal 方法并接受要编组的对象和 an 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。当编组到文件时，内部标记为的简单对象字段@XmlAttribute打印为：

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

while when marshaling to a String, they are:

而在编组到字符串时，它们是：

<element value="VALUE"/>

I would prefer the second format for both cases, but I am curious as to how to control the difference, and would settle for them being the same regardless. I even created one static marshaller that both methods use to eliminate different instance values. The formatting code follows:

对于这两种情况，我更喜欢第二种格式，但我很好奇如何控制差异，并且无论如何都会满足于它们是相同的。我什至创建了一个静态编组器，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;
}

The result of calling marshal(container, new FileOutputStream("output.xml"))and marshalToString(container)are as follows:

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

Output to file

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>

and

和

Output to String

输出到字符串

<?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>

采纳答案by Eric Thorbjornsen

Looks like this might be a "bug" in JAXB. Looking at the source, the calls for marshal() create different writers based on the output/writer type parameter:

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));
}

The implementations of the writers is different with regards to how they handle "empty elements". The above code is from:

编写器的实现在如何处理“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.

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

The two writers you are creating are:

您正在创建的两个作家是：

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

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

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

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

回答by mdma

I don't know why JAXB is doing this - or even if it is JAXB - if JAXB is outputting XML via a SAXContentHandler for example, then it has no direct control over how close tags are produced.

我不知道 JAXB 为什么要这样做 - 或者即使它是 JAXB - 例如，如果 JAXB 通过 SAXContentHandler 输出 XML，那么它无法直接控制关闭标记的生成方式。

To get consistent behaviour, you could wrap your OutputStream in a OutputStreamWriter, e.g.

为了获得一致的行为，您可以将您的 OutputStream 包装在一个 OutputStreamWriter 中，例如

   public void marshal(Marshalable obj, OutputStream os) throws XMLMarshalException {
        try {
            marshaller.marshal(obj, new OutputStreamWriter(os, "UTF-8"));
        } catch (JAXBException jaxbe) {
            throw new XMLMarshalException(jaxbe);
        }
    }

Along the same lines, you might see what happens if you wrap the StringWriter in a PrintWriter. Maybe there is some custom code that detects StringWriterto tries to keep the output short as possible. Sounds unlikely, but I have no other explanation.

同样，如果将 StringWriter 包装在 PrintWriter 中，您可能会看到会发生什么。也许有一些自定义代码会检测StringWriter到尝试使输出尽可能短。听起来不太可能，但我没有其他解释。

回答by bdoughan

The good news is that JAXB is a specification with more than one implementation (just like JPA). If one implementation is not meeting your needs, others are available such as EclipseLink JAXB (MOXy):

好消息是 JAXB 是一种具有多个实现的规范（就像 JPA 一样）。如果一种实现不能满足您的需求，则可以使用其他实现，例如 EclipseLink JAXB (MOXy)：

• http://www.eclipse.org/eclipselink/moxy.php

• http://www.eclipse.org/eclipselink/moxy.php