使用Guava的Optional与@XmlAttribute

I would like to setup a JAXB-annotated Java class to generate some XML in the following format:

我想设置一个带JAXB注释的Java类，以下列格式生成一些XML：

<page refId="0001">
    <title>The title of my page</title>
</page>

The "refId" field is optional, so I'd like to use Guava's Optional construct to reference the string in memory. I see Using generic @XmlJavaTypeAdapter to unmarshal wrapped in Guava's Optional, which gives a thorough example if you're using an element (even if that wasn't the original question), but how would you set up the annotations for an XML attribute?

“refId”字段是可选的，所以我想使用Guava的Optional构造来引用内存中的字符串。我看到使用泛型@XmlJavaTypeAdapter解压缩包装在Guava的Optional中，如果您使用的是元素（即使这不是原始问题），它会提供一个完整的示例，但是您如何设置XML属性的注释？

Here's what I have so far:

这是我到目前为止所拥有的：

@XmlRootElement(name="page")
public final class Page {
    @XmlAttribute
    @XmlJavaTypeAdapter(OptionalAdapter.class)
    private Optional<String> refId;

    @XmlElement
    private String title;

    ... getters/setters, default constructor, etc.
}

And OptionalAdapter is a simple XmlAdapter:

而OptionalAdapter是一个简单的XmlAdapter：

public class OptionalAdapter<T> extends XmlAdapter<T, Optional<T>> {

    @Override
    public Optional<T> unmarshal(T v) throws Exception {
        return Optional.fromNullable(v);
    }

    @Override
    public T marshal(Optional<T> v) throws Exception {
        if (v == null || !v.isPresent()) {
            return null;
        } else {
            return v.get();
        }
    }
}

When I try to load up a unit test against the above code, it fails instantly during initialization, but if I change the annotation to @XmlElement, the test will run and pass, but obviously sets the refId as a child element instead of an attribute.

当我尝试针对上面的代码加载单元测试时，它在初始化期间立即失败，但是如果我将注释更改为@XmlElement，测试将运行并通过，但显然将refId设置为子元素而不是属性。

Thanks in advance!

提前致谢！

1 个解决方案

#1

Xml-attribute can have only simple type (like String, Integer etc.), so you cann't use OptionalAdapter<T>.
If your field has type String then adapter should have type OptionalAdapter<String>.
You can do in next way:
- create additional class, and use is as XmlAdapter

Xml属性只能有简单类型（如String，Integer等），因此不能使用OptionalAdapter 。如果您的字段具有String类型，则适配器应具有OptionalAdapter 类型。您可以通过以下方式执行操作： - 创建其他类，并使用XmlAdapter

   public final class StringOptionalAdapter extends OptionalAdapter<String>
   {
   }  

Page.java

Page.java

   @XmlAttribute
   @XmlJavaTypeAdapter(StringOptionalAdapter.class)
   private Optional<String> refId;