使用JAXB解组时将空元素转换为null [英] Transforming empty element into null when unmarshalling with JAXB

问题描述

使用以下JAXB注释定义类：

  class Course {
 @XmlElement（name = book）
 List< Book> requiredBooks = new ArrayList< Book>（）; 
  

当解组包含此

的XML文档时pre> <当然>
...
< book />
< / course>


我最终将一本书添加到列表中，其所有属性都设置为null。我不控制XML输入。如何防止添加此空白书？我尝试拦截set ..（）或添加..（）方法，但是在处理集合时，JAXB绕过了setter。有什么建议吗？

解决方案

这是 工作的解决方案。同样，优雅被遗忘。

JAXB定义了两个回调：beforeUnmarshal和afterUnmarshal。通过实现afterUnmarshal来清理列表，我能够达到预期的结果。

  class课程
 {
 @XmlElement（name =book）
 List< Book> requiredBooks = new ArrayList< Book>（）; 
 ... 
 void afterUnmarshal（Unmarshaller aUnmarshaller，Object aParent）
 {
 if（requiredBooks！= null）
 {
 Iterator< Book> iterator = requiredBooks.iterator（）; 
 while（iterator.hasNext（））
 {
 Book book = iterator.next（）; 
 if（StringUtils.isEmpty（book.getTitle（）））
 {
 //没有标题的书被视为无效
 iterator.remove（）; 
} 
} 
} 
} 
} 
  

虽然这有效，但最大的问题是没有用于实现afterUnmarshal的接口。它是由JAXB使用反射查找的，但我认为如果JAXB只提供接口和/或抽象实现，那将会很方便（并且会减少调试/维护）。如果是，如果afterUnmarshal的签名将来会发生什么变化呢？这段代码将神秘地停止工作，因为它应该是。

A class is defined with the following JAXB annotation:

class Course {
@XmlElement (name = "book")
List<Book> requiredBooks = new ArrayList<Book>();

When unmarshalling an XML document that contains this

<course>
  ...
  <book/>
</course>

I end up with a Book added to the list, with all of its attributes set to null. I don't control the XML input. How can I prevent this empty book from being added? I tried intercepting in set..() or add..() methods, but turns out JAXB bypasses setters when dealing with collections. Any suggestions?

解决方案

Here's a solution that does work. Again, elegance was left behind.

There are two callbacks defined by JAXB: beforeUnmarshal and afterUnmarshal. By implementing afterUnmarshal to clean up the list, I was able to achieve the desired result.

  class Course
  {
    @XmlElement (name = "book")
    List<Book> requiredBooks = new ArrayList<Book>();  
    ...
    void afterUnmarshal(Unmarshaller aUnmarshaller, Object aParent)
    {
        if (requiredBooks != null)
        {
            Iterator<Book> iterator = requiredBooks.iterator();
            while (iterator.hasNext())
            {
                Book book = iterator.next();
                if (StringUtils.isEmpty(book.getTitle()))
                {
                    // a book without title is considered invalid
                    iterator.remove();
                }
            }
        }
    }
  }

While this works, by biggest issue with it is the absence of an interface for implementing afterUnmarshal. It's looked up by JAXB using reflection, but I think it would've been convenient (and would reduce debugging/maintenance) if JAXB simply supplied interfaces and/or absract implementations. As it is, what if the signature of afterUnmarshal changes in the future? This code will just mysteriously stop working as it's supposed to.

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