java JAXB 编组和泛型
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JAXB Marshalling and Generics
提问by Steve Skrla
I am trying to use JAXB's introspection to marshall and unmashall some existing domain objects marked up with JAXB annotations. Most things work as expected, but I am having quite a bit of trouble getting a fairly simple class to serialize. This class is used as an @XmlElement on a number of beans and looks something like:
我正在尝试使用 JAXB 的自省来编组和解组一些用 JAXB 注释标记的现有域对象。大多数事情都按预期工作，但我在序列化一个相当简单的类时遇到了很多麻烦。这个类在许多 bean 上用作 @XmlElement，看起来像：
public class Range<E extends Comparable<E>> implements Serializable {
protected boolean startInclusive, endInclusive;
protected E start, end;
public Range(){
startInclusive = endInclusive = true;
}
public boolean contains(E value){...}
public E getEnd() {
return end;
}
public void setEnd(E end) {
this.end = end;
}
public boolean isEndInclusive() {
return endInclusive;
}
public void setEndInclusive(boolean endInclusive) {
this.endInclusive = endInclusive;
}
public E getStart() {
return start;
}
public void setStart(E start) {
this.start = start;
}
public boolean isStartInclusive() {
return startInclusive;
}
public void setStartInclusive(boolean startInclusive) {
this.startInclusive = startInclusive;
}
}
I have tried to do the following, with no success, JAXB is still angry with the interface Comparable.
我尝试了以下操作，没有成功，JAXB 仍然对接口 Comparable 生气。
public class DoubleRange extends Range<Double> {}
Using both Range and DoubleRange as return types for the bean getter's yields an exception like:
使用 Range 和 DoubleRange 作为 bean getter 的返回类型会产生如下异常：
java.lang.Comparable is an interface, and JAXB can't handle interfaces.
this problem is related to the following location:
at java.lang.Comparable
at protected java.lang.Comparable com.controlpath.util.Range.start
at example.util.Range
at example.util.DoubleRange
at public example.util.DoubleRange example.domain.SomeBean.getRange()
at example.domain.SomeBean
I realize that in most cases List<T> and Map<T, U> only work because the JAXB specification has special provisions for those types when they are encountered on beans, but is there any way to convey what I want to the JAXB introspection engine without having to reimplement range with non-generic fields?
我意识到在大多数情况下 List<T> 和 Map<T, U> 仅起作用，因为 JAXB 规范对在 bean 上遇到这些类型时有特殊规定，但是有什么方法可以将我想要的内容传达给 JAXB 自省引擎而不必重新实现具有非通用字段的范围？
采纳答案by ivan_ivanovich_ivanoff
You could write a custom adapter (not using JAXB's XmlAdapter) by doing the following:
您可以通过执行以下操作来编写自定义适配器（不使用 JAXB 的 XmlAdapter）：
1) declare a class which accepts all kinds of elements and has JAXB annotations and handles them as you wish (in my example I convert everything to String)
1) 声明一个类，它接受各种元素并具有 JAXB 注释并根据需要处理它们（在我的示例中，我将所有内容都转换为字符串）
@YourJAXBAnnotationsGoHere
public class MyAdapter{
@XmlElement // or @XmlAttribute if you wish
private String content;
public MyAdapter(Object input){
if(input instanceof String){
content = (String)input;
}else if(input instanceof YourFavoriteClass){
content = ((YourFavoriteClass)input).convertSomehowToString();
}else if(input instanceof .....){
content = ((.....)input).convertSomehowToString();
// and so on
}else{
content = input.toString();
}
}
}
// I would suggest to use a Map<Class<?>,IMyObjToStringConverter> ...
// to avoid nasty if-else-instanceof things
2) use this class instead of E in your to-be-marshalled class
2）在您要编组的类中使用此类而不是 E
NOTES
笔记
- Of course this would notwork for complex (nested) data structures.
- You have to think how to unmarshall this back again, could be more tricky. If it's too tricky, wait for a better proposal than mine ;)
- 当然，这不适用于复杂的（嵌套）数据结构。
- 您必须考虑如何再次将其解组，这可能会更加棘手。如果太棘手，请等待比我更好的建议；)
回答by ivan_ivanovich_ivanoff
How about
怎么样
public class Range<**E extends Number**> implements Serializable { ...
Number is a class
I bet JAXB knows defaultmarshalling/unmarshalling rules for Number
数字是一个类
我敢打赌 JAXB 知道Number 的默认编组/解组规则
For unmarshalling to specific type, you need XmlAdapter as I described here: JAXB inheritance, unmarshal to subclass of marshaled class
要解组到特定类型，您需要 XmlAdapter，正如我在此处描述的：JAXB 继承，解组到已编组类的子类
回答by StaxMan
Actually, it is not quite clear to me why this would not work. It seems like JAXB should be able to resolve specific subtype correctly: if (and only if!) this type is NOT the root type (which it is not as per your description). I mean, it is just a Bean; so if bean with T replaced with direct type works, so should generic version iff using sub-classing to bind types (as is done in example).
实际上，我不太清楚为什么这行不通。似乎 JAXB 应该能够正确解析特定的子类型：如果（且仅当！）此类型不是根类型（这与您的描述不同）。我的意思是，它只是一个 Bean；因此，如果使用直接类型替换 T 的 bean 有效，那么泛型版本 iff 也应该使用子类来绑定类型（如示例中所做的那样）。
So perhaps it could be a bug in implementation?
那么也许它可能是实现中的错误？
回答by ng.
Try something like Simple XML Serializationit comes with support for generic types in XML elements with a number of annotations such as @Element and @ElementList. The programming model is very similar, but simpler than JAXB.
尝试像Simple XML Serialization这样的东西，它支持 XML 元素中的泛型类型，带有许多注释，例如 @Element 和 @ElementList。编程模型非常相似，但比 JAXB 简单。
回答by Tom Hawtin - tackline
So it looks like the problem is the erasure of Eon startand endis Comparable. If it can't handle interfaces you could try Object, but I would hope it complains at that too (either now or later). Possibly you could make Rangeabstract and specialise it for each specific E. I should know more about JAXB.
所以看起来问题是擦除Eonstart和endis Comparable。如果它无法处理接口，您可以尝试Object，但我希望它也抱怨（现在或以后）。可能您可以将其Range抽象化并针对每个特定的E. 我应该更多地了解 JAXB。
