规定JAXB中仅允许元素的一个实例？

Question

I am passing some XML to a RESTful web service:

<root>
    <element1/>
        <element2/>
    <element3/>
</root>

The web service will unmarshal this XML, and I want to ensure that the XML is valid.

I used a schema to create the Java objects, and in this schema element1 has a maxOccurs=1 attribute but this has no effect on the Java objects that are created (unlike, for example, adding minOccurs=1 which will result in "required=true" being added to the @XMLElement annotation). When unmarshalling the XML that is passed the web service, if an element that should appear only once appears multiple times, the last instance of the element will be parsed. I do not want this to happen, as I want to be able to alert the client that the XML they sent is not valid and that they should remove whatever element does not apply - not just arbitrarily accept the last element. I believe this will involve a little pre-validation before unmarshalling - and I may be able to make use of the beforeUnmarshal method.

After doing a bit of research, it is my understanding that there is no JAXB annotation that will allow me to specify that element1 can exist only once. Is this the case? Is it possible to validate this at all without a schema?

Answer 1

I have worked around this issue by allowing multiple instances of the element in the Java object:

@XmlElement(name = "element1", required = true)
private List<element1> element1;

This will allow me to check the size of my list in the afterUnmarshal method, and if the size of the list is greater than 1, I can throw a new ValidationException .

Answer 2

Which application calling your application as it is bit difficult to do that on Java layer but you can easily do that at your web service layer where in you can define your minOccurance to 1 ad that will serve.

But if you want to do that on Java layer you need to use XML parser and then count the Occurance of particular tag but it would take a lot of time.