使用Java中的Contract-First进行Web服务循环

Question

I'm working on an application with contract first web services ( wsimport and jaxws-maven-plugin ).

How should I write WSDL/XSD files in order to be able to process cycles? For example object department with reference to employees and employee with reference to department (as in this article http://jaxb.java.net/guide/Mapping_cyclic_references_to_XML.html ). The article notes a @XmlTransient annotation, but as I use contract-first, I'm unable to modify generated classes in any way.

If I just ignore those cycles, first time I run the webservice, I receive an error such as:

Caused by: com.sun.istack.SAXException2: 
A cycle is detected in the object graph. This will cause infinitely deep XML: 
Employee@18ac4d8 -> Department@aa35d5 -> Employee@18ac4d8

Answer 1

The problem is that webservice contracts (at least WS-I Basic Profile compliant ones) can not encode references to other objects in the message. That is, a field of reference type is always marshalled by marshalling the fields of the object it refers to. This recursion is limitless if the object graph contains a cycle.

That is, if you had:

class A {
    String name;
    A a;
}

and did:

A a = new A();
a.name = "hello";
a.a = a;
marshall(a);

The XML would look like

<a>
    <name>hello</name>
    <a>
        <name>hello</name>
        <a>
            <name>hello</name>
            <a>
                ...

To avoid this, the cycle needs to be broken. Typical approaches include to make the association navigable in only one direction, set backreferences to null prior to marshalling (tasking the recipient to reconstruct them), move the associations to seperate classes like in

class A {
    String name;
}
class B {
    String adress;   
}

class AWithB {
    A a;
    B b;
}

and a host of other options.