# Mapping cyclic references to XML

Object models designed in Java often have cycles, which prevent straight-forward conversion to XML by JAXB. In fact, when you try to marshal an object tree that contains a cycle, the JAXB marshaller reports an error, pointing out the objects that formed the cycle. This is because JAXB by itself cannot figure out how to cut cycles into a tree.

Thus it is your responsibility to annotate classes and use other means to "tell" JAXB how to handle a cycle. This chapter talks about various techniques to do this.

## Parent pointers

One of the very common forms of cycle is a parent pointer. The following example illustrates a typical parent pointer, and how this can be turned into "natural" XML:

Classes with parent pointer

    @XmlRootElement
    class Department {
      @XmlAttribute
      String name;
      @XmlElement(name="employee")
      List<Employee> employees;
    }

    class Employee {
      @XmlTransient
      Department department;  // parent pointer
      @XmlAttribute
      String name;

      public void afterUnmarshal(Unmarshaller u, Object parent) {
        this.department = (Department)parent;
      }
    }

This will produce the following XML:

XML view of department

    <department name="accounting">
      <employee name="Joe Chin" />
      <employee name="Adam Smith" />
      ...
    </department>

And reading this document back into Java objects will produce the expected tree with all the proper parent pointers set up correctly.

The first technique here is the use of `XmlTransient` on the parent pointer. This tells JAXB that you don't need this parent pointer to be represented explicitly in XML, because the fact that `employee` is always contained inside `department` implies the parent/child relationship. This causes the marshaller to produce the expected XML. However, when you unmarshal it, since this field is not bound, the `Employee.department` field will be left null.

That's where the second technique comes in, which is the use of the `afterUnmarshal` callback. This method is invoked by the JAXB implementation on each instance when the unmarshalling of a `Employee` object completes. Furthermore, the second parameter to the method is the parent object, which in this case is a `Department` object. So in this example, this sets up the parent pointer correctly.

This callback can be also used to perform other post-unmarshalling set up work.

## Many-to-many relationship

TBD

## `@XmlID` and `@XmlIDREF`

When a reference to another object is annotated with `XmlIDREF`, its corresponding XML it will be referenced by `xs:IDREF`, instead of containment. See below for an example:

Example of `@XmlID` and `@XmlIDREF`

    @XmlRootElement
    class Root {
      List<Foo> foos;
      List<Bar> bars;
    }
    class Foo {
      // you don't have to make it an attribute, but that's more common
      @XmlAttribute @XmlIDREF Bar bar;
    }
    class Bar {
      // you don't have to make it an attribute, but that's more common
      @XmlAttribute @XmlID String id;
    }

Schema for above

    <xs:complexType name="foo">
      <xs:sequence/>
      <xs:attribute name="bar" type="xs:IDREF"/>
      </xs:sequence>
    </xs:complexType>
    <xs:complexType name="bar">
      <xs:sequence/>
      <xs:attribute name="id" type="xs:ID"/>
    </xs:complexType>

A sample instance

    <root>
      <foo bar="x"/>
      <foo bar="y"/>
      <bar id="x"/>
      <bar id="y"/>
    </root>

There are a few things to consider when you do this. First, the object to be referenced must have an ID that is unique within the whole document. You'd also need to ensure that the referenced objects are contained somewhere else (like in the `Root` class in this case), or else `Bar` objects will never be marshalled. This technique can be used to remove the cyclic references, but it's only possible when your object model has an easy cut point.

## Use the CycleRecoverable interface

Starting 2.1 EA2, the JAXB RI exposes `CycleRecoverable` interface. Applications can choose to implement this interface in some of its objects. When a cyclic reference is detected during marshalling, and if the object that formed a cycle implements this interface, then the method on this interface is called to allow an application to nominate its replacement to be written to XML. In this way, the application can recover from a cycle gracefully.

This technique allows you to cope with a situation where you cannot easily determine upfront as to where a cycle might happen. On the other hand, this feature is a JAXB RI feature. Another downside of this is that unless you nominate your replacement carefully, you can make the marshalling output invalid with respect to the schema, and thus you might hit another problem when you try to read it back later.