Latest revision as of 11:36, 18 June 2012

Cycle Recoverable

In the current JAXB RI, developed by Sun, there is a series of "proprietary" JAXB extensions which provide advanced functionality outside of the JAXB specification (these extension classes and properties reside in the com.sun.xml.bind package).

The CycleRecoverable interface allows the user to write their own logic to be used when object cycles are detected during marshal operations. When a cycle is detected, MOXy will invoke the onCycleDetected method, which returns a "replacement" object for the one that caused the cycle.

Example

Consider the following two classes:

import java.util.*;
import javax.xml.bind.annotation.*;
 
@XmlRootElement
public class Department {
 
   @XmlAttribute
   public int id;
   public String name;
   public List<Employee> employees = new ArrayList<Employee>();
 
}

public class Employee {
 
   @XmlAttribute
   public int id;
   public String name;
   public Department dept;
 
}

An object cycle would be created if you constructed the following objects:

Employee emp1 = new Employee();
emp1.name = "Fred Smith";
 
Department dept1 = new Department();
dept1.name = "Accounting";
 
emp1.dept = dept1;
dept1.employees.add(emp1);
 
JAXBContext ctx = JAXBContext.newInstance(new Class[] { Department.class });
ctx.createMarshaller().marshal(dept1, System.out);

A cycle is detected in the object graph. This will cause an infinite loop: Department@122cdb6 -> Employee@1ef9157 -> Department@122cdb6

By implementing the CycleRecoverable interface on the Department class, the user has the opportunity to provide a new Object for marshalling, replacing the duplicate Department. In this example, we will return a DepartmentPointer object that holds the id of the real Department.

import java.util.*;
import javax.xml.bind.annotation.*;
import com.sun.xml.bind.CycleRecoverable;
 
@XmlRootElement
public class Department implements CycleRecoverable {
 
   @XmlAttribute
   public int id;
   public String name;
   public List<Employee> employees = new ArrayList<Employee>();
 
   public Object onCycleDetected(Context arg0) {
       // Context provides access to the Marshaller being used:
       System.out.println("JAXB Marshaller is: " + cycleRecoveryContext.getMarshaller());
 
       DepartmentPointer p = new DepartmentPointer();
       p.id = this.id;
       return p;
   }
 
}

public class DepartmentPointer {
 
    public int id;
 
}

Note: In this case, DepartmentPointer must be explicitly specified in the list of classes when creating the JAXBContext, as it is not reachable from the Department class:

JAXBContext ctx = JAXBContext.newInstance(new Class[] { Department.class, DepartmentPointer.class });

This time, marshalling dept1 will succeed:

JAXB Marshaller is: org.eclipse.persistence.jaxb.JAXBMarshaller@18fd984
<?xml version="1.0" encoding="UTF-8"?>
<department>
   <id>123</id>
   <name>Accounting</name>
   <employees>
      <id>111</id>
      <name>Fred Smith</name>
      <dept xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="departmentPointer">
         <id>123</id>
      </dept>
   </employees>
</department>