How to map Java classes to a specific XML format?

Question

I’m developing a framework that manages system calls and needs to save/load the state of the instance variables in a specific XML format.

I looked for tools on the Internet and found the JAXB 2. This tool allows you to map a XML Schema for Java classes, however I already own the classes and want to do a one-to-one match with the expected XML format.

Below are the three classes of framework:

public class Experiment {
    private String name;
    private ArrayList<Task> tasks;

    public Experiment(String name, ArrayList<Task> tasks) {}
    public Experiment(String name, Task ... tasks) {}
}

public class Requirements {
    private int cores;
    private int ram;
    private int storage;
    private long timeout;

    public Requirements(int cores, int ram, int storage, int timeout) {}
}

public class Task implements Runnable {
    private String id;
    private String command;

    private List<Task> dependencies;
    private Requirements requirements;
    private List<Task> observers;

    public Task(String id, String command) {}
}

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Example of a XML expected:

<?xml version="1.0" encoding="UTF-8" ?>
<experiment>
  <name>Test</name>
  <vars>
    <var>
      <id>message</id>
      <value>Hello World!!!</value>
    </var>
    <var>
      <id>string</id>
      <value>String test</value>
    </var>
  </vars>
  <tasks>
    <task>
      <id>print</id>
      <command>echo $(message)</command>
      <dependencies/>
      <requirements>
        <cores>1</cores>
        <ram>0</ram>
        <storage>0</storage>
        <timeout>0</timeout>
      </requirements>
    </task>
  </tasks>
</experiment>

Schema generated from the example by IntelliJ:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="experiment" type="experimentType"/>
  <xs:complexType name="varType">
    <xs:sequence>
      <xs:element type="xs:string" name="id"/>
      <xs:element type="xs:string" name="value"/>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="varsType">
    <xs:sequence>
      <xs:element type="varType" name="var" maxOccurs="unbounded" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="requirementsType">
    <xs:sequence>
      <xs:element type="xs:string" name="cores"/>
      <xs:element type="xs:string" name="ram"/>
      <xs:element type="xs:string" name="storage"/>
      <xs:element type="xs:string" name="timeout"/>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="taskType">
    <xs:sequence>
      <xs:element type="xs:string" name="id"/>
      <xs:element type="xs:string" name="command"/>
      <xs:element type="xs:string" name="dependencies"/>
      <xs:element type="requirementsType" name="requirements"/>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="tasksType">
    <xs:sequence>
      <xs:element type="taskType" name="task"/>
    </xs:sequence>
  </xs:complexType>
  <xs:complexType name="experimentType">
    <xs:sequence>
      <xs:element type="xs:string" name="name"/>
      <xs:element type="varsType" name="vars"/>
      <xs:element type="tasksType" name="tasks"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

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Can someone explain to me how to convert an object Experiment for XML and vice versa?