Your problem is coming from the misuse of the xml annotations. You are defining a Tuple2 to be an xml root element by annotating it with @XmlRootElement, and its fields to be xml attributes by annotating the get methods with @XmlAttribute. Which translates to:

<tuple2 first="first_attributes_vale" second="second_attributes_value" />

Now, both fields are of type Coords, which is declared to be another xml element by annotating the Coords class with @XmlRootElement, and its fields to be xml attributes. When Coords gets serialised to xml, it will be:

<coords x="value" y="value" />

The problem occurs when serialising Tuple2. Its fields are supposed to be xml attributes, but Coords is another xml element. Xml attributes cannot contain nested elements, but only values.

Solution

Depending on what you want, you can solve this in two different ways. Although, I wouldn't recommend the second approach, because it's odd (even though it works) and will incur extra effort on the client side (see explanations below).

First Approach

Annotate the getFirst() and getSecond() methods with the @XmlElement annotation.

package model;

import static javax.xml.bind.annotation.XmlAccessType.NONE;

import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
@XmlAccessorType(NONE)
public class Tuple2 {
    private Coords c1;
    private Coords c2;

    public Tuple2(Coords c1, Coords c2) {
        this.c1 = c1;
        this.c2 = c2;
    }

    public Tuple2() {
        c1 = new Coords(0, 0);
        c2 = new Coords(0, 0);
    }

    @XmlElement
    public Coords getFirst() {
        return this.c1;
    }

    @XmlElement
    public Coords getSecond() {
        return this.c2;
    }
}

This will produce a result that looks like this:

<tuple2>
    <first x="2" y="4"/>
    <second x="12" y="12"/>
</tuple2>

Second Approach

This is the odd way of solving it. It works, but it incurs extra effort on the client side, because the values of Coords are encoded as string values and will require parsing on the receiving side.

Change the return type of getFirst() and getSecond() methods to be String and override the toString() method of Coords.

package model;

import static javax.xml.bind.annotation.XmlAccessType.NONE;

import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
@XmlAccessorType(NONE)
public class Tuple2 {
    private Coords c1;
    private Coords c2;

    public Tuple2(Coords c1, Coords c2) {
        this.c1 = c1;
        this.c2 = c2;
    }

    public Tuple2() {
        c1 = new Coords(0, 0);
        c2 = new Coords(0, 0);
    }

    @XmlAttribute
    public String getFirst() {
        return this.c1.toString();
    }

    @XmlAttribute
    public String getSecond() {
        return this.c2.toString();
    }
}

Override the toString() method of Coords:

package model;

import static javax.xml.bind.annotation.XmlAccessType.NONE;

import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
@XmlAccessorType(NONE)
public class Coords {
    @XmlAttribute private int x;
    @XmlAttribute private int y;

    public Coords(final int x, final int y) {
        this.x = x;
        this.y = y;
    }

    public Coords() {
        this.x = 0;
        this.y = 0;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("Coords [x=");
        builder.append(x);
        builder.append(", y=");
        builder.append(y);
        builder.append("]");
        return builder.toString();
    }
}

This will produce a result similar to this:

<tuple2 first="Coords [x=2, y=4]" second="Coords [x=12, y=12]"/>

The values of the attributes first and second will be whatever the toString() method of Coords returns.