How Add element to XML file?

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XML

Java

I used JAXB in generate my class but I have problem when insert element in my XML file, so I insert first time element in my file XML, but when insert second time all file is clear and re-write XML. I have classes (project,layer) and I am write method for insert Project and layer to XML file. Please help me how I can insert project in XML then insert layer in XML with out clear xml file each time write XML.

This my class project :

@XmlAccessorType(XmlAccessType.FIELD)
@XmlRootElement(name = "Project")
public class Project {

    @XmlElement(name = "Layer")
    protected List<Layer> layer;
    @XmlAttribute(name = "name")
    protected String name; 

   public List<Layer> getLayer() {
        if (layer == null) {
            layer = new ArrayList<Layer>();
        }
        return this.layer;
    }
    public String getName() {
        return name;
    }
    public void setName(String value) {
        this.name = value;
    }
}

this my class Layer:

@XmlAccessorType(XmlAccessType.FIELD)
public class Layer {
    @XmlElement(name = "LayerName", required = true)
    protected String layerName;
    @XmlAttribute(name = "idLayer")
    protected int idLayer;
    public String getLayerName() {
        return layerName;
    }
    public void setLayerName(String value) {
        this.layerName = value;
    }
    public int getIdLayer() {
        return idLayer;
    }
    public void setIdLayer(int i) {
        this.idLayer = i;
    }
}

Method insert project:

 public boolean Insert(Project entity) {
    Project pr = new Project();
    pr.setName(entity.getName());
    try {
         JAXBContext jc = JAXBContext.newInstance(Project.class);
         javax.xml.bind.Marshaller marshaller = jc.createMarshaller();
         marshaller.setProperty(marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
         File XMLfile = new File("Projects//asdad//ProjectDataBase.xml");
         marshaller.marshal(pr, XMLfile);
    } catch (JAXBException e) {
        e.printStackTrace();
    }
    return false;
}

Method insert Layer:

 public boolean Insert(Layer entity) {
    Project pr = new Project();
    pr.setName(pr.getName());
    Layer addLayer = new Layer();
    addLayer.setIdLayer(entity.getIdLayer());
    addLayer.setLayerName(entity.getLayerName());
    pr.getLayer().add(addLayer);

    try {
         JAXBContext jc = JAXBContext.newInstance(Project.class);
         javax.xml.bind.Marshaller marshaller = jc.createMarshaller();
         marshaller.setProperty(marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
         File XMLfile = new File("Projects//asdad//ProjectDataBase.xml");
         marshaller.marshal(pr, XMLfile);
    } catch (JAXBException e) {
        e.printStackTrace();
    }
    return false;
}