xml - ObservableList and StringProperty are not marshalled in xml file

Question

I want to write/read an XML-File via JAXB using Java 1.7. In my Files I'm using StringProperty, ObservableList and SimpleObjectProperty. I have an xml adapter for each of this classes. When i try to marshall the classes i only get an neary empty xml-file.

ModelDescriptorHelper (stores the project list and marshal and unmarshall the xml file)

@XmlRootElement
public class ModelDescriptorHelper {

private static ModelDescriptorHelper helper = new ModelDescriptorHelper();
private File file;
@XmlTransient private ObservableList<Project> projects =       FXCollections.observableArrayList();
private SimpleListProperty<Project> projectsProperty = new SimpleListProperty(this, "projects", projects);



public ObservableList<project> getProjects() {
    return projectsProperty.get();
}


public void setCreoViewProjects(ObservableList<CreoViewProject> creoViewProjects) {
    creoViewProjectsProperty.set(creoViewProjects);
}

public SimpleListProperty<CreoViewProject> creoViewProjectsProperty()
{
    return creoViewProjectsProperty;
}


private ModelDescriptorHelper readData() {
    try {
        File file = this.file;
        helper = JAXB.unmarshal(file, ModelDescriptorHelper.class);
        helper.file = file;

    } catch (Exception ex) {
    }
    return helper;
}

public void saveData() {
    try {
        if (!file.exists())
        {
            file.getParentFile().mkdirs();

        }
        else
        {
            file.delete();
            file.createNewFile();
        }
        try (BufferedOutputStream bf = new BufferedOutputStream(new FileOutputStream(file, false))) {
            JAXB.marshal(this, bf);
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

@XmlTransient
public File getFile() {
    return file;
}

public void setFile(File file) {
    this.file = file;
    readData();
}

public static ModelDescriptorHelper getInstance(File file) {
    helper.setFile(file);
    return helper;
}
}

Project Class:

public class Project {

private SimpleStringProperty wtContextName = new SimpleStringProperty();
private SimpleStringProperty projectName = new SimpleStringProperty();

public String getProjectName() {
    return projectName.get();
}

public void setProjectName(String projectName) {
    this.projectName.set(projectName);
}

public StringProperty projectNameProperty() {
    return projectName;
}

public String getWtContextName() {
    return wtContextName.get();
}

public void setWtContextName(String wtContextName) {
    this.wtContextName.set(wtContextName);
}

public StringProperty wtContextNameProperty() {
    return wtContextName;
}

}

StringPropertyAdapter:

public class SimpleStringPropertyAdatper extends XmlAdapter<String, SimpleStringProperty> {
@Override
public SimpleStringProperty unmarshal(String v) throws Exception {
    return new SimpleStringProperty(v);
}

@Override
public String marshal(SimpleStringProperty v) throws Exception {
    if(null == v) {
        return null;
    }
    return v.get(); // Or whatever the correct method is
} 

}

ObservableListAdatper:

public class ObservableListAdatper<T> extends XmlAdapter<LinkedList<T>, ObservableList<T>> {
@Override
public ObservableList<T> unmarshal(LinkedList<T> v) throws Exception {
    return FXCollections.observableList(v);
}

@Override
public LinkedList<T> marshal(ObservableList<T> v) throws Exception {
    LinkedList<T> list = new LinkedList();
    list.addAll(v);
    return list; // Or whatever the correct method is
} 

}

Package info:

@XmlJavaTypeAdapters({
@XmlJavaTypeAdapter(value=SimpleStringPropertyAdatper.class, type=String.class),
@XmlJavaTypeAdapter(value=ObservableListAdatper.class, type=ObservableList.class)})
package org.project;
import javafx.collections.ObservableList;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapters;

Create Project and save data to xml:

ModelDescriptorHelper helper = ModelDescriptorHelper.getInstance(new File ("/test/test.xml"));
    Project project = new Project();
    project.setProjectName("Test Project");
    helper.getProjects().add(project);
    helper.getProjects().add(project);
    helper.saveData();

The Xml file after marshall:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ModelDescriptorHelper>
     <projects/>
 </ModelDescriptorHelper>

When i debug it seems that the marshaller calls 2 times the ObservableListAdatper. 1 Time the list has 2 projects, second time the list is empty.

What do i wrong?