Save XML file with XStream

Question

I use XStream to write an object in a xml file.
Then I deserializing the file again to use the objects.
My Problem is, that after I close my program, the xml "file" is gone. So how can I save this xml file to a specific directory? I already tried FileOutputStream but it doesn't work... I also google it, but found not the right solution for me...

Method savePerson

public void savePerson(String uNummer, Person person) {
    System.out.println("save person");
    try{            
        xml = xstream.toXML(person);

    }catch (Exception e){
        System.err.println("Error in XML Write: " + e.getMessage());
    }
}

And the Method readPerson

public Person readPerson(String uNummer) {
    System.out.println("read person");
     Person person = new Person();
    try{
        person = (Person) xstream.fromXML(file_path + uNummer + ".xml");       
    }catch(Exception e){
        System.err.println("Error in XML Read: " + e.getMessage());
    }
    return person;
}

Directory: \\releasearea\ToolReleaseArea\PersistenceSave

EDIT
Correct Code: (by ppeterka)

public void savePerson(String uNummer, Person person) {
    System.out.println("save person XML");
    FileOutputStream fos = null;
    try{            
        xml = xstream.toXML(person);
        fos = new FileOutputStream(file_path + uNummer + ".xml");
        fos.write("<?xml version=\"1.0\"?>".getBytes("UTF-8"));
        byte[] bytes = xml.getBytes("UTF-8");
        fos.write(bytes);

    }catch (Exception e){
        System.err.println("Error in XML Write: " + e.getMessage());
    }
    finally{
        if(fos != null){
            try{
                fos.close();
            }catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Answer 1

You didn't write the file, just obtained the serialized content...

FileOutputStream fos = null;
try {
    fos = new FileOutputStream("myfilename");
    fos.write("<?xml version=\"1.0\"?>".getBytes("UTF-8")); //write XML header, as XStream doesn't do that for us
    byte[] bytes = xml.getBytes("UTF-8");
    fos.write(bytes);

} catch(Exception e) {
    e.printStackTrace(); // this obviously needs to be refined.
} finally {
    if(fos!=null) {
        try{ 
            fos.close();
        } catch (IOException e) {
            e.printStackTrace(); // this obviously needs to be refined.
        }
    }
}

Also, your reading function has an error too: the xstream.fromXML(String) accepts a String, but it does not interpret it as a file name, but as the XML content itself... You have to use the fromXML(File) function:

public Person readPerson(String uNummer) {
    System.out.println("read person");
    Person person = new Person(); //if there is an error during deserialization, this is going to be returned, is this what you want?
    try{
        File xmlFile = new File(file_path + uNummer + ".xml");
        person = (Person) xstream.fromXML(xmlFile);       
    }catch(Exception e){
        System.err.println("Error in XML Read: " + e.getMessage());
    }
    return person;
}