Confused as how to use JAXB XML Adapter for my requirement

I am Using JAXB for unmarshalling process , for which the request comes from the UI to our service class . The below is the format of XML request .

<SampleRequest  user="testUser"  account="testAccount"    Specifier=  "value1a,value1b,value1c : name2a,value2b,value2c"/>

My requirement is that , the Specifier attribute has got Multiple series of values (: colon separated) i need to map each series of values to my custom java class

I tried this way

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class SampleRequest {

    @XmlAttribute
    private String user;

    @XmlAttribute
    private String account;


    @XmlAttribute(name="Specifier")
    private List<Specifier> specifier;


}

Specifier.java

@XmlJavaTypeAdapter(SpecifierAdapter.class)
public class Specifier {



}

SpecifierAdapter.java

public class SpecifierAdapter  extends XmlAdapter{

    @Override
    public Object marshal(Object arg0) throws Exception {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public Object unmarshal(Object arg0) throws Exception {
        // TODO Auto-generated method stub
        return null;
    }

}

Edited part

The class Specifier has got 3 String properties .

class Specifier
{
String value1;
String value2;
String value3;
}

And i need each series of Specifier for example (value1a,value1b,value1c) should be mapped to value1 , value2 , value3 respectively

Edited Part 3

Hi , Thanks for the response , i tried to unmarshall this example , what i found is that , i am getting null

This is the request i passed

<sampleRequest user="user" account="account" Specifier="v1,v2,v3 : a1,a2,a3"/>

Just want to make sure that , is my Specifier class is correct or not ?? (As i did not use any Annotations here )

package com;

import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlJavaTypeAdapter(SpecifierAdapter.class)
public class Specifier {


    Specifier(String v1 , String v2 , String v3)
    {

    }

    String value1;

    public String getValue1() {
        return value1;
    }

    public void setValue1(String value1) {
        this.value1 = value1;
    }

    public String getValue2() {
        return value2;
    }

    public void setValue2(String value2) {
        this.value2 = value2;
    }

    public String getValue3() {
        return value3;
    }

    public void setValue3(String value3) {
        this.value3 = value3;
    }

    String value2;
    String value3;

}