Parsing xml with jaxb return body always null

Question

Hi I'm trying parse xml with jaxb but its body always return null. I tried to create object of that class and convert it to xml that is same fine result but when try to parse it that is not working.

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <soap:Body>
        <St_PurchaseResponse xmlns="http://tempuri.org/">
            <St_PurchaseResult>
                <string>RESULT STRING</string>
            </St_PurchaseResult>
        </St_PurchaseResponse>
    </soap:Body>
</soap:Envelope>

with following code:

 val responseObj: PurchaseResponseEnvelope? =
            JAXBContext.newInstance(PurchaseResponseEnvelope::class.java).createUnmarshaller()
                .unmarshal(StreamSource(StringReader(response)), PurchaseResponseEnvelope::class.java).value


@XmlRootElement(name = "soap:Envelope")
@XmlAccessorType(XmlAccessType.FIELD)
open class ApiEnvelope {
    @XmlAttribute(name = "xmlns:soap")
    val soapAttr = "http://schemas.xmlsoap.org/soap/envelope/"

    @XmlAttribute(name = "xmlns:xsi")
    val xsiAttr = "http://www.w3.org/2001/XMLSchema-instance"

    @XmlAttribute(name = "xmlns:xsd")
    val xsdAttr = "http://www.w3.org/2001/XMLSchema"

    constructor()
}


@XmlRootElement(name = "soap:Envelope")
@XmlAccessorType(XmlAccessType.FIELD)
class PurchaseResponseEnvelope : ApiEnvelope {
    @XmlElement(name = "soap:Body")
    var body: PurchaseResponseBody? = null

    constructor(body: PurchaseResponseBody) {
        this.body = body
    }

    constructor() : super()
}

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class PurchaseResponseBody {
    @XmlElement(name = "St_PurchaseResponse")
    var body: PurchaseResponseBodyWrap? = null
}


@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class PurchaseResponseBodyWrap {
    @XmlAttribute(name = "xmlns")
    val namespace = "http://tempuri.org/"

    @XmlElement(name = "St_PurchaseResult")
    var result: PurchaseResponseResult? = null
}


@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class PurchaseResponseResult {
    @XmlElement(name = "string")
    var result: String? = null
}

But this is not working body always null i tried to marshall object that is fine same xml result but when unmarshall always null body