I have to create following xml string,
<TRX>
    <SVC>CheckoutStartSession</SVC>
    <PRJ>PRJ</PRJ>
    <CTY>ABC</CTY>
    <COM>Internet</COM>
    <CUR>DEF</CUR>
    <ORD>1234</ORD>
    <NET>1.0</NET>
    <TAX>0.0</TAX>
    <GRS>1.0</GRS>
    <SES>
        <SUU>SCHEME://IP:PORT/CONTEXT_PATH/SUU</SUU>
        <FAU>SCHEME://IP:PORT/CONTEXT_PATH/FAU</FAU>
        <STY>SCHEME://IP:PORT/CONTEXT_PATH/STY</STY>
        <CVI>Required</CVI>
        <ACT>AUTHORIZE</ACT>
        <LAN>en-US</LAN>
    </SES>
    <CUS>
        <CTP>PRIVATE</CTP>
        <FNM>PRATEEK</FNM>
    </CUS>
</TRX>
XML-String creation using JAXB
public class PGReqConfig {
    public static void main(String[] args) throws JAXBException {
        RequestSession sessionConfig = new RequestSession();
        sessionConfig.setSuccessNotifURL("SCHEME://IP:PORT/CONTEXT_PATH/SUU");
        sessionConfig.setFailureNotifURL("SCHEME://IP:PORT/CONTEXT_PATH/FAU");
        sessionConfig.setStyleSheetURL("SCHEME://IP:PORT/CONTEXT_PATH/STY");
        sessionConfig.setAction("AUTHORIZE");
        sessionConfig.setCvcInput("Required");
        sessionConfig.setLaguage("en-US");
        CustomerDetails customerData = new CustomerDetails();
        customerData.setCustomerType("PRIVATE");
        customerData.setFirstName("PRATEEK");
        TransactionRequest transactionConfig = new TransactionRequest();
        transactionConfig.setService("CheckoutStartSession");
        transactionConfig.setConsentMedium("Internet");
        transactionConfig.setProjectCode("PRJ");
        transactionConfig.setCountryCode("ABC");
        transactionConfig.setCurrencyCode("DEF");
        transactionConfig.setNet("1.0");
        transactionConfig.setTax("0.0");
        transactionConfig.setGross("1.0");
        transactionConfig.setOrderId("1234");
        transactionConfig.setCustomerData(customerData);
        transactionConfig.setPaymentSession(sessionConfig);
        StringWriter xmlData = new StringWriter();
        JAXBContext context = JAXBContext.newInstance(TransactionRequest.class);
        Marshaller marshal = context.createMarshaller();
        marshal.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshal.setProperty(Marshaller.JAXB_FRAGMENT, true);
        marshal.marshal(transactionConfig, xmlData);
        System.out.println(xmlData.toString());
    }
}
@XmlRootElement(name = "TRX")
@XmlAccessorType(XmlAccessType.FIELD)
class TransactionRequest{
    @XmlElement(name = "SVC")
    private String service;
    @XmlElement(name = "PRJ")
    private String projectCode;
    @XmlElement(name = "CTY")
    private String countryCode;
    @XmlElement(name = "COM")
    private String consentMedium;
    @XmlElement(name = "CUR")
    private String currencyCode;
    @XmlElement(name = "ORD")
    private String orderId;
    @XmlElement(name = "NET")
    private String netAmount;
    @XmlElement(name = "TAX")
    private String taxAmount;
    @XmlElement(name = "GRS")
    private String grossAmount;
    @XmlElement(name = "SES")
    private RequestSession paymentSession;
    @XmlElement(name = "CUS")
    private CustomerDetails customerData;
    ......
}
@XmlAccessorType(XmlAccessType.FIELD)
class RequestSession{
    @XmlElement(name = "SUU")
    private String successNotifURL;
    @XmlElement(name = "FAU")
    private String failureNotifURL;
    @XmlElement(name = "STY")
    private String styleSheetURL;
    @XmlElement(name = "CVI")
    private String cvcInput; 
    @XmlElement(name = "ACT")
    private String action; 
    @XmlElement(name = "LAN")
    private String laguage;
    ......
}
@XmlAccessorType(XmlAccessType.FIELD)
class CustomerDetails{
    @XmlElement(name = "CTP")
    private String customerType; 
    @XmlElement(name = "FNM")
    private String firstName;
    ......
}
XML-String creation using StringBuffer
StringBuffer trxTag = new StringBuffer();
trxTag.append("<TRX>" + "<SVC>CheckoutStartSession</SVC>");
trxTag.append("<PRJ>PRJ</PRJ>");
trxTag.append("<CTY>ABC</CTY>");
trxTag.append("<COM>Internet</COM>");
trxTag.append("<CUR>DEF</CUR>");
StringBuffer cusTag = new StringBuffer("<CUS>");
cusTag.append("<CTP>Private</CTP>")
        .append("<FNM>PRATEEK</FNM>")
        .append("</CUS>");
StringBuffer sesTag = new StringBuffer("<SES>");
sesTag.append("<SUU>SCHEME://IP:PORT/CONTEXT_PATH/SUU</SUU>")
        .append("<FAU>SCHEME://IP:PORT/CONTEXT_PATH/FAU</FAU>")
        .append("<STY>SCHEME://IP:PORT/CONTEXT_PATH/STY</STY>")
        .append("<CVI>Required</CVI>")
        .append("<LAN>en-US</LAN>")
        .append("<ACT>Authorize</ACT>" + "</SES>");
trxTag.append("<ORD>1234</ORD>")
        .append("<NET>1.0</NET>")
        .append("<TAX>0.0</TAX>")
        .append("<GRS>1.0</GRS>")
        .append(cusTag)
        .append(sesTag)
        .append("</TRX>");
System.out.println(trxTag.toString());
 
     
    