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public class loginsalesforce {
    private static final String NS_SOAP = 'http://schemas.xmlsoap.org/soap/envelope/';
    private static final String NS_SF = 'urn:partner.soap.sforce.com';

    public static String login() {
        string user = 'email';
        string password = '***';

        HttpRequest req = new HttpRequest();
        req.setMethod('POST');
        req.setTimeout(60000);
        req.setEndpoint('https://login.salesforce.com/services/Soap/u/29.0');
        req.setHeader('Content-Type', 'text/xml;charset=UTF-8');
        req.setHeader('SOAPAction', '""');
        req.setBody('<Envelope xmlns="http://schemas.xmlsoap.org/soap/envelope/"><Header/><Body><login xmlns="urn:partner.soap.sforce.com"><username>' + user + '</username><password>' + password + '</password></login></Body></Envelope>');
        HttpResponse res =  new Http().send(req);
        system.debug('response' + res);
        system.debug('response' + res.getBody());
        if (res.getStatusCode() != 200) {
            Dom.Document responseDocument = res.getBodyDocument();
            Dom.Xmlnode rootElm = responseDocument.getRootElement(); // soapenv:Envelope
            Dom.Xmlnode bodyElm = rootElm.getChildElement('Body', NS_SOAP); // soapenv:Body
            Dom.Xmlnode faultElm = bodyElm.getChildElement('Fault', NS_SOAP); // soapenv:Fault
            Dom.Xmlnode faultStringElm = faultElm.getChildElement('faultstring', null); // faultstring
            throw new LoginException(faultStringElm.getText());
        }
        // As per http://wiki.developerforce.com/page/Enterprise_Login
        Dom.Document responseDocument = res.getBodyDocument();
        Dom.Xmlnode rootElm = responseDocument.getRootElement(); // soapenv:Envelope
        Dom.Xmlnode bodyElm = rootElm.getChildElement('Body', NS_SOAP); // soapenv:Body
        Dom.Xmlnode loginResponseElm = bodyElm.getChildElement('loginResponse', NS_SF); // loginResponse
        Dom.Xmlnode resultElm = loginResponseElm.getChildElement('result', NS_SF); // result
        Dom.Xmlnode sessionIdElm = resultElm.getChildElement('sessionId', NS_SF); // sessionId
        return sessionIdElm.getText();
    }
    public class LoginException extends Exception {}
}