Values assigned to variables are cleaned

Good morning, I need your support. What happens is that I have an application that uses connections to web service. Everything works very well, but from time to time something strange happens to me: the values that I have assigned to main variables in a class (class that I use constantly) are cleaned. They take an empty value. the memory has free space, there are no more open applications, or processes in the background. I chose to use the ending, but the same thing happens.

Something I may be ignoring? I put the class where I declared the variables.

public class Usuario {

//Campos
public static String imei;
public static String companiaSesion;
public static String usuarioSesion;
public static String nombreSesion;
public static String numeroPedido;
public static String listcode;


/////////////////////
public static String plant;
public static String almacen;
public static String contado;
public static String credito;
public static String length;
public static String latitude;


public static String getImei() {
    return imei;
}

public static void setImei(String imei) {
    Usuario.imei = imei;
}

public static String getNumeroPedido() {
    return numeroPedido;
}

public static void setNumeroPedido(String numeroPedido) {
    Usuario.numeroPedido = numeroPedido;
}

public static String getCompaniaSesion() {
    return companiaSesion;
}

public static void setCompaniaSesion(String companiaSesion) {
    Usuario.companiaSesion = companiaSesion;
}

public static String getUsuarioSesion() {
    return usuarioSesion;
}

public static void setUsuarioSesion(String usuarioSesion) {
    Usuario.usuarioSesion = usuarioSesion;
}

public static String getListcode() {
    return listcode;
}

public static void setListcode(String listcode) {
    Usuario.listcode = listcode;
}

public static String getNombreSesion() {
    return nombreSesion;
}

public static void setNombreSesion(String nombreSesion) {
    Usuario.nombreSesion = nombreSesion;
}

public static String getPlant() {
    return plant;
}

public static void setPlant(String plant) {
    Usuario.plant = plant;
}

public static String getAlmacen() {
    return almacen;
}

public static void setAlmacen(String almacen) {
    Usuario.almacen = almacen;
}

public static String getContado() {
    return contado;
}

public static void setContado(String contado) {
    Usuario.contado = contado;
}

public static String getCredito() {
    return credito;
}

public static void setCredito(String credito) {
    Usuario.credito = credito;
}

public static String getLength() {
    return length;
}

public static void setLength(String length) {
    Usuario.length = length;
}

public static String getLatitude() {
    return latitude;
}

public static void setLatitude(String latitude) {
    Usuario.latitude = latitude;
}
}