code: public static Bson geoWithinCenterSphere(final String fieldName, final double x, final double y, final double radius) {
          BsonDocument centerSphere = new BsonDocument("$centerSphere",
                                                              new BsonArray(Arrays.<BsonValue>asList(new BsonArray(asList(new BsonDouble(x),
                                                                      new BsonDouble(y))),
                                                                      new BsonDouble(radius))));
          return new OperatorFilter<BsonDocument>("$geoWithin", fieldName, centerSphere);
      }
docs: Creates a filter that matches all documents containing a field with geospatial data (GeoJSON or legacy coordinate pairs) that exist
  entirely within the specified circle, using spherical geometry.  If using longitude and latitude, specify longitude first.
  
  @param fieldName the field name
  @param x         the x coordinate of the circle
  @param y         the y coordinate of the circle
  @param radius    the radius of the circle, in radians
  @return the filter
  @mongodb.driver.manual reference/operator/query/geoWithin/ $geoWithin
  @mongodb.driver.manual reference/operator/query/centerSphere/#op._S_centerSphere $centerSphere
  @mongodb.server.release 2.4
  @since 3.1