Java Practices -> Parse parameters into domain objects

Parse parameters into domain objects

All HTTP request parameter names and values are entirely textual. The problem always arises in a web application of how to translate or parse such data into more meaningful objects in the problem domain.

Such translation varies with the tool you are using. Here is an example taken from an application built with the WEB4J tool.


/** Build a Model Object from request parameters. */
public final class RestoAction extends ActionTemplateListAndEdit {

  public static final RequestParameter RESTO_ID = RequestParameter.withLengthCheck("Id");
  public static final RequestParameter NAME = RequestParameter.withLengthCheck("Name");
  public static final RequestParameter LOCATION = RequestParameter.withLengthCheck("Location");
  public static final RequestParameter PRICE = RequestParameter.withLengthCheck("Price");
  public static final RequestParameter COMMENT = RequestParameter.withLengthCheck("Comment");

  protected void validateUserInput() {
    try {
      ModelFromRequest builder = new ModelFromRequest(getRequestParser());
      fResto = builder.build(Resto.class, RESTO_ID, NAME, LOCATION, PRICE, COMMENT);
    }
    catch (ModelCtorException ex){
      //displays error message to end user
      addError(ex);
    }    
  }
  
  //..many items elided

  //target Model Object, built from user input
  private Resto fResto;
}

In this case, the parsing of raw HTTP request parameters is shared between these classes :

RequestParameter represents the raw underlying parameters accepted by the Action
RequestParser defines default policies for translating single request parameters into String, Date, Boolean, Integer, and BigDecimal objects, and Collections thereof.
ModelFromRequest translates sets of request parameters into Model Objects. While a RequestParser returns single "base" objects (Integer, Date, etc.), ModelFromRequest goes a step further, and returns an entire Model Object, which more or less encapsulates a number of "base" objects.

Validation and parsing of HTTP request parameters are related topics. See the Repel invalid requests topic for further information.