GWT-VL (Validation library and framework for GWT)


From an architectural view we can divide the library into two parts: client side and server side validation.

Client side validation architecture

On the client side we foremost have the usability aspect of validation. The users who will mostly be not malicious (hopefully) need a way of knowing that they entered some data that the application can tell is not correct. The requirements of the application might also require, that the validations should be shown in a "nice" and/or contextual way to the end user. So here is where probably the most work is needed from the developers point of view (even though its useless from a security point of view).
In order to support this kind of usage the client side part of the library has the following functionality:

  • Validate input fields
  • Show "nice" error messages
  • Show validation errors coming from the server side

There are three main objects that you will use while doing validation with the GWT Validation Library.

  • Validators
  • The validators are used to implement a validation constraint. These validators can be instantiated with widgets which act as an input for the validators validation algorithm. For example you can use an IntegerValidator and add a TextBox instance to it. It will then validate that the value entered into the TextBox instance is an integer value. The only thing that validators do is to inform your application that one or more values failed to meet the required validation constraints (what you do with this information is completely up to you).

  • Actions
  • The actions define a specific functionality that should be carried out when a specific validator fails. An action or even multiple actions can be added to a validator, so you have the freedom of reacting differently for different validation constraints or widgets, additionally you are able to build your actions as you require them once and reuse them everywhere. An example action would be the StyleAction, which will add and, optionally, remove a CSS-style from the validated widget if the validation failed (E.g. adding a style that adds a red border to the widget which failed input validation).

  • Transformers
  • Transformers also fall into the category "usability". They might be used to assist the user in the data entry process. For example there is an UpperCaseTransformer. It is used to transform the data entered into some TextBox into upper case. So the user is assisted by this functionality in his data entry process.

While the library already comes with some validators, actions and transformers it is easy for you to create your own. So you have the freedom of implementing the validation cases you really need and are not provided for by the library (this is what makes this library also a validation framework).

One core element of the actual validation process was left out so far. The ValidationProcessor. It is responsible for invoking all the necessary validations and invoking the actions for all the failed validations. It is also responsible for handling errors that occured on the server side of the application (more on that later). For each window, dialog, etc. there is usually one instance that you invoke to validate all the fields. The result will tell you whether client side validation failed or succeeded. If it failed all the actions added to validators that failed will be invoked automatically. If it succeeded you propably want to communicate the entered data to the server (You don't have to, the library doesn't bother with how you communicate your data to the server).

Look at the schematic drawing below. It shows you how the components (your input fields and GWT-VL) interact with each other on the client side.

Server side validation architecture

The server side validation does not care about things like usability or how the validation errors are presented to the user. Its purpose is to ensure syntactically and semantically correct user input. This can only be achieved at the server side, as we can never trust the data that gets sent from the client side. That means that on the server side we need to be aware that possibly every single data input may be wrong or even malicious, so we need to check each and every input. Furthermore some validations can only be carried out on the server side. As a result the server side part of the library has the following functionality:

  • Validate user input
  • Hand over possible validation errors to the client

As you can see the server side has nothing to do with how or even if that information is presented to the end user.

As a result the server side is not quite so complex as the client side part of the library. The main part you will use will be the ServerValidation class. It provides methods such as ServerValidation.notNull etc. that will help you to check correct data on the server side and already gives you the functionality for talking back validation errors to the client which will then be visually rich (as you defined them) shown to the end user. The good thing is, that you are not limited to the ServerValidation class validation functionality. You can easily do your own possibly more advanced checks (like checking if some property value already exists in the database, eg. a username). The ServerValidation class gives you an really easy way to talk even these back to the client with (ServerValidation.exception()!) and you still profit from the automatic action invocation on the client side.

If you are using Hibernate-Validator you can even use some of the hibernate validation constraints. If they fail it will also be talked back to the client automatically without you needing to plugin some custom code everywhere to get the profit from the hibernate validations. Actually you can use all of the Hibernate-Validator annotations. Although there are some issues with some of the validation constraints that prevent the GWT Validation Library from knowing which of the properties caused the failure. Right now there is nothing that the library can do, the problem occurs only with annotations that need to query the database in order to know if the validation failed or not (e.g. unique=true on a column). So you should take care of these cases yourself (which is not so complicated after all). Annotations like @NotNull or @Length are automatically taken care of by GWT-VL. In the future their might be additions to that behaviour, so stay tuned.

For connecting the client side with the server side validation there exist two possible ways.

  • If you can use RPC, than all the methods of the services that do validation and want to talk the validation information back to the client, are required declaring to throw ValidationException's. After an failed async call this exception can be passed to the ValidationProcessor who will parse it and invoke the failed validation actions according to the exception.

  • Generic plain text
  • If you are not using GWT-RPC there is the possibility to use a functionality provided by GWT-VL that can translate a ValidationException into a simple String representation. This can then be send to the client and the ValidationProcessor is able to reconstruct the ValidationException from this String.

To the user it should be fairly transparent whether the validation error was rooted on a client side validation or on a server side validation. The only "feelable" difference might be a short delay in response time depending on how fast the client-server communication is.

The schematic drawing below shows you how the server side part of the library is architectured: