# servlet-filterlistener: How to Write Servlet Filters and Listeners

Jonathan Fuerth

> The `servlet-filterlistener` quickstart demonstrates how to use Servlet filters and listeners in an application.

## What is it?

The `servlet-filterlistener` quickstart demonstrates how to use Servlet filters and listeners in WildFly Application Server.

This example contains the following classes:

- `FilterExampleServlet`: A simple servlet that prints a form that contains an input field and a Send button.

- `VowelRemoverFilter`: A servlet filter that removes the vowels from the text entered in the form.

- `ParameterDumpingRequestListener`: A simple servlet request listener that creates a map of the original HTTP request parameter values before the filter is applied.

## System Requirements

The application this project produces is designed to be run on WildFly Application Server 13 or later.

All you need to build this project is Java 8.0 (Java SDK 1.8) or later and Maven 3.3.1 or later. See Configure Maven to Build and Deploy the Quickstarts to make sure you are configured correctly for testing the quickstarts.

## Use of WILDFLY_HOME

In the following instructions, replace `WILDFLY_HOME` with the actual path to your WildFly installation. The installation path is described in detail here: Use of WILDFLY_HOME and JBOSS_HOME Variables.

## Start the WildFly Standalone Server

1. Open a terminal and navigate to the root of the WildFly directory.

2. Start the WildFly server with the default profile by typing the following command.

       $ WILDFLY_HOME/bin/standalone.sh

   Note | For Windows, use the `WILDFLY_HOME\bin\standalone.bat` script.

## Build and Deploy the Quickstart

1. Make sure you start the WildFly server as described above.

2. Open a terminal and navigate to the root directory of this quickstart.

3. Type the following command to build the artifacts.

       $ mvn clean package wildfly:deploy

This deploys the `servlet-filterlistener/target/servlet-filterlistener.war` to the running instance of the server.

You should see a message in the server log indicating that the archive deployed successfully.

## Access the Application

The application will be running at the following URL http://localhost:8080/servlet-filterlistener/.

You are presented with a form containing an input field and a Send button. To test the quickstart:

1. Enter some text in the field, for example:

       This is only a test!

2. Click Send.

3. The servlet filter intercepts and removes the vowels from the text entered in the form and displays:

       You Typed: Ths s nly tst!

4. The server console displays the following log messages.

   The following messages appear in the log when you access the application URL. This is because the ParameterDumpingRequestListener, which is a ServletRequestListener, and the VowelRemoverFilter both map to the application context and print logs for every request. Note that the VowelRemoveFilter contains logic to handle empty input field arguments.

       INFO  [io.undertow.servlet] (default task-46) ParameterDumpingRequestListener: request has been initialized. It has 0 parameters:
       INFO  [io.undertow.servlet] (default task-46) VowelRemoverFilter initialized
       INFO  [io.undertow.servlet] (default task-46) VowelRemoverFilter invoking filter chain...
       INFO  [io.undertow.servlet] (default task-46) VowelRemoverFilter done filtering request
       INFO  [io.undertow.servlet] (default task-46) ParameterDumpingRequestListener: request has been destroyed
       INFO  [io.undertow.servlet] (default task-48) ParameterDumpingRequestListener: request has been initialized. It has 0 parameters:
       INFO  [io.undertow.servlet] (default task-48) VowelRemoverFilter invoking filter chain...
       INFO  [io.undertow.servlet] (default task-48) VowelRemoverFilter done filtering request
       INFO  [io.undertow.servlet] (default task-48) ParameterDumpingRequestListener: request has been destroyed

   The following messages appear in the log when you type "This is only a test!" in the input field and click `Send`.

       INFO  [io.undertow.servlet] (default task-50) ParameterDumpingRequestListener: request has been initialized. It has 1 parameters:
       INFO  [io.undertow.servlet] (default task-50)   userInput=This is only a test!
       INFO  [io.undertow.servlet] (default task-50) VowelRemoverFilter invoking filter chain...
       INFO  [io.undertow.servlet] (default task-50) VowelRemoverFilter done filtering request
       INFO  [io.undertow.servlet] (default task-50) ParameterDumpingRequestListener: request has been destroyed

## Undeploy the Quickstart

When you are finished testing the quickstart, follow these steps to undeploy the archive.

1. Make sure you start the WildFly server as described above.

2. Open a terminal and navigate to the root directory of this quickstart.

3. Type this command to undeploy the archive:

       $ mvn wildfly:undeploy

## Run the Quickstart in Red Hat JBoss Developer Studio or Eclipse

You can also start the server and deploy the quickstarts or run the Arquillian tests in Red Hat JBoss Developer Studio or from Eclipse using JBoss tools. For general information about how to import a quickstart, add a WildFly server, and build and deploy a quickstart, see Use JBoss Developer Studio or Eclipse to Run the Quickstarts.

## Debug the Application

If you want to debug the source code of any library in the project, run the following command to pull the source into your local repository. The IDE should then detect it.

    $ mvn dependency:sources