Servlet extensions allow you to hook into the Servlet deployment process, and modify aspect of a Servlet deployment. In some ways they are similar to ServletContainerInitializer or ServletContextListener, however they provides much more flexibility over what can be modified.

In order to create a ServletExtension it is necessary to implement io.undertow.servlet.ServletExtension, and then add the name of your implementation class to META-INF/services/io.undertow.servlet.ServletExtension. When Undertow is deploying a Servlet deployment it will load all such services from the deployments class loader, and then invoke their handleDeployment method.

This method is passed an Undertow DeploymentInfo structure, which contains a complete and mutable description of the deployment, by modifying this structure it is possible to change any aspect of the deployment.

The DeploymentInfo structure is the same structure that is used by the embedded API, so in effect a ServletExtension has the same amount of flexibility that you have when using Undertow in embedded mode.

There are many possible use cases for this, a common one would be to add additional authentication mechanisms to a deployment, or to use native Undertow handlers as part of a Servlet deployment.