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Karate Mock Servlet

Test any Java Servlet without a Container

That’s right, you can test Spring MVC or Spring Boot controllers and even Jersey JAX-RS resource end-points without having to start Tomcat, JBoss, WebSphere, Glassfish or the like.

And you can re-use your traditional HTTP integration tests without changes - just switch your environment, and Karate can run tests and bypass HTTP on the wire.

This can be a huge time-saver as you don’t have to spend time waiting for your app-server to start and stop. You also don’t need to worry about having free ports on your local machine, there’s no more fiddling with HTTPS and certificates, and if you do things right - you can achieve TDD and code-coverage for all your application layers, starting from the web-controllers.

So yes, you can test HTTP web-services with the same ease that you expect from traditional unit-tests. Especially for micro-services - when you combine this approach with Karate’s data-driven and data-matching capabilities, you can lean towards having more integration tests without losing any of the benefits of unit-tests.




Switching the HTTP Client

Karate actually allows you to switch the implementation of the Karate HttpClient even during a test. For mocking a servlet container, you don’t need to implement it from scratch and you just need to over-ride one or two methods of the mock-implementation that Karate provides.

If you need to create a completely new HttpClient implementation from scratch, the MockHttpClient is a good reference. There are many possibilities here, you can add support for other HTTP clients besides Apache and Jersey, or mock a stack that is not based on Java servlets.

Let’s take a closer look at the following configure keys:

Key Type Description
httpClientClass string The class name of the implementation you wish to use. By default the Karate Apache (or Jersey) HTTP client is used. You need a zero-argument constructor. If you need more control over construction, refer to the next row.
httpClientInstance Java Object A fully constructed instance of an HttpClient implementation. Useful if you need more dynamic control over things like dependency-injection.
userDefined JSON You normally would not need this general-purpose extension mechanism where you can pass custom JSON data to the HTTP Client instance via the configure keyword. Refer to this test for an idea of how you can use this for advanced needs, such as if you wanted to customize your HTTP Client implementation during a test.

Mocking Your Servlet

You only need to over-ride two methods:

  • Servlet getServlet(HttpRequestBuilder request)
  • ServletContext getServletContext()

Once you refer to the following examples, you should be able to get up and running for your project.

Note that the first example above tests the whole of the karate-demo Spring Boot application, and using a parallel runner


Everything is typically tied together in bootstrap configuration. If you do this right, your *.feature files can be re-used for mock as well as real integration tests.

Use the test configuration for this karate-mock-servlet project as a reference: karate-config.js


Most teams would not run into these, but if you do, please consider contributing !

  • File Upload is not supported.
  • Other similar edge-cases (such as redirects) are not supported.

Teams typically use the mock-servlet for simple JSON / XML request-response use cases. If you find some file-upload or negative-test scenarios failing because of the above limitations, you can choose to tag those tests to run only in your end-to-end integration test environment.