Cactus is an open-source tool used for testing software components and web applications. Its primary goal is to provide a simple, extendible framework for writing and running repeatable tests. Cactus also allows developers to easily test how their code interacts with other components, such as databases, web services, and JSPs. In order to write a Cactus test, you first need to create a class that extends org.apache.cactus.ServletTestCase. This class provides the methods needed to run your tests. You will typically configure your test environment and initialize any objects that are needed by your test case & clean up after your test case has been run. Once you have created your ServletTestCase subclass, you can begin writing individual test methods. These methods must be annotated with @TestMethod so that Cactus knows they are meant to be run as part of a test suite. Within each test method, you can invoke any of the protected methods provided by ServletTestCase. These methods take care of setting up and tearing down the HTTP session for each individual test. Cactus provides a number of other features that can be used to further customize your tests. For example, you can use the @BeforeMethod and @AfterMethod annotations to specify methods that should be run before and after each individual test method. You can also pass configuration information to your tests via the cactus.contextURL system property. Overall, Cactus is a powerful tool for testing web applications. Its simple yet extensible framework makes it easy to write repeatable tests, and its support for sessions and configurable settings ensures that your tests will accurately reflect your application's behavior.