Dependency injection (DI) is a design pattern that is commonly used in Java to improve the maintainability and testability of software. By using a DI container, you can easily manage the dependencies between objects in your application, and make it easier to write unit tests.

In Java, there are several popular DI containers, such as Spring and Guice. These containers allow you to define the dependencies between objects in a central configuration file or class, and then inject these dependencies into objects at runtime.

Here are the basic steps involved in testing with a DI container:

1. Define the dependencies: The first step is to define the dependencies between the objects in your application. This is typically done in a central configuration file or class, using the DI container's syntax.

2. Create a test configuration: Once you have defined the dependencies, you can create a separate configuration for testing. This configuration should define mock or test objects for any dependencies that would be difficult or impossible to test in a real environment.

3. Inject the dependencies: During testing, the DI container will use the test configuration to inject the appropriate dependencies into the objects being tested. This makes it easy to test the behavior of individual objects in isolation, without having to worry about complex dependencies.

4. Test the objects: With the dependencies injected, you can write tests to ensure that the objects in your application behave as expected. Because the dependencies are managed by the DI container, you can focus on testing the behavior of individual objects, rather than worrying about complex interactions between objects.

Testing with a DI container can help make your code more modular, maintainable, and testable. By managing the dependencies between objects in a central configuration, you can easily swap out dependencies during testing, and focus on testing individual objects in isolation. This can make it easier to write robust, high-quality tests for your Java applications.