Unit Testing

Dependency injection should make your code less dependent on the container than it would be with traditional J2EE / Java EE development. The POJOs that make up your application should be testable in JUnit or TestNG tests, with objects instantiated by using the new operator, without Spring or any other container. You can use mock objects (in conjunction with other valuable testing techniques) to test your code in isolation. If you follow the architecture recommendations for Spring, the resulting clean layering and componentization of your codebase facilitate easier unit testing. For example, you can test service layer objects by stubbing or mocking DAO or repository interfaces, without needing to access persistent data while running unit tests.

True unit tests typically run extremely quickly, as there is no runtime infrastructure to set up. Emphasizing true unit tests as part of your development methodology can boost your productivity. You may not need this section of the testing chapter to help you write effective unit tests for your IoC-based applications. For certain unit testing scenarios, however, the Spring Framework provides mock objects and testing support classes, which are described in this chapter.

Mock Objects

Spring includes a number of packages dedicated to mocking:

Environment

The org.springframework.mock.env package contains mock implementations of the Environment and PropertySource abstractions (see Bean Definition Profiles and PropertySource Abstraction). MockEnvironment and MockPropertySource are useful for developing out-of-container tests for code that depends on environment-specific properties.

JNDI

The org.springframework.mock.jndi package contains a partial implementation of the JNDI SPI, which you can use to set up a simple JNDI environment for test suites or stand-alone applications. If, for example, JDBC DataSource instances get bound to the same JNDI names in test code as they do in a Jakarta EE container, you can reuse both application code and configuration in testing scenarios without modification.

The mock JNDI support in the org.springframework.mock.jndi package is officially deprecated as of Spring Framework 5.2 in favor of complete solutions from third parties such as Simple-JNDI.

Servlet API

The org.springframework.mock.web package contains a comprehensive set of Servlet API mock objects that are useful for testing web contexts, controllers, and filters. These mock objects are targeted at usage with Spring’s Web MVC framework and are generally more convenient to use than dynamic mock objects (such as EasyMock) or alternative Servlet API mock objects (such as MockObjects).

Since Spring Framework 6.0, the mock objects in org.springframework.mock.web are based on the Servlet 6.0 API.

The Spring MVC Test framework builds on the mock Servlet API objects to provide an integration testing framework for Spring MVC. See MockMvc.

Spring Web Reactive

The org.springframework.mock.http.server.reactive package contains mock implementations of ServerHttpRequest and ServerHttpResponse for use in WebFlux applications. The org.springframework.mock.web.server package contains a mock ServerWebExchange that depends on those mock request and response objects.

Both MockServerHttpRequest and MockServerHttpResponse extend from the same abstract base classes as server-specific implementations and share behavior with them. For example, a mock request is immutable once created, but you can use the mutate() method from ServerHttpRequest to create a modified instance.

In order for the mock response to properly implement the write contract and return a write completion handle (that is, Mono<Void>), it by default uses a Flux with cache().then(), which buffers the data and makes it available for assertions in tests. Applications can set a custom write function (for example, to test an infinite stream).

The WebTestClient builds on the mock request and response to provide support for testing WebFlux applications without an HTTP server. The client can also be used for end-to-end tests with a running server.

Unit Testing Support Classes

Spring includes a number of classes that can help with unit testing. They fall into two categories:

General Testing Utilities

The org.springframework.test.util package contains several general purpose utilities for use in unit and integration testing.

AopTestUtils is a collection of AOP-related utility methods. You can use these methods to obtain a reference to the underlying target object hidden behind one or more Spring proxies. For example, if you have configured a bean as a dynamic mock by using a library such as EasyMock or Mockito, and the mock is wrapped in a Spring proxy, you may need direct access to the underlying mock to configure expectations on it and perform verifications. For Spring’s core AOP utilities, see AopUtils and AopProxyUtils.

ReflectionTestUtils is a collection of reflection-based utility methods. You can use these methods in testing scenarios where you need to change the value of a constant, set a non-public field, invoke a non-public setter method, or invoke a non-public configuration or lifecycle callback method when testing application code for use cases such as the following:

  • ORM frameworks (such as JPA and Hibernate) that condone private or protected field access as opposed to public setter methods for properties in a domain entity.

  • Spring’s support for annotations (such as @Autowired, @Inject, and @Resource), that provide dependency injection for private or protected fields, setter methods, and configuration methods.

  • Use of annotations such as @PostConstruct and @PreDestroy for lifecycle callback methods.

TestSocketUtils is a simple utility for finding available TCP ports on localhost for use in integration testing scenarios.

TestSocketUtils can be used in integration tests which start an external server on an available random port. However, these utilities make no guarantee about the subsequent availability of a given port and are therefore unreliable. Instead of using TestSocketUtils to find an available local port for a server, it is recommended that you rely on a server’s ability to start on a random ephemeral port that it selects or is assigned by the operating system. To interact with that server, you should query the server for the port it is currently using.

Spring MVC Testing Utilities

The org.springframework.test.web package contains ModelAndViewAssert, which you can use in combination with JUnit, TestNG, or any other testing framework for unit tests that deal with Spring MVC ModelAndView objects.

Unit testing Spring MVC Controllers
To unit test your Spring MVC Controller classes as POJOs, use ModelAndViewAssert combined with MockHttpServletRequest, MockHttpSession, and so on from Spring’s Servlet API mocks. For thorough integration testing of your Spring MVC and REST Controller classes in conjunction with your WebApplicationContext configuration for Spring MVC, use the Spring MVC Test Framework instead.