Unit testing is crucial to Java code quality. Using JUnit 5 as the mainstream framework, we introduce dependencies through Maven or Gradle and write test cases; tests should cover normal processes, boundary values, error input and exception handling; use Mockito to simulate external dependencies to avoid real calls; follow clear naming specifications and organizational structures, such as "Method Name_Scenario_Expected Behavior", and place the test class under src/test/java to maintain the package structure consistent with the class being tested; insist on writing tests simultaneously in development to improve efficiency.
Unit testing is an important part of ensuring the quality of Java code. Writing a good unit test will not only help you discover logical errors, but also provide security guarantees during subsequent refactoring. Here are some practical suggestions for most Java projects.
Using JUnit is the mainstream choice
JUnit is the most commonly used unit testing framework in the Java field. Currently the mainstream version is JUnit 5, which has clear structure and rich features. You can introduce dependencies through Maven or Gradle, and then create specialized test classes to write test cases.
For example, a simple test method might look like this:
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
class CalculatorTest {
@Test
void add_returnsSumOfTwoNumbers() {
Calculator calculator = new Calculator();
assertEquals(5, calculateator.add(2, 3));
}
} In this example, the @Test annotation mark test method is used, and assertEquals is used to verify whether the results meet expectations.
Tests should cover core logic and boundary situations
Write tests not to make up coverage, but to grasp the critical path. For example, a method of processing strings not only needs to measure normal input, but also needs to consider boundary situations such as empty strings, null, and special characters.
Common practices include:
- Normal process (such as entering legal parameters)
- Boundary value (such as the first/last element of the array)
- Incorrect input (such as null, illegal numbers)
- Exception handling (exception expected to be thrown)
For example: If you have a method to parse an entire string of numbers, you need at least three test cases:
- Normal string ("123" → 123)
- Empty string ("" → throw an exception or return the default value)
- Non-numeric string ("abc" → exception throwing)
Use Mockito to simulate external dependencies
In a real environment, many classes rely on databases, network services or other complex components. At this time, you need to use Mock tools, and Mockito is a very popular simulation framework.
For example, if you want to test a service class that calls a remote API, but don't want to really initiate a network request every time you run a test, you can do this:
@Test
void fetchUser_returnsUserName() {
UserRepository mockRepo = Mockito.mock(UserRepository.class);
Mockito.when(mockRepo.findById(1)).thenReturn(new User("Alice"));
UserService service = new UserService(mockRepo);
String result = service.getUserName(1);
assertEquals("Alice", result);
}This way, even if there is no real database connection, it can verify whether the business logic is correct.
Naming specifications and organizational structures must be clear
A good naming allows others to see at a glance what you are testing. The recommended format is "Method Name_Scenario_Expected Behavior", such as:
-
add_negativeNumbers_throwsException -
parse_validInput_returnsCorrectValue
The test class is generally placed under src/test/java , and the package structure is the same as the tested class. This way the IDE and build tools can automatically identify and run tests.
Basically that's it. It is not difficult to write unit tests well. The key is to insist on writing code while writing tests during the development process. After forming habits, the efficiency will be higher.
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