@NullSource and @EmptySource in JUnit - Framework Patterns

Framework Mode - @NullSource and @EmptySource

Folder Structure

project-root/
├── src/
│   ├── main/
│   │   └── java/
│   │       └── com/example/app/
│   │           └── Service.java
│   └── test/
│       └── java/
│           └── com/example/app/
│               ├── ServiceTest.java  <-- Tests using @NullSource and @EmptySource
│               └── utils/
│                   └── TestUtils.java
├── build.gradle or pom.xml
└── junit-platform.properties

Test Framework Layers

• Test Classes: Contain test methods annotated with @ParameterizedTest, using @NullSource and @EmptySource to provide null and empty inputs.
• Test Utilities: Helper methods for common assertions or test data setup.
• Application Code: The main code under test, e.g., Service.java.
• Configuration: JUnit platform properties and build tool configs to run tests.

Configuration Patterns

• JUnit Platform Properties: Configure test discovery and execution in junit-platform.properties if needed.
• Build Tool: Use Gradle or Maven to include JUnit Jupiter dependencies for parameterized tests.
• Environment: No special environment setup needed for @NullSource and @EmptySource as they are part of JUnit Jupiter.
• Test Data: Use annotations directly on test methods to supply null and empty values without external files.

Test Reporting and CI/CD Integration

• JUnit Reports: Standard JUnit XML reports generated by Gradle or Maven for CI tools.
• CI Integration: Configure CI pipelines (GitHub Actions, Jenkins, GitLab CI) to run tests and publish reports.
• Test Output: Clear pass/fail results for parameterized tests including cases with null and empty inputs.
• Failure Analysis: Reports show which input (null or empty) caused failure for easy debugging.

Best Practices

• Use @NullSource and @EmptySource to test edge cases simply without writing extra code.
• Combine @NullSource and @EmptySource with other sources like @ValueSource for comprehensive input coverage.
• Keep test methods focused on one behavior and use descriptive names to clarify what null or empty input tests.
• Use assertions that clearly check behavior when inputs are null or empty to avoid false positives.
• Organize parameterized tests in the test package alongside other unit tests for easy maintenance.

Self Check

Where in this folder structure would you add a new parameterized test method that uses @NullSource and @EmptySource to test a new input validation method?

Key Result

Use @NullSource and @EmptySource annotations in JUnit parameterized tests to easily test null and empty inputs for robust input validation.

Practice

1. What is the main purpose of the @NullSource annotation in JUnit parameterized tests?

easy

A. To provide a null value as input to the test method parameter

B. To provide an empty string as input to the test method parameter

C. To provide multiple non-null values to the test method

D. To skip the test when the input is null

Solution

1. Step 1: Understand @NullSource role

   @NullSource is used to supply a null value as input to a parameterized test method.

2. Step 2: Differentiate from other sources

   @EmptySource provides empty values like empty strings or collections, not null. Other options do not match @NullSource behavior.

3. Final Answer:

   To provide a null value as input to the test method parameter -> Option A

4. Quick Check:

   @NullSource = null input [OK]

Hint: NullSource always means null input for tests [OK]

Common Mistakes:

• Confusing @NullSource with @EmptySource
• Thinking @NullSource provides empty strings
• Assuming @NullSource skips tests

2. Which of the following is the correct way to use both @NullSource and @EmptySource together in a JUnit 5 parameterized test?

easy

A. @ParameterizedTest
@NullSource
void testMethod(@EmptySource String input) { ... }

B. @ParameterizedTest
@EmptySource
@NullSource
void testMethod() { ... }

C. @ParameterizedTest
@NullSource
@EmptySource
void testMethod(String input) { ... }

D. @ParameterizedTest
@NullSource
@EmptySource
void testMethod() { ... }

Solution

1. Step 1: Check annotation placement

   Both @NullSource and @EmptySource must be placed above the test method along with @ParameterizedTest.

2. Step 2: Verify method signature

   The test method must accept a parameter matching the input type (e.g., String input) to receive values from sources.

3. Final Answer:

   @ParameterizedTest @NullSource @EmptySource void testMethod(String input) { ... } -> Option C

4. Quick Check:

   Annotations above method with parameter = correct [OK]

Hint: Put both sources above method with matching parameter [OK]

Common Mistakes:

• Placing @EmptySource inside method parameters
• Missing parameter in test method
• Wrong order or missing @ParameterizedTest

3. Given the following test method, what inputs will it receive during execution?

@ParameterizedTest
@NullSource
@EmptySource
void testInput(String input) {
    System.out.println(input == null ? "null" : "'" + input + "'");
}

medium

A. Only an empty string printed as: ''

B. null and an empty string printed as: null and ''

C. Only null printed as: null

D. null, empty string, and a string with spaces

Solution

1. Step 1: Identify inputs from annotations

   @NullSource provides a null input, and @EmptySource provides an empty string input.

2. Step 2: Understand output behavior

   The method prints "null" if input is null, otherwise prints the input in quotes. So outputs will be "null" and "''" for empty string.

3. Final Answer:

   null and an empty string printed as: null and '' -> Option B

4. Quick Check:

   NullSource + EmptySource = null + "" [OK]

Hint: NullSource gives null, EmptySource gives empty string [OK]

Common Mistakes:

• Assuming EmptySource provides null
• Expecting extra inputs like spaces
• Ignoring null input from NullSource

4. Consider this test code snippet:

@ParameterizedTest
@NullSource
@EmptySource
void test(String input) {
    assertNotNull(input);
}

What will happen when this test runs?

medium

A. Test fails with AssertionError due to null input

B. Test passes for all inputs

C. Compilation error because of missing imports

D. Test is skipped because of null input

Solution

1. Step 1: Analyze inputs from sources

   @NullSource provides a null input, and @EmptySource provides an empty string input.

2. Step 2: Check assertion behavior

   assertNotNull(input) will fail when input is null, causing an AssertionError.

3. Final Answer:

   Test fails with AssertionError due to null input -> Option A

4. Quick Check:

   assertNotNull fails on null input [OK]

Hint: assertNotNull fails if input is null from NullSource [OK]

Common Mistakes:

• Thinking test passes because empty string is valid
• Assuming test skips null input
• Confusing compilation errors with runtime failures

5. You want to test a method that should accept null, empty, and non-empty strings. Which combination of annotations correctly tests all these cases in one parameterized test?

hard

A. @ParameterizedTest
@NullSource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

B. @ParameterizedTest
@EmptySource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

C. @ParameterizedTest
@NullSource
@EmptySource
void testMethod() { ... }

D. @ParameterizedTest
@NullSource
@EmptySource
@ValueSource(strings = {"test"})
void testMethod(String input) { ... }

Solution

1. Step 1: Identify needed inputs

   You need null (from @NullSource), empty string (from @EmptySource), and a non-empty string (from @ValueSource).

2. Step 2: Verify method signature and annotations

   Method must accept a parameter to receive inputs. @ParameterizedTest @NullSource @EmptySource @ValueSource(strings = {"test"}) void testMethod(String input) { ... } includes all three annotations and a parameter.

3. Final Answer:

   @ParameterizedTest @NullSource @EmptySource @ValueSource(strings = {"test"}) void testMethod(String input) { ... } -> Option D

4. Quick Check:

   Combine NullSource, EmptySource, and ValueSource for all cases [OK]

Hint: Combine NullSource, EmptySource, and ValueSource for full coverage [OK]

Common Mistakes:

• Omitting NullSource or EmptySource
• Missing method parameter
• Using annotations without @ParameterizedTest