Why @NullSource and @EmptySource in JUnit? - Purpose & Use Cases

The Big Idea

Discover how a few simple annotations can save you hours of tedious testing work!

The Scenario

Imagine testing a method that processes user input. You have to check how it behaves when given null or empty values. Doing this by hand means writing separate test cases for each scenario, running them one by one, and carefully watching for mistakes.

The Problem

Manually writing tests for null and empty inputs is slow and repetitive. It's easy to forget a case or make typos. Running tests separately wastes time and makes your test suite bulky and hard to maintain.

The Solution

The @NullSource and @EmptySource annotations in JUnit let you automatically supply null and empty values to your parameterized tests. This means one test method can check many edge cases quickly and cleanly, reducing errors and saving time.

Before vs After

✗ Before

void testInput() {
  testMethod(null);
  testMethod("");
  testMethod("data");
}

✓ After

@ParameterizedTest
@NullSource
@EmptySource
@ValueSource(strings = {"data"})
void testInput(String input) {
  testMethod(input);
}

What It Enables

It enables fast, clear, and reliable testing of edge cases like null and empty inputs without extra code clutter.

Real Life Example

When testing a login form, you want to ensure the system handles empty usernames or passwords gracefully. Using @NullSource and @EmptySource lets you cover these cases effortlessly in one test.

Key Takeaways

Manual tests for null and empty inputs are slow and error-prone.

@NullSource and @EmptySource automate supplying these edge cases.

This leads to cleaner, faster, and more reliable tests.

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