@CsvFileSource for external CSV in JUnit

Introduction

@CsvFileSource helps you run the same test many times with different data from a CSV file. This saves time and avoids repeating code.

You want to test a function with many input values stored in a CSV file.

You have test data maintained separately from your code for easy updates.

You want to check how your code behaves with different sets of data without writing multiple tests.

You want to keep your test code clean and simple by loading data externally.

You want to share test data files across multiple test classes.

Syntax

JUnit

@ParameterizedTest
@CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)
void testMethod(String input, int expected) {
    // test code here
}

resources points to the CSV file path inside your test resources folder.

numLinesToSkip is used to skip header lines in the CSV file.

Examples

This test runs once for each number in numbers.csv, checking if it is positive.

JUnit

@ParameterizedTest
@CsvFileSource(resources = "/numbers.csv")
void testWithNumbers(int number) {
    assertTrue(number > 0);
}

This test skips the first line (header) and tests login data from users.csv.

JUnit

@ParameterizedTest
@CsvFileSource(resources = "/users.csv", numLinesToSkip = 1)
void testUserLogin(String username, String password) {
    assertNotNull(username);
    assertNotNull(password);
}

Sample Program

This test reads three numbers per line from test-data.csv (skipping the header). It checks if the sum of a and b equals expectedSum.

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvFileSource;

public class CsvFileSourceTest {

    @ParameterizedTest
    @CsvFileSource(resources = "/test-data.csv", numLinesToSkip = 1)
    void testSum(int a, int b, int expectedSum) {
        int actualSum = a + b;
        assertEquals(expectedSum, actualSum);
    }
}

Important Notes

Place your CSV file in src/test/resources folder so it is on the test classpath.

Use forward slashes '/' in the resource path, even on Windows.

Make sure the CSV file encoding is UTF-8 to avoid reading errors.

Summary

@CsvFileSource lets you run parameterized tests with data from CSV files.

It helps keep test data separate and easy to update.

Remember to skip headers if your CSV has them using numLinesToSkip.

Practice

1. What is the main purpose of using @CsvFileSource in JUnit tests?

easy

A. To run the same test multiple times with different data from a CSV file

B. To write data into a CSV file during test execution

C. To generate random test data automatically

D. To skip tests based on CSV file content

Solution

1. Step 1: Understand the role of @CsvFileSource

   @CsvFileSource provides external CSV data to parameterized tests, allowing multiple runs with different inputs.

2. Step 2: Compare options with this role

   Only To run the same test multiple times with different data from a CSV file correctly describes running tests multiple times with CSV data. Others describe unrelated actions.

3. Final Answer:

   To run the same test multiple times with different data from a CSV file -> Option A

4. Quick Check:

   @CsvFileSource runs tests with CSV data = B [OK]

Hint: Remember: @CsvFileSource feeds test data, not writes or skips [OK]

Common Mistakes:

• Thinking @CsvFileSource writes data instead of reading
• Confusing it with random data generation
• Assuming it controls test skipping

2. Which of the following is the correct syntax to skip the header line in a CSV file when using @CsvFileSource?

easy

A. @CsvFileSource(resources = "/data.csv", skipLines = 1)

B. @CsvFileSource(resources = "/data.csv", skipHeader = true)

C. @CsvFileSource(resources = "/data.csv", ignoreFirstLine = true)

D. @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)

Solution

1. Step 1: Recall the correct attribute for skipping lines

   The correct attribute to skip lines in @CsvFileSource is numLinesToSkip.

2. Step 2: Match options with correct attribute

   Only @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1) uses numLinesToSkip = 1 which skips the first line (header).

3. Final Answer:

   @CsvFileSource(resources = "/data.csv", numLinesToSkip = 1) -> Option D

4. Quick Check:

   Skip header line = numLinesToSkip = 1 [OK]

Hint: Use numLinesToSkip to skip headers, not skipHeader or ignoreFirstLine [OK]

Common Mistakes:

• Using non-existent attributes like skipHeader or ignoreFirstLine
• Confusing skipLines with numLinesToSkip
• Forgetting to skip header causing test failures

3. Given the CSV file data.csv with content:

name,age
Alice,30
Bob,25

and the test method:

@ParameterizedTest
@CsvFileSource(resources = "/data.csv", numLinesToSkip = 1)
void testUser(String name, int age) {
    System.out.println(name + "-" + age);
}

What will be the printed output when the test runs?

medium

A. name-age
Alice-30
Bob-25

B. Alice-30
Bob-25

C. Alice-30

D. Test fails due to header line

Solution

1. Step 1: Understand numLinesToSkip effect

   numLinesToSkip=1 skips the header line "name,age" so only data lines remain.

2. Step 2: Identify printed lines

   Test runs twice, printing "Alice-30" and "Bob-25" each on separate lines.

3. Final Answer:

   Alice-30 Bob-25 -> Option B

4. Quick Check:

   Header skipped, prints data lines only = D [OK]

Hint: Skip header to avoid printing it as data [OK]

Common Mistakes:

• Not skipping header causing it to print as data
• Expecting header line to print
• Confusing output format with CSV file content

4. Consider this test snippet:

@ParameterizedTest
@CsvFileSource(resources = "/users.csv")
void testUser(String name, int age) {
    assertTrue(age > 0);
}

The CSV file users.csv has a header line. What is the likely problem when running this test?

medium

A. Test will fail because header line is treated as data causing NumberFormatException

B. Test will pass because header is ignored automatically

C. Test will fail due to missing @Test annotation

D. Test will skip all lines and run zero times

Solution

1. Step 1: Check if header is skipped

   No numLinesToSkip is set, so header line is treated as data.

2. Step 2: Understand impact on parsing

   Trying to convert header "age" to int causes NumberFormatException, failing the test.

3. Final Answer:

   Test will fail because header line is treated as data causing NumberFormatException -> Option A

4. Quick Check:

   Header not skipped causes parse error = A [OK]

Hint: Always skip header lines to avoid parsing errors [OK]

Common Mistakes:

• Assuming header is skipped automatically
• Forgetting to add numLinesToSkip=1
• Confusing @ParameterizedTest with @Test

5. You want to test a method that calculates discounts using data from a CSV file with columns: product,price,discountRate. The CSV has a header line. Which is the best way to write the parameterized test method signature and annotation to correctly read and use this data?

hard

A. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1)
void testDiscount(String product, String price, String discountRate) { ... }

B. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv")
void testDiscount(String product, int price, int discountRate) { ... }

C. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1)
void testDiscount(String product, double price, double discountRate) { ... }

D. @ParameterizedTest
@CsvFileSource(resources = "/discounts.csv", skipHeader = true)
void testDiscount(String product, double price, double discountRate) { ... }

Solution

1. Step 1: Identify header skipping attribute

   Use numLinesToSkip = 1 to skip the CSV header line.

2. Step 2: Match method parameters to CSV columns and types

   Product is String, price and discountRate are decimal numbers, so use double for price and discountRate.

3. Step 3: Check options for correctness

   @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, double price, double discountRate) { ... } correctly uses numLinesToSkip=1 and matching parameter types. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv") void testDiscount(String product, int price, int discountRate) { ... } misses skipping header and uses int instead of double. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, String price, String discountRate) { ... } uses String for numeric fields. @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", skipHeader = true) void testDiscount(String product, double price, double discountRate) { ... } uses invalid attribute skipHeader.

4. Final Answer:

   @ParameterizedTest @CsvFileSource(resources = "/discounts.csv", numLinesToSkip = 1) void testDiscount(String product, double price, double discountRate) { ... } -> Option C

5. Quick Check:

   Skip header with numLinesToSkip=1 and match types = C [OK]

Hint: Use numLinesToSkip=1 and match parameter types to CSV columns [OK]

Common Mistakes:

• Using skipHeader instead of numLinesToSkip
• Not skipping header causing parse errors
• Mismatching parameter types with CSV data