How to Create Dynamic JUnit Tests for Each File in a Directory?

Question

How can I implement JUnit tests that execute for each file in a directory without writing explicit test methods for each file?

import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import java.io.File;
import java.util.Arrays;
import java.util.Collection;

@RunWith(Parameterized.class)
public class FileTest {
    private String fileName;

    public FileTest(String fileName) {
        this.fileName = fileName;
    }

    @Parameterized.Parameters
    public static Collection<Object[]> data() {
        File dir = new File("path/to/directory");
        return Arrays.stream(dir.listFiles())
                     .filter(File::isFile)
                     .map(file -> new Object[]{file.getName()})
                     .toList();
    }

    @Test
    public void testFileProcessing() {
        // Load file and perform test logic
        System.out.println("Testing file: " + fileName);
        // Add assertions based on file processing
    }
}

Answer

To create dynamic JUnit tests that treat each file in a specified directory as an individual test case, you can utilize JUnit 4's Parameterized Test feature. This allows you to run the same test logic with different parameters (in this case, the names of files).

import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;
import java.io.File;
import java.util.Arrays;
import java.util.Collection;

@RunWith(Parameterized.class)
public class FileTest {
    private String fileName;

    public FileTest(String fileName) {
        this.fileName = fileName;
    }

    @Parameterized.Parameters
    public static Collection<Object[]> data() {
        File dir = new File("path/to/directory");
        return Arrays.stream(dir.listFiles())
                     .filter(File::isFile)
                     .map(file -> new Object[]{file.getName()})
                     .toList();
    }

    @Test
    public void testFileProcessing() {
        // Logic for processing each file
        System.out.println("Testing file: " + fileName);
        // Add assertions based on file processing
    }
}

Causes

Using a single test method in JUnit leads to aggregate results instead of individual test outputs.
Lack of knowledge on creating parameterized tests may restrict the flexibility of unit testing with JUnit.

Solutions

Utilize the Parameterized test runner in JUnit 4 to dynamically generate test cases from the files in a directory.
Implement a method that gathers files from the specified directory and returns them as parameters for the test method.

Common Mistakes

Mistake: Not filtering for files, resulting in directories being included in the tests.

Solution: Ensure to use `File::isFile` in the filtering process.

Mistake: Hardcoding the directory path instead of making it configurable.

Solution: Consider parameterizing the path to allow for flexibility.

Helpers

JUnit tests
dynamic JUnit tests
parameterized tests in JUnit
JUnit TestRunner
testing files in directory