Creating Temporary Files with JUnit 5

This post shows you how to perform unit testing using temporary files with JUnit 5. If you're still on JUnit 4, please check out my previous post!

In JUnit 5, the @TempDir annotation is used to indicate that a field or method parameter of type Path or File is a temporary directory. Each test will use its own temporary directory and when the test method has finished executing, the directory and all its contents will be deleted. (If you want to share a temporary directory between tests, you need to make the field static.)

Here is an example:

import static org.hamcrest.CoreMatchers.*;
import static org.hamcrest.MatcherAssert.*;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.io.TempDir;

public class MyTest {

  @TempDir
  Path tempDir;

  @Test
  public void testWrite() throws IOException {

    // Create a temporary file.
    // This is guaranteed to be deleted after the test finishes.
    final Path tempFile = Files.createFile(tempDir.resolve("myfile.txt"));

    // Write something to it.
    Files.writeString(tempFile, "Hello World");

    // Read it.
    final String s = Files.readString(tempFile);

    // Check that what was written is correct.
    assertThat("Hello World", is(s));
  }
}

Related post: JUnit: Creating Temporary Files using the TemporaryFolder @Rule

Parameterized Tests in JUnit 5

A parameterized test allows you to run a test against a varying set of data. If you find yourself calling the same test but with different inputs, over and over again, a parameterized test would help make your code cleaner. To create one in JUnit 5 you need to:

• Annotate the test method with @ParameterizedTest
• Annotate the test method with at lease one source e.g. @ValueSource
• Consume the arguments in the test method

The sections below describe some of the commonly used source annotations you can use to provide inputs to your test methods.

@ValueSource
This annotation lets you specify a single array of literal values that will be passed to your test method one by one, as shown in the example below:

@ParameterizedTest
@ValueSource(ints = {2, 4, 6})
void testIsEven(final int i) {
  assertTrue(i % 2 == 0);
}

@CsvSource
This annotation allows you to specify an array of comma-separated values, which is useful if your test method takes multiple arguments. If you have a large number of arguments, you can use an ArgumentsAccessor to extract the arguments as opposed to creating a method with a long parameter list. For example:

@ParameterizedTest(name = "Person with name {0} and age {1}")
@CsvSource({ "Alice, 28",
             "Bob, 30" })
void testPerson(final String name, final int age) {
  final Person p = new Person(name, age);
  assertThat(p.getName(), is(name));
  assertThat(p.getAge(), is(age));
}

@ParameterizedTest(name = "Person with name {0} and age {1}")
@CsvSource({ "Alice, 28",
             "Bob, 30" })
void testPersonWithArgumentAccessor(final ArgumentsAccessor arguments) {
  final String name = arguments.getString(0);
  final int age = arguments.getInteger(1);
  final Person p = new Person(name, age);
  assertThat(p.getName(), is(name));
  assertThat(p.getAge(), is(age));
}

By the way, note how I have also customised the display name of the test using the {0} and {1} argument placeholders.

@CsvFileSource
This annotation is similar to CsvSource but allows you to load your test inputs from a CSV file on the classpath. For example:

@ParameterizedTest(name = "Person with name {0} and age {1}")
@CsvFileSource(resources = { "data.csv" })
void testPerson(final String name, final int age) {
  final Person p = new Person(name, age);
  assertThat(p.getName(), is(name));
  assertThat(p.getAge(), is(age));
}

@MethodSource
This annotation allows you to specify a factory method which returns a stream of objects to be passed to your test method. If your test method has multiple arguments, your factory method should return a stream of Arguments instances as shown in the example below:

import static org.junit.jupiter.params.provider.Arguments.*;

@ParameterizedTest(name = "{0} is sorted to {1}")
@MethodSource("dataProvider")
void testSort(final int[] input, final int[] expected) {
  Arrays.sort(input);
  assertArrayEquals(expected, input);
}

static Stream<Arguments> dataProvider() {
  return Stream.of(
      arguments(new int[] { 1, 2, 3 }, new int[] { 1, 2, 3 }),
      arguments(new int[] { 3, 2, 1 }, new int[] { 1, 2, 3 }),
      arguments(new int[] { 5, 5, 5 }, new int[] { 5, 5, 5 }));
}

For more information, see the JUnit 5 User Guide on Parameterized Tests.

If you're still on JUnit 4 (why?!), check out my previous post on Parameterized Tests in JUnit 4.

Testing Expected Exceptions with JUnit 5

This post shows how to test for expected exceptions using JUnit 5. If you're still on JUnit 4, please check out my previous post.

Let's start with the following class that we wish to test:

public class Person {
  private final String name;
  private final int age;
    
  /**
   * Creates a person with the specified name and age.
   *
   * @param name the name
   * @param age the age
   * @throws IllegalArgumentException if the age is not greater than zero
   */
  public Person(String name, int age) {
    this.name = name;
    this.age = age;
    if (age <= 0) {
      throw new IllegalArgumentException("Invalid age:" + age);
    }
  }
}

To test that an IllegalArgumentException is thrown if the age of the person is less than zero, you should use JUnit 5's assertThrows as shown below:

import static org.hamcrest.CoreMatchers.*;
import static org.hamcrest.MatcherAssert.*;
import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.Test;

class PersonTest {

  @Test
  void testExpectedException() {
    assertThrows(IllegalArgumentException.class, () -> {
      new Person("Joe", -1);
    });
  }

  @Test
  void testExpectedExceptionMessage() {
    final Exception e = assertThrows(IllegalArgumentException.class, () -> {
      new Person("Joe", -1);
    });
    assertThat(e.getMessage(), containsString("Invalid age"));
  }
}

Related post: Testing Expected Exceptions with JUnit 4 Rules