Getting NullPointerException when using @TempDir on instance variable

Question

I have a test class with an instance variable annotated with @TempDir. Many of the tests in the class use this variable and expected that a new TempDir will be created for each test (and this looks too be true).

When I run the tests in Idea they all complete successfully. However when I run them from a terminal they fail because the TempDir is null. If I move the @TempDir to each test as an argument to each method the test runs successfully in both Idea and in a terminal.

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

import java.nio.file.Path;

public class FileTest {
    @TempDir
    private Path tempDir;

    @Test
    public void testConstructor_FileDoesNotExist_ExpectIllegalArgumentException() {
        Path nonExistingFile = Path.of(tempDir.toString(), "NonExistingFile.xls");
        Assertions.assertFalse(() -> nonExistingFile.toFile().exists());
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.test</groupId>
    <artifactId>file-test</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>5.9.2</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
            <version>5.9.2</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

I have tried using different scopes for the tests and instance variable but the results where the same.

What is the reason for this?

Answer 1

You'll have to add either Maven Surefire Plugin or Maven Failsafe Plugin to your pom.xml:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>3.0.0</version>
        </plugin>
    </plugins>
</build>

Without one of these plugins, Maven does not support the JUnit Platform.