2.20. Built-in Extensions

While the JUnit team encourages reusable extensions to be packaged and maintained in separate libraries, the JUnit Jupiter API artifact includes a few user-facing extension implementations that are considered so generally useful that users shouldn’t have to add another dependency.

2.20.1. The TempDirectory Extension

The built-in [TempDirectory](https://github.com/junit-team/junit5/tree/r5.7.0/junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/TempDirectory.java) extension is used to create and clean up a temporary directory for an individual test or all tests in a test class. It is registered by default. To use it, annotate a non-private field of type java.nio.file.Path or java.io.File with [@TempDir](https://junit.org/junit5/docs/current/api/org.junit.jupiter.api/org/junit/jupiter/api/io/TempDir.html) or add a parameter of type java.nio.file.Path or java.io.File annotated with @TempDir to a lifecycle method or test method.

For example, the following test declares a parameter annotated with @TempDir for a single test method, creates and writes to a file in the temporary directory, and checks its content.

A test method that requires a temporary directory

@Test
void writeItemsToFile(@TempDir Path tempDir) throws IOException {
    Path file = tempDir.resolve("test.txt");
    new ListWriter(file).write("a", "b", "c");
    assertEquals(singletonList("a,b,c"), Files.readAllLines(file));
}

The following example stores a shared temporary directory in a static field. This allows the same sharedTempDir to be used in all lifecycle methods and test methods of the test class.

A test class that shares a temporary directory across test methods

class SharedTempDirectoryDemo {
    @TempDir
    static Path sharedTempDir;
    @Test
    void writeItemsToFile() throws IOException {
        Path file = sharedTempDir.resolve("test.txt");
        new ListWriter(file).write("a", "b", "c");
        assertEquals(singletonList("a,b,c"), Files.readAllLines(file));
    }
    @Test
    void anotherTestThatUsesTheSameTempDir() {
        // use sharedTempDir
    }
}