JUnit: Creating Temporary Files using the TemporaryFolder @Rule

If you have an application that writes out files, how do you test that the generated file is correct?

One approach, is to configure the application to write out to some pre-defined temporary location such as /tmp (on *nix based systems) and then delete the files after the test. But this requires a lot of boilerplate code in your unit tests and can be error prone. Sometimes, developers forget to clean up these temporary files and leave a mess behind. I have also seen cases where unit tests have written temporary files to the current directory (which contains test code) and developers have accidently checked them into source control, which definitely shouldn't happen!

The right way to deal with temporary files in unit tests is by using JUnit's TemporaryFolder Rule. With it, you no longer need to worry about where to create your temporary files or deleting them after the test succeeds or fails. JUnit handles all of that for you.

The following example shows you how to use the TemporaryFolder Rule:

import static org.hamcrest.Matchers.is;
import static org.junit.Assert.assertThat;

import java.io.File;
import java.io.IOException;

import org.apache.commons.io.FileUtils;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

public class MyTest {

  @Rule
  public TemporaryFolder tempFolder = new TemporaryFolder();

  @Test
  public void testWrite() throws IOException {
    // Create a temporary file.
    // This is guaranteed to be deleted after the test finishes.
    final File tempFile = tempFolder.newFile("myfile.txt");

    // Write something to it.
    FileUtils.writeStringToFile(tempFile, "hello world");

    // Read it.
    final String s = FileUtils.readFileToString(tempFile);

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