JUnit Rules

This is how it works

Test class

public class TestClass {
    @Test
    public void test_one() {...}

    @Test
    public void test_two() {...}

    @Test
    public void test_three() {...}
}

This is how it works

Test wrapped in Statement

This is how it works

Statement

public abstract class Statement {
    public abstract void evaluate() throws Throwable;
}

This is how it works

Rule takes and returns Statement

This is how it works

Rule

public interface TestRule {
    Statement apply(Statement base, Description description);
}

Use a Rule

Method

public class SuperTestUsingRules {

    @Rule
    public LoggingRule loggingRule = new LoggingRule();

    @Test
    public void test_one() {...}

    @Test
    public void test_two() {...}
}

Use a Rule

Class

public class SuperTestUsingRules {

    @ClassRule
    public static LoggingRule loggingRule = new LoggingRule();

    @Test
    public void test_one() {...}

    @Test
    public void test_two() {...}
}

Example: Expected Exception

public class ExpectedExceptionTest {

    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void throws_nullpointer_exception() {
        thrown.expect(NullPointerException.class);
        thrown.expectMessage(endsWith("null pointer"));
        throwNullPointerException();
    }

    private void throwNullPointerException() {
        throw new NullPointerException("This is a null pointer");
    }
}

Example: External Resource

Extending TestRule

public class LoggingRule implements TestRule {

  @Override
  public Statement apply(Statement base, Description description) {

    return new Statement() {
      @Override
      public void evaluate() throws Throwable {
        System.out.println("Before executing original Statement");
        try {
          base.evaluate();
        } finally {
          System.out.println("After executing original Statement");
        }
      }
    };
  }
}

Example External Resource

Extending ExternalResource

public class LoggingRule extends ExternalResource {

  @Override
  protected void before() throws Throwable {
    System.out.println("Before executing original Statement");
  }

  @Override
  protected void after() {
    System.out.println("After executing original Statement");
  }
}

Chain Rules