JUnit: Possible to 'expect' a wrapped exception?

Question

Hello!

I know that one can define an 'expected' exception in JUnit, doing:

@Test(expect=MyException.class)
public void someMethod() { ... }

But what if there is always same exception thrown, but with different 'nested' causes.

Any suggestions?

Answer 1

You could wrap the testing code in a try / catch block, catch the thrown exception, check the internal cause, log / assert / whatever, and then rethrow the exception (if desired).

Answer 2

You could always do it manually:

@Test
public void someMethod() {
    try{
        ... all your code
    } catch (Exception e){
        // check your nested clauses
        if(e.getCause() instanceof FooException){
            // pass
        } else {
            Assert.fail("unexpected exception");
        }
    }

Answer 3

I wrote a little JUnit extension for that purpose. A static helper function takes a function body and an array of expected exceptions:

import static org.junit.Assert.assertTrue;
import static org.junit.Assert.fail;

import java.util.Arrays;

public class AssertExt {
    public static interface Runnable {
     void run() throws Exception;
    }

    public static void assertExpectedExceptionCause( Runnable runnable, @SuppressWarnings("unchecked") Class[] expectedExceptions ) {
     boolean thrown = false;
     try {
      runnable.run();
     } catch( Throwable throwable ) {
      final Throwable cause = throwable.getCause();
      if( null != cause ) {
       assertTrue( Arrays.asList( expectedExceptions ).contains( cause.getClass() ) );
       thrown = true;
      }
     }
     if( !thrown ) {
      fail( "Expected exception not thrown or thrown exception had no cause!" );
     }
    }
}

You can now check for expected nested exceptions like so:

import static AssertExt.assertExpectedExceptionCause;

import org.junit.Test;

public class TestExample {
    @Test
    public void testExpectedExceptionCauses() {
     assertExpectedExceptionCause( new AssertExt.Runnable(){
      public void run() throws Exception {
       throw new Exception( new NullPointerException() );
      }
     }, new Class[]{ NullPointerException.class } );
    }
}

This saves you writing the same boiler plate code again and again.

Answer 4

If you're using the latest version of JUnit you can extend the default test runner to handle this for you (without having to wrap each of your methods in a try/catch block)

ExtendedTestRunner.java - New test runner:

public class ExtendedTestRunner extends BlockJUnit4ClassRunner
{
    public ExtendedTestRunner( Class<?> clazz )
        throws InitializationError
    {
        super( clazz );
    }

    @Override
    protected Statement possiblyExpectingExceptions( FrameworkMethod method,
                                                     Object test,
                                                     Statement next )
    {
        ExtendedTest annotation= method.getAnnotation( ExtendedTest.class );
        return expectsCauseException( annotation ) ?
                new ExpectCauseException( next, getExpectedCauseException( annotation ) ) :
                super.possiblyExpectingExceptions( method, test, next );
    }

    @Override
    protected List<FrameworkMethod> computeTestMethods()
    {
        List<FrameworkMethod> testMethods = new ArrayList<FrameworkMethod>( super.computeTestMethods() );
        testMethods.addAll( getTestClass().getAnnotatedMethods( ExtendedTest.class ) );
        return testMethods;
    }

    @Override
    protected void validateTestMethods( List<Throwable> errors )
    {
        super.validateTestMethods( errors );
        validatePublicVoidNoArgMethods( ExtendedTest.class, false, errors );
    }

    private Class<? extends Throwable> getExpectedCauseException( ExtendedTest annotation )
    {
        if (annotation == null || annotation.expectedCause() == ExtendedTest.None.class)
            return null;
        else
            return annotation.expectedCause();
    }

    private boolean expectsCauseException( ExtendedTest annotation) {
        return getExpectedCauseException(annotation) != null;
    }

}

ExtendedTest.java - annotation to mark test methods with:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface ExtendedTest
{

    /**
     * Default empty exception
     */
    static class None extends Throwable {
        private static final long serialVersionUID= 1L;
        private None() {
        }
    }

    Class<? extends Throwable> expectedCause() default None.class;
}

ExpectCauseException.java - new JUnit Statement:

public class ExpectCauseException extends Statement
{
    private Statement fNext;
    private final Class<? extends Throwable> fExpected;

    public ExpectCauseException( Statement next, Class<? extends Throwable> expected )
    {
        fNext= next;
        fExpected= expected;
    }

    @Override
    public void evaluate() throws Exception
    {
        boolean complete = false;
        try {
            fNext.evaluate();
            complete = true;
        } catch (Throwable e) {
            if ( e.getCause() == null || !fExpected.isAssignableFrom( e.getCause().getClass() ) )
            {
                String message = "Unexpected exception cause, expected<"
                            + fExpected.getName() + "> but was<"
                            + ( e.getCause() == null ? "none" : e.getCause().getClass().getName() ) + ">";
                throw new Exception(message, e);
            }
        }
        if (complete)
            throw new AssertionError( "Expected exception cause: "
                    + fExpected.getName());
    }
}

Usage:

@RunWith( ExtendedTestRunner.class )
public class MyTests
{
    @ExtendedTest( expectedCause = MyException.class )
    public void someMethod()
    {
        throw new RuntimeException( new MyException() );
    }
}