If you've ever written Java code that uses reflection, you've surely been confronted with what to do in case a reflected Method or Constructor raises an IllegalAccessException when invoke()d. How do you test your code's reaction to IllegalAccessException? Do you just say, "Bah, that'll never happen", and press on? Or do you find the gap in your test suite's code coverage gnawing at you until plugged?

Recently, I found myself writing a JSP custom tag that took a bean in some scope, and wrote its readable properties out as HTML hidden fields. The tag's doEndTag() method located the bean, discovered its properties, and for each property got its value by reflectively invoking an accessor method. Such reflective invocation can raise the checked exceptions InvocationTargetException and IllegalAccessException. The tag code, in the presence of either exception, I decided, should wrap the exception in a JspException and raise the wrapper.

Testing the handling of InvocationTargetException was no big deal: feed the tag a bean with a public property that raises some unchecked exception always. Testing the handling of IllegalAccessException was a little trickier. Here's what I did: make a bean in its own package, with a single property whose type is an interface, and whose property accessor returns an anonymous-inner-class implementation of said interface. Reflection can then discover the property on the interface, since it's public; but when invoked, at runtime the system will realize that the target class is an anonymous inner class, inaccessible to the outside world, and raise IllegalAccessException:

package pholser.taglib;

public class HiddenFieldsFromBeanTagTest extends TestCase {
    private HiddenFieldsFromBeanTag tag;
    private FakePageContext pageContext;

    protected void setUp() throws Exception {
        super.setUp();

        pageContext = new FakePageContext();
        tag = new HiddenFieldsFromBeanTag();
        tag.setPageContext( pageContext );
    }

    protected void tearDown() throws Exception {
        try {
            tag.release();
        }
        finally {
            super.tearDown();
        }
    }

    public void testAccessorAccessExceptionShouldTranslateToJspException()
        throws Exception {

        tag.setName( "sessionBean" );
        tag.setScope( "session" );
        pageContext.setAttribute( "sessionBean", Faker.getStringMaker(),
            PageContext.SESSION_SCOPE );
        assertEquals( Tag.SKIP_BODY, tag.doStartTag() );

        try {
            tag.doEndTag();
            fail();
        }
        catch ( JspException expected ) {
            assertEquals(
                IllegalAccessException.class,
                expected.getRootCause().getClass() );
        }
    }
}


package pholser.beans;

public interface StringMaker {
    String getString();
}

public class Faker {
    public static StringMaker getStringMaker() {
        return new StringMaker() {
            public String getString() {
                return "x";
            }
        };
    }
}