Java 8 magic exception copying

Since I can in Java 8 now parameterize constructors as functions, I can write a generic exception copier:

<E extends Throwable>
E copy(final Throwable from, final Function<String, E> ctor) {
    final E to = ctor.apply(from.getMessage());
    to.setStackTrace(from.getStackTrace());
    for (final Throwable s : from.getSuppressed())
        to.addSuppressed(s);
    return to;
}

Example:

try {
    // Something throws
} catch (final AnException | BeException | KatException e) {
    throw copy(e, IOException::new);
}

This is not a common strategy but one I sometimes use to reduce the incredible size of layered exceptions, especially for logging. It is also handy for shoehorning 3^rd-party exceptions into a common framework exception, a nice feature for APIs to simplify calling code. Copy helps reduce boilerplate code.

Lambda annotations

I am still startled by Java. While playing with Byte Buddy I noodled with adding annotations to a class at runtime. I wrote this:

final Class<? extends X> dynamicType
            = (Class<< extends X>) new ByteBuddy().
        subclass(Object.class).
        implement(X.class).
        annotateType(() -> Inject.class).
        method(named("toString")).intercept(value("Hello World!")).
        method(named("foo")).intercept(value(3)).
        make().
        load(ByteBuddyMain.class.getClassLoader(), WRAPPER).
        getLoaded();

Get a load of "annotatedType"! It wants an annotation instance. Originally I tried:

new Inject() {
    @Override
    public Class<Inject> annotationType() {
        return Inject.class;
    }
}

What the hey, then I thought to try the lambda expression and live dangerously. It works!

Modern XML to Java

How many frameworks are there for converting XML to Java? Hard to count. As an experiment I tried my hand at one. I have two top-level classes plus an annotation:

public final class XMLFuzzy
        implements InvocationHandler {
    private static final XPath xpath = XPathFactory.newInstance().
            newXPath();
    private static final Map<Method, XPathExpression> expressions
     = new ConcurrentHashMap<>();

    private final Node node;
    private final Converter converter;

    public static final class Factory {
        private final Converter converter;

        public Factory(final Converter converter) {
            this.converter = converter;
        }

        public <T> T of(@Nonnull final Class<T> itf,
                @Nonnull final Node node) {
            return XMLFuzzy.of(itf, node, converter);
        }
    }

    public static <T> T of(@Nonnull final Class<T> itf,
     @Nonnull final Node node,
            @Nonnull final Converter converter) {
        return itf.cast(newProxyInstance(itf.getClassLoader(),
                new Class[]{itf},
                new XMLFuzzy(node, converter)));
    }

    private XMLFuzzy(final Node node, final Converter converter) {
        this.node = node;
        this.converter = converter;
    }

    @Override
    public Object invoke(final Object proxy, final Method method,
         final Object[] args)
            throws Throwable {
        return converter.convert(method.getReturnType(), expressions.
                computeIfAbsent(method, XMLFuzzy::compile).
                evaluate(node));
    }

    private static XPathExpression compile(@Nonnull final Method method) {
        final String expression = asList(method.getAnnotations()).stream().
                filter(From.class::isInstance).
                map(From.class::cast).
                findFirst().
                orElseThrow(() -> new MissingAnnotation(method)).
                value();
        try {
            return xpath.compile(expression);
        } catch (final XPathExpressionException e) {
            throw new BadXPath(method, expression, e);
        }
    }

    public static final class MissingAnnotation
            extends RuntimeException {
        private MissingAnnotation(final Method method) {
            super(format("Missing @X(xpath) annotation: %s", method));
        }
    }

    public static final class BadXPath
            extends RuntimeException {
        private BadXPath(final Method method, final String expression,
                final XPathExpressionException e) {
            super(format("Bad @X(xpath) annotation on '%s': %s: %s",
                    method, expression, e.getMessage()));
            setStackTrace(e.getStackTrace());
        }
    }
}

I have left out Converter; it turns strings into objects of a given type, another example of overimplemented framework code in Java. And the annotation:

@Documented
@Inherited
@Retention(RUNTIME)
@Target(METHOD)
public @interface From {
    String value();
}

The idea is straight-forward: drive the object mapping from XML with XPaths. Credit to XMLBeam for introducing to me the elegant use of JDK proxies for this purpose.

Of course tests:

public final class XMLFuzzyTest {
    private Top top;

    @Before
    public void setUp()
            throws ParserConfigurationException, IOException, SAXException {
        final Document document = DocumentBuilderFactory.newInstance().
                newDocumentBuilder().
                parse(new InputSource(new StringReader(XML)));
        top = new XMLFuzzy.Factory(new Converter()).of(Top.class, document);
    }

    @Test
    public void shouldHandleString() {
        assertThat(top.a(), is(equalTo("apple")));
    }

    @Test
    public void shouldHandlePrimitiveInt() {
        assertThat(top.b(), is(equalTo(3)));
    }

    @Test
    public void shouldHandleRURI() {
        assertThat(top.c(), is(equalTo(URI.create("http://some/where"))));
    }

    @Test(expected = MissingAnnotation.class)
    public void shouldThrowOnMissingAnnotation() {
        top.d();
    }

    @Test(expected = BadXPath.class)
    public void shouldThrowOnBadXPath() {
        top.e();
    }

    public interface Top {
        // For the purposes of this blog post, pretend Java supports
 // multiline string literals
        @Language("XML")
        String XML = "<top>
                    <a>apple</a>
      <b>3</b>
      <c>http://some/where</c>
         </top>";

        @From("//top/a")
        String a();

        @From("//top/b")
        int b();

        @From("//top/c")
        URI c();

        void d();

        @From("dis' ain't xpath")
        void e();
    }
}