Saturday, February 28, 2015

Architects, engineers?

Delightful passage from Sam Newman's new book, Building Microservices:

Part of us wants recognition, so we borrow names from other professions that already have the recognition we as an industry crave. But this can be doubly harmful. First, it implies we know what we are doing, when we plainly don’t. I wouldn’t say that buildings and bridges never fall down, but they fall down much less than the number of times our programs will crash, making comparisons with engineers quite unfair. Second, the analogies break down very quickly when given even a cursory glance. To turn things around, if bridge building were like programming, halfway through we’d find out that the far bank was now 50 meters farther out, that it was actually mud rather than granite, and that rather than building a footbridge we were instead building a road bridge. Our software isn’t constrained by the same physical rules that real architects or engineers have to deal with, and what we create is designed to flex and adapt and evolve with user requirements.

A lot of times I really don't understand my own job.

Monday, February 02, 2015

Dice, Test First and saving time

tl;dr — Test first saves time. Also a dice expression calculator.

The setup

Larry Wall famously remarked, "the three great virtues of a programmer: laziness, impatience, and hubris." A skill earned by good programmers is knowing when to favor which virtue. I failed while writing a parser for "dice expressions" ala Dungeons & Dragons. (Example: "3d6 + 1" means roll 3 6-sided dice, sum the results and add 1.)

The parser is straight-forward using the popular ANTLR4 tools:

grammar DiceExpression;

expr: left=expr op=('*' | '/') right=expr #opExpr
    | left=expr op=('+' | '-') right=expr #opExpr
    | '(' group=expr ')' #groupExpr
    | van=('adv' | 'dis')? number=NUMBER? sides=SIDES #diceExpr
    | number=NUMBER #numberExpr
    ;

dice: number=NUMBER? sides=SIDES;

NUMBER: '1'..'9' '0'..'9'*;
SIDES: 'd' ('4' | '6' | '8' | '10' | '12' | '20' | '100')
    { setText(getText().substring(1)); };

WS: [ \t]+ -> skip;

The grammar supports a 5th Edition feature, Advantage/Disadvantage—where you get to roll twice and pick the better/worse result—, with the keywords adv/dis. A less-5th Edition-specific grammar would leave out that term from "#diceExpr".

Enter the rube

All tests pass: good! Then I thought, "What about whitespace in dice rolls or with adv/dis?" I was concerned with expressions such as "1 +disd6" or "3 d4". Surely those are invalid, but I don't check for them. So I began adding explicit whitespace parsing in the #diceExpr rule. Mistake!

Several of my tests failed for perfectly good dice expressions.

Okay, so I reverted my grammar changes. To understand what was going on, I added tests I should have started with before editing (lines marked "FAIL"):

@RunWith(Parameterized.class)
public final class DiceExpressionTest {
    @Parameters(name = "{index}: '{0}'")
    public static Collection<Object> parameters() {
        return asList(args("1", true),
                args("d6", true),
                args("3d6", true),
                args("1+3d6", true),
                args("adv d6", true),
                args("dis 3d6", true),
                args("disd6", false), // FAIL
                args("1 + 3 d6", false)); // FAIL
    }

    private final ExpectedException thrown = ExpectedException.none();

    @Parameter(0)
    public String in;
    @Parameter(1)
    public boolean ok;

    @Test
    public void shouldParse() {
        if (!ok)
            thrown.expect(ParseCancellationException.class);

        // Just check parsing works, ignore results
        DiceExpression.valueOf(in).evalWith(Roll.die());
    }

    private static Object[] args(final Object... args) {
        return args;
    }
}

And ... the "FAIL" tests pass with the original grammar. This got me to thinking more about how ANTLR works. It is a lexer/parser. The lexer grabs strings from the input and hands them to the parser to figure out. The lexer has no brains, it's an expert at chopping up input. (Great post on this.)

The cases I worried about? The lexer does not know how to chop up the first one ("disd6"): it matches no token pattern. The parser does not understand the tokens in the second one ("1 + 3 d6"): it expects an arithmetic operator between "3" and "d6". My grammar already did the right thing.

Had I started with tests for these cases, instead of jumping right to coding, I would have not spent time messing with whitespace in the grammar. Coding first cost time rather than saved it.

Enlightenment by coding Kōan

Write tests first! No, really. Even when you "know" the solution, write tests first. There are many ways it saves time in the long run, it helps you think about your API, and it can save time in the short run. My dice expression example is not unique: test-first avoids unneeded work.

Epilogue

What does the dice expression implementation look like?

public final class DiceExpression {
    private final ExprContext expression;

    @Nonnull
    public static DiceExpression valueOf(@Nonnull final String expression) {
        final DiceExpressionLexer lexer = new DiceExpressionLexer(
                new ANTLRInputStream(expression));
        final DiceExpressionParser parser = new DiceExpressionParser(
                new CommonTokenStream(lexer));
        parser.setErrorHandler(new BailErrorStrategy());

        return new DiceExpression(parser.expr());
    }

    private DiceExpression(final ExprContext expression) {
        this.expression = expression;
    }

    public int evalWith(final Roll roll) {
        return expression.accept(new EvalWithVisitor(roll));
    }

    private static final class EvalWithVisitor
            extends DiceExpressionBaseVisitor<Integer> {
        private final Roll roll;

        public EvalWithVisitor(final Roll roll) {
            this.roll = roll;
        }

        @Override
        public Integer visitOpExpr(final OpExprContext ctx) {
            final int left = visit(ctx.left);
            final int right = visit(ctx.right);
            switch (ctx.op.getText().charAt(0)) {
            case '+': return left + right;
            case '-': return left - right;
            case '*': return left * right;
            case '/': return left / right;
            }
            throw new ArithmeticException(
                    "Unknown operator: " + ctx.getText());
        }

        @Override
        public Integer visitGroupExpr(final GroupExprContext ctx) {
            return visit(ctx.group);
        }

        @Override
        public Integer visitDiceExpr(final DiceExprContext ctx) {
            return Dice.valueOf(ctx).evalWith(roll);
        }

        @Override
        public Integer visitNumberExpr(final NumberExprContext ctx) {
            return Integer.valueOf(ctx.number.getText());
        }
    }
}

And Dice, overly complicated until I can think through Advantage/Disadvantage further:

public class Dice
        implements Comparable<Dice> {
    @FunctionalInterface
    public interface Roll {
        int roll(final int sides);

        static Roll die() {
            final Random random = new Random();
            return sides -> random.nextInt(sides) + 1;
        }
    }

    public enum Advantage {
        DIS(2, Math::min),
        NONE(1, (a, b) -> a),
        ADV(2, Math::max);

        private final int rolls;
        private final IntBinaryOperator op;

        Advantage(final int rolls, final IntBinaryOperator op) {
            this.rolls = rolls;
            this.op = op;
        }

        public final int evalWith(final Roll roll, final Dice dice) {
            return range(0, rolls).
                    map(n -> dice.rollWith(roll)).
                    reduce(op).
                    getAsInt();
        }

        public final String toString(final Dice dice) {
            switch (this) {
            case NONE:
                return dice.stringOf();
            default:
                return name().toLowerCase() + ' ' + dice.stringOf();
            }
        }
    }

    private final Optional<Integer> number;
    private final int sides;
    private final Advantage advantage;

    @Nonnull
    public static Dice valueOf(@Nonnull final String expression) {
        final DiceExpressionLexer lexer = new DiceExpressionLexer(
                new ANTLRInputStream(expression));
        final DiceExpressionParser parser = new DiceExpressionParser(
                new CommonTokenStream(lexer));
        parser.setErrorHandler(new BailErrorStrategy());

        return valueOf((DiceExprContext) parser.expr());
    }

    @Nonnull
    static Dice valueOf(@Nonnull final DiceExprContext ctx) {
        final Optional<Integer> number = Optional.ofNullable(ctx.number).
                map(Token::getText).
                map(Integer::valueOf);
        final Integer sides = Integer.valueOf(ctx.sides.getText());
        final Advantage advantage = Optional.ofNullable(ctx.van).
                map(Token::getText).
                map(String::toUpperCase).
                map(Advantage::valueOf).
                orElse(NONE);
        return new Dice(number, sides, advantage);
    }

    public Dice(@Nonnull final Optional<Integer> number, final int sides,
            final Advantage advantage) {
        this.number = number;
        this.sides = sides;
        this.advantage = advantage;
    }

    public Dice(@Nullable final Integer number, final int sides,
            final Advantage advantage) {
        this(Optional.ofNullable(number), sides, advantage);
    }

    public Dice(@Nullable final Integer number, final int sides) {
        this(Optional.ofNullable(number), sides, NONE);
    }

    public int number() {
        return number.orElse(1);
    }

    public int sides() {
        return sides;
    }

    public Advantage advantage() {
        return advantage;
    }

    public int evalWith(final Roll roll) {
        return advantage.evalWith(roll, this);
    }

    protected int rollWith(final Roll roll) {
        return rangeClosed(1, number()).
                map(ignored -> roll.roll(sides())).
                sum();
    }

    @Override
    public int compareTo(@Nonnull final Dice that) {
        // Sort by roll average - adv/dis messes this up though
        final int compare = Integer.compare(number() + (sides() + 1),
                that.number() * (that.sides() + 1));
        return 0 == compare ? advantage().compareTo(that.advantage())
                : compare;
    }

    @Override
    public String toString() {
        return advantage.toString(this);
    }

    protected String stringOf() {
        return number.
                map(number -> number + "d" + sides()).
                orElse("d" + sides());
    }

    @Override
    public boolean equals(final Object o) {
        if (this == o)
            return true;
        if (o == null || getClass() != o.getClass())
            return false;

        final Dice that = (Dice) o;

        return sides() == that.sides() && number() == that.number()
                && advantage() == that.advantage();
    }

    @Override
    public int hashCode() {
        return 31 * (31 * sides() + number()) * advantage().hashCode();
    }
}
UPDATE: If you wonder about the parser visitor, you have to explicitly enable it in ANTLR4 (even though it is recommended over tree rewriting). Use the command line flag, or in Maven

<plugin>
    <groupId>org.antlr</groupId>
    <artifactId>antlr4-maven-plugin</artifactId>
    <version>${antlr4.version}</version>
    <configuration>
        <visitor>true</visitor>
    </configuration>
    <executions>
        <execution>
            <id>antlr4</id>
            <goals>
                <goal>antlr4</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Friday, December 26, 2014

CompletableFuture and ExecutorService

Introduction

CompletableFuture was one of the "small gifts" in Java 8. It is a clever class but not well-integrated into the rest of the JDK. Particularly, ExecutorService still returns Futures rather than CompletableFutures. No class in the JDK references completable futures.

The other odd thing about CompletableFuture is that methods such as get() declare throwing InterruptedException but do not do so except under narrow circumstances: tasks which are interrupted and themselves throw InterruptedException have those exceptions wrapped by ExecutionException, making is difficult to handle interrupts in a general way. This is "baked into" the API, which provides only static factory methods accepting Runnable or Supplier (e.g., supplyAsync), and clashes with standard ExecutorService implementations.

Oddly the source for CompletableFuture shows interrupts could have been addressed in a straight-forward way:

public T get() throws InterruptedException, ExecutionException {
    Object r; Throwable ex, cause;
    if ((r = result) == null && (r = waitingGet(true)) == null)
        throw new InterruptedException();
    if (!(r instanceof AltResult)) {
        @SuppressWarnings("unchecked") T tr = (T) r;
        return tr;
    }
    if ((ex = ((AltResult)r).ex) == null)
        return null;
    if (ex instanceof CancellationException)
        throw (CancellationException)ex;
    // Hypothetical approach to exposing interrupts, NOT in the JDK:
    // if (ex instanceof InterruptedException)
    //     throw (InterruptedException)ex;
    if ((ex instanceof CompletionException) &&
        (cause = ex.getCause()) != null)
        ex = cause;
    throw new ExecutionException(ex);
}

I suspect there is some deeper interaction I am missing that such an easy solution was avoided. (This also shows off nicely the new ability in Java 8 to annotate assignments.)

That I can tell CompletableFuture was modeled on other libraries and languages, especially Guava's SettableFuture and Akka's Promise (formerly named CompletableFuture). Tomasz Nurkiewicz points out the considerable value-add in the Java 8 variant. Koji Lin provides the slides.

Solution

Let's integrate CompletableFuture into ExecutorService.

The natural approach is to extend ExecutorService, overriding methods which return Future to return CompletableFuture (covariant return from Java 5). This means updating methods which construct or return ExecutorService to return, say, CompletableExecutorService. My ideal solution uses a non-existent Java language feature, assignment to this for delegation (alas not in this timeline). A practical solution is mixins. So let's write that:

public interface CompletableExecutorService extends ExecutorService {
    /**
     * @return a completable future representing pending completion of the
     * task, never missing
     */
    @Nonnull
    @Override
    <T> CompletableFuture<T> submit(@Nonnull final Callable<T> task);

    /**
     * @return a completable future representing pending completion of the
     * task, never missing
     */
    @Nonnull
    @Override
    <T> CompletableFuture<T> submit(@Nonnull final Runnable task,
            @Nullable final T result);

    /**
     * @return a completable future representing pending completion of the
     * task, never missing
     */
    @Nonnull
    @Override
    CompletableFuture<?> submit(@Nonnull final Runnable task);
}

A static factory method turns any ExecutorService into a CompletableExecutorService:

@Nonnull
public static CompletableExecutorService completable(
        @Nonnull final ExecutorService threads) {
    return newMixin(CompletableExecutorService.class,
            new Overrides(threads), threads);
}

The grunt work is in Overrides:

public static final class Overrides {
    private final ExecutorService threads;

    private Overrides(final ExecutorService threads) {
        this.threads = threads;
    }

    @Nonnull
    public <T> CompletableFuture<T> submit(
            @Nonnull final Callable<T> task) {
        final CompletableFuture<T> cf = new UnwrappedCompletableFuture<>();
        threads.submit(() -> {
            try {
                cf.complete(task.call());
            } catch (final CancellationException e) {
                cf.cancel(true);
            } catch (final Exception e) {
                cf.completeExceptionally(e);
            }
        });
        return cf;
    }

    @Nonnull
    public <T> CompletableFuture<T> submit(@Nonnull final Runnable task,
            @Nullable final T result) {
        return submit(callable(task, result));
    }

    @Nonnull
    public CompletableFuture<?> submit(@Nonnull final Runnable task) {
        return submit(callable(task));
    }
}

What is UnwrappedCompletableFuture? It handles the pesky issue mentioned above with interrupts:

private static final class UnwrappedCompletableFuture<T>
        extends CompletableFuture<T> {
    @Override
    public T get() throws InterruptedException, ExecutionException {
        return UnwrappedInterrupts.<T, RuntimeException>unwrap(super::get);
    }

    @Override
    public T get(final long timeout, final TimeUnit unit)
            throws InterruptedException, ExecutionException,
            TimeoutException {
        return UnwrappedInterrupts.<T, TimeoutException>unwrap(
                () -> super.get(timeout, unit));
    }

    @FunctionalInterface
    private interface UnwrappedInterrupts<T, E extends Exception> {
        T get() throws InterruptedException, ExecutionException, E;

        static <T, E extends Exception> T unwrap(
                final UnwrappedInterrupts<T, E> wrapped)
                throws InterruptedException, ExecutionException, E {
            try {
                return wrapped.get();
            } catch (final ExecutionException e) {
                final Throwable cause = e.getCause();
                if (cause instanceof InterruptedException)
                    throw (InterruptedException) cause;
                throw e;
            }
        }
    }
}

Thursday, December 18, 2014

Blog code 0.5

I've published to Maven Central a set of Java jars capturing code and ideas from this blog and Internet reading. The maven coordinates are hm.binkley:*:0.5. Other vital statistics:

I still need to update the javadoc pages hosted by GitHub. I'm particularly happy to have finally worked out how to make a lombok processor.

Wednesday, December 17, 2014

Where I fit in

While reading on how to improve recruiting for Macquarie, I ran across an interesting job candidate description. Not a particular applicant, a description of a type of applicant: Five Tips to Hiring a Generalizing Specialist. Apparently there is a name for people like me. (More at Generalizing Specialists: Improving Your IT Career Skills.)

My career path has been atypical. I graduated a with a degree in classical music and jumped into programming out of need. I was very fortunate to have smart, capable friends who recommended the right books. And I wound up one of those, a "generalizing specialist".

It makes for a non-linear work life, which is challenging, and leads to opportunities less available otherwise. It is never dull.

Wednesday, December 10, 2014

Java validation

Martin Fowler posted Replacing Throwing Exceptions with Notification in Validations discussing alternatives to data validation than throwing exceptions. There are off-the-shelf solutions such as Commons Validator (XML driven) or Bean Validation (annotation driven) which are complete frameworks.

There is more to these frameworks than I suggest, but to explore Fowler's post better I quickly coded up my own simple-minded approach:

public final class ValidationMain {
    public static void main(final String... args) {
        final Notices notices = new Notices();
        notices.add("Something went horribly wrong %d time(s)", 1);
        try {
            foo(null);
        } catch (final Exception e) {
            notices.add(e);
        }
        notices.forEach(err::println);
        notices.fail(IllegalArgumentException::new);
    }

    public static String foo(final Object missing) {
        return missing.toString();
    }
}

Output on stderr:

lab.Notices$Notice@27f8302d
lab.Notices$Notice@4d76f3f8
Exception in thread "main" java.lang.IllegalArgumentException: 2 notices:
 Something went horribly wrong 1 time(s)
 at lab.ValidationMain.main(ValidationMain.java:21)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:483)
 at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
 Suppressed: java.lang.IllegalArgumentException: Only 1 reason(s)
  at lab.ValidationMain.main(ValidationMain.java:14)
 Suppressed: java.lang.IllegalArgumentException
  at lab.ValidationMain.main(ValidationMain.java:18)
 Caused by: java.lang.NullPointerException
  at lab.ValidationMain.foo(ValidationMain.java:25)
  at lab.ValidationMain.main(ValidationMain.java:16)

And the Notices class:

public final class Notices
        implements Iterable<Notice> {
    private final List<Notice> notices = new ArrayList<>(0);

    public void add(final String reason, final Object... args) {
        // Duplicate code so stack trace keeps same structure
        notices.add(new Notice(null, reason, args));
    }

    public void add(final Throwable cause) {
        // Duplicate code so stack trace keeps same structure
        notices.add(
                new Notice(cause, null == cause ? null : cause.getMessage()));
    }

    public void add(final Throwable cause, final String reason,
            final Object... args) {
        // Duplicate code so stack trace keeps same structure
        notices.add(new Notice(cause, reason, args));
    }

    public <E extends Exception> void fail(
            final BiFunction<String, Throwable, E> ctor)
            throws E {
        final E e = ctor.apply(toString(), null);
        notices.forEach(n -> e.addSuppressed(n.as(ctor)));
        final List<StackTraceElement> frames = asList(e.getStackTrace());
        // 2 is the magic number: lambda, current
        e.setStackTrace(frames.subList(2, frames.size())
                .toArray(new StackTraceElement[frames.size() - 2]));
        throw e;
    }

    @Override
    public Iterator<Notice> iterator() {
        return unmodifiableList(notices).iterator();
    }

    @Override
    public String toString() {
        if (notices.isEmpty())
            return "0 notices";
        final String sep = lineSeparator() + "\t";
        return notices.stream().
                map(Notice::reason).
                filter(Objects::nonNull).
                collect(joining(sep,
                        format("%d notices:" + sep, notices.size()), ""));
    }

    public static final class Notice {
        // 4 is the magic number: Thread, init, init, addError, finally the
        // user code
        private final StackTraceElement location = currentThread()
                .getStackTrace()[4];
        private final Throwable cause;
        private final String reason;
        private final Object[] args;

        private Notice(final Throwable cause, final String reason,
                final Object... args) {
            this.cause = cause;
            this.reason = reason;
            this.args = args;
        }

        public Throwable cause() {
            return cause;
        }

        public String reason() {
            return null == reason ? null : format(reason, args);
        }

        private <E extends Exception> E as(
                final BiFunction<String, Throwable, E> ctor) {
            final E e = ctor.apply(reason(), cause);
            e.setStackTrace(new StackTraceElement[]{location});
            return e;
        }
    }
}

Comments:

  • I manipulate the stack traces to focus on the caller's point of view. This is the opposite of, say, Spring Framework.
  • I haven't decided on what an intelligent toString() should look like for Notice
  • Java 8 lambdas really shine here. Being able to use exception constructors as method references is a win.

Saturday, December 06, 2014

Writing your own lombok annotation

It took me quite a while to get around to writing my own lombok annotation and processor. This took more effort than I expected, hopefully this post will save someone else some.

tl;dr — Look at the source in my Github repo.

Motivation

Reading the excellent ExecutorService - 10 tips and tricks by Tomasz Nurkiewicz, I thought about tip #2, Switch names according to context, which recommends wrapping important methods and code blocks with custom thread names to aid in logging and debugging.

"This is a great use case for annotations!" I thought. The code screams boilerplate:

public void doNiftyThings() {
    final Thread thread = Thread.currentThread();
    final String oldName = thread.getName();
    thread.setName("Boy this is nifty!");
    try {
        // Do those nifty things - the actual work
    } finally {
        thread.setName(oldName);
    }
}

The whole point of the method is indented out of focus, wrapped with bookkeeping. I'd rather write this:

@ThreadNamed("Boy this is nifty!")
public void doNiftyThings() {
    // Do those nifty thing - the actual work
}

Bonus: simple text search finds those places in my code where I change the thread name based on context.

Writing the annotation

Ok, let's make this work. I started with cloning the @Cleanup annotation and processor, and editing from there. First the annotation, the easy bit. I include the javadoc to emphasize the importance of documenting your public APIs.

/**
 * {@code ThreadNamed} sets the thread name during method execution, restoring
 * it when the method completes (normally or exceptionally).
 *
 * @author <a href="mailto:binkley@alumni.rice.edu">B. K. Oxley (binkley)</a>
 */
@Documented
@Retention(SOURCE)
@Target({CONSTRUCTOR, METHOD})
public @interface ThreadNamed {
    /** The name for the thread while the annotated method executes. */
    String value();
}

Nothing special here. I've made the decision to limit the annotation to methods and constructors. Ideally I'd include blocks but that isn't an option (yet) in Java, and you can always refactor out a block to a method.

Writing the processor

This is the serious part. First some preliminaries:

  1. I have only implemented support for JDK javac. Lombok also supports the Eclipse compiler, which requires a separate processor class. I have nothing against Eclipse, but it's not in my toolkit.
  2. I'll discuss library dependencies below. For now pretend these are already working for you.
  3. I'm a big fan of static imports, the diamond operator, etc. I don't like retyping what the compiler is already thinking. You should note List below is not java.util.List; it's com.sun.tools.javac.util.List. Yeah, I don't know this class either.
  4. The implementation is hard to follow. Most of us don't spend much time with expression trees, which is how most compilers (including javac) see your source code. A language like LISP lets you write you code as the expression tree directly, which is both nifty and challenging (macros being like annotation processors).

Without further ado:

/**
 * Handles the {@code lombok.ThreadNamed} annotation for javac.
 */
@MetaInfServices(JavacAnnotationHandler.class)
@HandlerPriority(value = 1024)
// 2^10; @NonNull must have run first, so that we wrap around the
// statements generated by it.
public class HandleThreadNamed
        extends JavacAnnotationHandler<ThreadNamed> {
    /**
     * lombok configuration: {@code lab.lombok.threadNamed.flagUsage} = {@code
     * WARNING} | {@code ERROR}.
     * <p>
     * If set, <em>any</em> usage of {@code @ThreadNamed} results in a warning
     * / error.
     */
    public static final ConfigurationKey<FlagUsageType>
            THREAD_NAMED_FLAG_USAGE = new ConfigurationKey<FlagUsageType>(
            "lab.lombok.threadNamed.flagUsage",
            "Emit a warning or error if @ThreadNamed is used.") {
    };

    @Override
    public void handle(final AnnotationValues<ThreadNamed> annotation,
            final JCAnnotation ast, final JavacNode annotationNode) {
        handleFlagUsage(annotationNode, THREAD_NAMED_FLAG_USAGE,
                "@ThreadNamed");

        deleteAnnotationIfNeccessary(annotationNode, ThreadNamed.class);
        final String threadName = annotation.getInstance().value();
        if (threadName.isEmpty()) {
            annotationNode.addError("threadName cannot be the empty string.");
            return;
        }

        final JavacNode owner = annotationNode.up();
        switch (owner.getKind()) {
        case METHOD:
            handleMethod(annotationNode, (JCMethodDecl) owner.get(),
                    threadName);
            break;
        default:
            annotationNode.addError(
                    "@ThreadNamed is legal only on methods and constructors"
                            + ".");
            break;
        }
    }

    public void handleMethod(final JavacNode annotation,
            final JCMethodDecl method, final String threadName) {
        final JavacNode methodNode = annotation.up();

        if ((method.mods.flags & Flags.ABSTRACT) != 0) {
            annotation.addError(
                    "@ThreadNamed can only be used on concrete methods.");
            return;
        }

        if (method.body == null || method.body.stats.isEmpty()) {
            generateEmptyBlockWarning(annotation, false);
            return;
        }

        final JCStatement constructorCall = method.body.stats.get(0);
        final boolean isConstructorCall = isConstructorCall(constructorCall);
        List<JCStatement> contents = isConstructorCall
                ? method.body.stats.tail : method.body.stats;

        if (contents == null || contents.isEmpty()) {
            generateEmptyBlockWarning(annotation, true);
            return;
        }

        contents = List
                .of(buildTryFinallyBlock(methodNode, contents, threadName,
                        annotation.get()));

        method.body.stats = isConstructorCall ? List.of(constructorCall)
                .appendList(contents) : contents;
        methodNode.rebuild();
    }

    public void generateEmptyBlockWarning(final JavacNode annotation,
            final boolean hasConstructorCall) {
        if (hasConstructorCall)
            annotation.addWarning(
                    "Calls to sibling / super constructors are always "
                            + "excluded from @ThreadNamed;"
                            + " @ThreadNamed has been ignored because there"
                            + " is no other code in " + "this constructor.");
        else
            annotation.addWarning(
                    "This method or constructor is empty; @ThreadNamed has "
                            + "been ignored.");
    }

    public JCStatement buildTryFinallyBlock(final JavacNode node,
            final List<JCStatement> contents, final String threadName,
            final JCTree source) {
        final String currentThreadVarName = "$currentThread";
        final String oldThreadNameVarName = "$oldThreadName";

        final JavacTreeMaker maker = node.getTreeMaker();
        final Context context = node.getContext();

        final JCVariableDecl saveCurrentThread = createCurrentThreadVar(node,
                maker, currentThreadVarName);
        final JCVariableDecl saveOldThreadName = createOldThreadNameVar(node,
                maker, currentThreadVarName, oldThreadNameVarName);

        final JCStatement changeThreadName = setThreadName(node, maker,
                maker.Literal(threadName), currentThreadVarName);
        final JCStatement restoreOldThreadName = setThreadName(node, maker,
                maker.Ident(node.toName(oldThreadNameVarName)),
                currentThreadVarName);

        final JCBlock tryBlock = setGeneratedBy(maker.Block(0, contents),
                source, context);
        final JCTry wrapMethod = maker.Try(tryBlock, nil(),
                maker.Block(0, List.of(restoreOldThreadName)));

        if (inNetbeansEditor(node)) {
            //set span (start and end position) of the try statement and
            // the main block
            //this allows NetBeans to dive into the statement correctly:
            final JCCompilationUnit top = (JCCompilationUnit) node.top()
                    .get();
            final int startPos = contents.head.pos;
            final int endPos = Javac
                    .getEndPosition(contents.last().pos(), top);
            tryBlock.pos = startPos;
            wrapMethod.pos = startPos;
            Javac.storeEnd(tryBlock, endPos, top);
            Javac.storeEnd(wrapMethod, endPos, top);
        }

        return setGeneratedBy(maker.Block(0,
                        List.of(saveCurrentThread, saveOldThreadName,
                                changeThreadName, wrapMethod)), source,
                context);
    }

    private static JCVariableDecl createCurrentThreadVar(final JavacNode node,
            final JavacTreeMaker maker, final String currentThreadVarName) {
        return maker.VarDef(maker.Modifiers(FINAL),
                node.toName(currentThreadVarName),
                genJavaLangTypeRef(node, "Thread"), maker.Apply(nil(),
                        genJavaLangTypeRef(node, "Thread", "currentThread"),
                        nil()));
    }

    private static JCVariableDecl createOldThreadNameVar(final JavacNode node,
            final JavacTreeMaker maker, final String currentThreadVarName,
            final String oldThreadNameVarName) {
        return maker.VarDef(maker.Modifiers(FINAL),
                node.toName(oldThreadNameVarName),
                genJavaLangTypeRef(node, "String"),
                getThreadName(node, maker, currentThreadVarName));
    }

    private static JCMethodInvocation getThreadName(final JavacNode node,
            final JavacTreeMaker maker, final String currentThreadVarNAme) {
        return maker.Apply(nil(),
                maker.Select(maker.Ident(node.toName(currentThreadVarNAme)),
                        node.toName("getName")), nil());
    }

    private static JCStatement setThreadName(final JavacNode node,
            final JavacTreeMaker maker, final JCExpression threadName,
            final String currentThreadVarName) {
        return maker.Exec(maker.Apply(nil(),
                maker.Select(maker.Ident(node.toName(currentThreadVarName)),
                        node.toName("setName")), List.of(threadName)));
    }
}

Wasn't that easy?

Dependencies

Of course the code depends on lombok. I'm using version 1.14.8. It also needs tools.jar from the JDK for compiler innards like expression trees. (An Eclipse processor needs an equivalent.)

Unfortunately lombok itself uses "mangosdk" to generate a META-INF/services/lombok.javac.JavacAnnotationHandler file for autodiscovery of processors. I say 'unfortunately' because this library is not in maven and is unsupported. Happyily Kohsuke Kawaguchi wrote the excellent metainf-services library a while back, maintains it, and publishes to Maven central. If you're new to annotation processors it's a good project to learn from.

Conclusion

Ok, that was not actually so easy. On the other hand, finding a starting point was the biggest hurdle for me in writing a lombok annotation. Please browse my source and try your hand at one.

UPDATE — A little bonus. This code:

@ThreadNamed("Slot #%2$d")
public void doSomething(final String name, final int slot) {
    // Do something with method params
}

Produces the thread name "Slot #2" when called with "Foo", 2. Strings without formatting or methods with params treat the annotation value as a plain string.

Monday, December 01, 2014

The ORM anti-pattern

Yegor Bugayenko writes on ORM Is an Offensive Anti-Pattern, offering a pure OO alternative.

(I've posted too many links to my Google+ account. It's a ghetto over there as my public posts are science-oriented, programming is shared privately.)

Thursday, November 13, 2014

Aiming for the right level

Vinod Kumaar Ramakrishnan writes It is just a road not a roadmap making a strong point visually: software needs a map, not a road. Roadmap

This is important to understand at any level of an organization. The problem comes as you perform larger roles.

You can track only so much detail—minds have a capacity. Using a Road rather than a Map overcomes this for viewing lower down the organization. Pull back your view and substitutes rough pictures for details. Then build a bigger map, less granular, covering a larger area. It's still a map, but a map of a country rather than a region or a place.

Managing calls for finding the right level of detail, be it the application or team or project or programme or department. But remember to keep rebuilding your map and explore some.

Sunday, August 31, 2014

Overcoming Java 8 streams

Java 8 streams provide functional and LINQ-like features in a fluent API. But streams are not without drawbacks:

  • Referenced methods and lambdas cannot throw checked exceptions
  • Controlling the threads used, especially for parallel streams, is awkward
  • Streams are not designed for extension

Overcoming these drawbacks requires a "look-a-like" API. For example, implementing java.util.stream.Stream does not help: none of the existing methods throw checked exceptions, and none of the existing stream factory helpers would return your implementation with new methods.

So I wrote my own, copying the existing stream API, updating the methods to throw checked exceptions:

hm.binkley.util.stream.CheckedStream ('develop' branch for now)

From the javadoc:

CheckedStream is a throwing Stream look-a-like with control over thread pool. It cannot be a Stream as it takes throwing versions of suppliers, functions and consumers. Otherwise it is a faithful reproduction.

Write this:

   long beanCount() throws SomeException, OtherException {
       checked(Stream.of(1, 2, 3)).
           map(this::someThrowingFunction).
           peek(That::oldBean).
           count();
   }

not this:

   long beanCount() throws SomeException, OtherException {
       try {
           Stream.of(1, 2, 3).
               map(i -> {
                   try {
                       someThrowingFunction(i);
                   } catch (final SomeException e) {
                       throw new RuntimeException(e);
                   }
               }).
               peek(i -> {
                   try {
                       That.oldBean(i);
                   } catch (final OtherException e) {
                       throw new RuntimeException(e);
                   }
               }).
               count();
       } catch (final RuntimeException e) {
           final Throwable x = e.getCause();
           if (x instanceof SomeException)
               throw (SomeException) x;
           if (x instanceof OtherException)
               throw (OtherException) x;
           throw e;
       }
   }

"Intentional" exceptions (checked exceptions plus CancellationException) have "scrubbed" stacktraces: frames from framework/glue packages are removed before the intentional exception is rethrown to calling code. Scrubbed stacktraces are much easier to understand, the framework and glue code having been removed.

To see the unscrubbed stacktrace, set the system property "hm.binkley.util.stream.CheckedStream.debug" to "true".

Controlling the thread pool used by Stream is a challenge. Deep in the implementation, it checks if being run in a ForkJoinTask, and uses that thread if so, otherwise using the common pool. So with CheckedStream write this:

       checked(stream, new ForkJoinPool()).
           map(currentThread()).
           forEach(System.out::println);

not this:

       try {
           new ForkJoinPool().submit(() -> stream.
                   map(currentThread()).
                   forEach(System.out::println)).
               get();
       } catch (final ExecutionException e) {
           final Throwable x = e.getCause();
           if (x instanceof Error)
               throw (Error) x;
           if (x instanceof RuntimeException)
               // Much tricker when stream functions throw runtime
               throw (RuntimeException) x;
           throw new Error(e); // We have no checked exceptions in this example
       }

Care is taken to respect lazy and terminal operations in using thread pools. Changing thread pool or thread mode mid-stream is supported, and are "immediate" operations: they terminate the existing stream, and start a new one with the changes:

stream.sequential().
    filter(this::someFilter).
    parallel(threads). // Existing lazy operations terminated
    map(this:someMapper).
    forEach(System.out::println);

Immediate operations ensure stream methods are run in the correct threading context.

I hope you'll agree: CheckedStream is nicer to use, especially with existing code using checked exceptions.

Suggestions, bug fixes, improvements welcome!

Tuesday, July 15, 2014

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 3rd-party exceptions into a common framework exception, a nice feature for APIs to simplify calling code. Copy helps reduce boilerplate code.