Thursday, December 25, 2014

CompletableFuture and ExecutorService


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.


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
    <T> CompletableFuture<T> submit(@Nonnull final Callable<T> task);

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

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

A static factory method turns any ExecutorService into a CompletableExecutorService:

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;

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

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

    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> {
    public T get() throws InterruptedException, ExecutionException {
        return UnwrappedInterrupts.<T, RuntimeException>unwrap(super::get);

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

    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;

Wednesday, December 17, 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.

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.

Tuesday, December 09, 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 {
        } catch (final Exception e) {

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

Output on stderr:

Exception in thread "main" java.lang.IllegalArgumentException: 2 notices:
 Something went horribly wrong 1 time(s)
 at lab.ValidationMain.main(
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(
 at java.lang.reflect.Method.invoke(
 at com.intellij.rt.execution.application.AppMain.main(
 Suppressed: java.lang.IllegalArgumentException: Only 1 reason(s)
  at lab.ValidationMain.main(
 Suppressed: java.lang.IllegalArgumentException
  at lab.ValidationMain.main(
 Caused by: java.lang.NullPointerException
  at lab.ValidationMain.main(

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
                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(;
        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;

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

    public String toString() {
        if (notices.isEmpty())
            return "0 notices";
        final String sep = lineSeparator() + "\t";
                        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()
        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;


  • 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.

Friday, December 05, 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.


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 {

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="">B. K. Oxley (binkley)</a>
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 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.
@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>(
            "Emit a warning or error if @ThreadNamed is used.") {

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

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

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

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

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

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

        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);

        contents = List
                .of(buildTryFinallyBlock(methodNode, contents, threadName,

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

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

        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)
            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,

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

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

    private static JCMethodInvocation getThreadName(final JavacNode node,
            final JavacTreeMaker maker, final String currentThreadVarNAme) {
        return maker.Apply(nil(),
                        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(),
                        node.toName("setName")), List.of(threadName)));

Wasn't that easy?


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.


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.