What is the best way to handle an ExecutionException?

Here's what I do in this situation. This accomplishes the following:

• Re-throws checked exceptions without wrapping them
• Glues together the stack traces

Code:

public <V> V waitForThingToComplete(Future<V> future) {
    boolean interrupted = false;
    try {
        while (true) {
            try {
                return future.get();
            } catch (InterruptedException e) {
                interrupted = true;
            }
        }
    } catch (ExecutionException e) {
        final Throwable cause = e.getCause();
        this.prependCurrentStackTrace(cause);
        throw this.<RuntimeException>maskException(cause);
    } catch (CancellationException e) {
        throw new RuntimeException("operation was canceled", e);
    } finally {
        if (interrupted)
            Thread.currentThread().interrupt();
    }
}

// Prepend stack frames from the current thread onto exception trace
private void prependCurrentStackTrace(Throwable t) {
    final StackTraceElement[] innerFrames = t.getStackTrace();
    final StackTraceElement[] outerFrames = new Throwable().getStackTrace();
    final StackTraceElement[] frames = new StackTraceElement[innerFrames.length + outerFrames.length];
    System.arraycopy(innerFrames, 0, frames, 0, innerFrames.length);
    frames[innerFrames.length] = new StackTraceElement(this.getClass().getName(),
      "<placeholder>", "Changed Threads", -1);
    for (int i = 1; i < outerFrames.length; i++)
        frames[innerFrames.length + i] = outerFrames[i];
    t.setStackTrace(frames);
}

// Checked exception masker
@SuppressWarnings("unchecked")
private <T extends Throwable> T maskException(Throwable t) throws T {
    throw (T)t;
}

Seems to work.

I've looked at this problem in depth, and it's a mess. There is no easy answer in Java 5, nor in 6 or 7. In addition to the clumsiness, verbosity and fragility that you point out, your solution actually has the problem that the ExecutionException that you are stripping off when you call getCause() actually contains most of the important stack trace information!

That is, all the stack information of the thread executing the method in the code you presented is only in the ExcecutionException, and not in the nested causes, which only cover frames starting at call() in the Callable. That is, your doSomethingWithTimeout method won't even appear in the stack traces of the exceptions you are throwing here! You'll only get the disembodied stack from the executor. This is because the ExecutionException is the only one that was created on the calling thread (see FutureTask.get()).

The only solution I know is complicated. A lot of the problem originates with the liberal exception specification of Callable - throws Exception. You can define new variants of Callable which specify exactly which exceptions they throw, such as:

public interface Callable1<T,X extends Exception> extends Callable<T> {

    @Override
    T call() throws X; 
}

This allows methods which executes callables to have a more precise throws clause. If you want to support signatures with up to N exceptions, you'll need N variants of this interface, unfortunately.

Now you can write a wrapper around the JDK Executor which takes the enhanced Callable, and returns an enhanced Future, something like guava's CheckedFuture. The checked exception type(s) are propagated at compile time from the creation and type of the ExecutorService, to the returned Futures, and end up on the getChecked method on the future.

That's how you thread the compile-time type safety through. This means that rather than calling:

Future.get() throws InterruptedException, ExecutionException;

You can call:

CheckedFuture.getChecked() throws InterruptedException, ProcessExecutionException, IOException

So the unwrapping problem is avoided - your method immediately throws the exceptions of the required type and they are available and checked at compile time.

Inside getChecked, however you still need to solve the "missing cause" unwrapping problem described above. You can do this by stitching the current stack (of the calling thread) onto the stack of the thrown exception. This a stretching the usual use of a stack trace in Java, since a single stack stretches across threads, but it works and is easy to understand once you know what is going on.

Another option is to create another exception of the same thing as the one being thrown, and set the original as the cause of the new one. You'll get the full stack trace, and the cause relationship will be the same as how it works with ExecutionException - but you'll have the right type of exception. You'll need to use reflection, however, and is not guaranteed to work, e.g., for objects with no constructor having the usual parameters.