Throwing exception from lambda

Question

Given this java 8 code

public Server send(String message) {
    sessions.parallelStream()
        .map(Session::getBasicRemote)
        .forEach(basic -> {
          try {
            basic.sendText(message);
          } catch (IOException e) {
            e.printStackTrace();
          }
        });

    return this;
}

how do we properly make this IOException be delegated up the stack of the method call? (in nutshell how to make this method throw this IOException ?)

Lambdas in java does not look very friendly to error handling...

score 12 · Accepted Answer · edited May 23 '17 at 12:08

My approach would be to sneakily throw it from the lambda, but take care to have the send method declare it in its throws clause. Using the Exceptional class I posted here:

public Server send(String message) throws IOException {
  sessions.parallelStream()
          .map(Session::getBasicRemote)
          .forEach(basic -> Exceptional.from(() -> basic.sendText(message)).get());
  return this;
}

This way you're effectively making the compiler "look away" for just a bit, disabling its exception checking at one spot in your code, but by declaring the exception on your send method, you restore the regular behavior for all its callers.

Oh now I like that. That is indeed very sneaky. – T.J. Crowder Jul 26 '15 at 14:39 — T.J. Crowder, Jul 26 '15 at 14:39

score 6 · Answer 2 · answered Jul 29 '15 at 16:32

6

I wrote an extension to the Stream API which allows for checked exceptions to be thrown.

public Server send(String message) throws IOException {
    ThrowingStream.of(sessions, IOException.class)
        .parallelStream()
        .map(Session::getBasicRemote)
        .forEach(basic -> basic.sendText(message));

    return this;
}

answered Jul 29 '15 at 16:32

Jeffrey

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thanks, now this is really nice interface :) will definitely check out your source code – vach Jul 30 '15 at 15:49

score 3 · Answer 3 · answered Jul 26 '15 at 14:11

3

The problem is indeed that all @FunctionalInterfaces used in lambdas do not allow exceptions to be thrown, save for unchecked exceptions.

One solution is using a package of mine; with it, your code can read:

sessions.parallelStream()
    .map(Session::getBasicRemote)
    .forEach(Throwing.consumer(basic -> basic.sendText(message)));
return this;

answered Jul 26 '15 at 14:11

fge

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How does that let `send` throw the exception? – T.J. Crowder Jul 26 '15 at 14:14
@MarkoTopolnik: I understand that. But a **lot** more work is required to then make `send` throw the `IOException`. If I'm not mistaken, you basically have to catch the `ThrownByLambdaException`, and then have a series of `instanceof` to figure out what the actual exception was so you can rethrow it. Maybe not that bad for one or two exceptions, but still pretty verbose. – T.J. Crowder Jul 26 '15 at 14:35
@T.J.Crowder To stay fair to our colleague fge (and as you discovered in our chat), `throwing-lambdas` does support the sneaky throwing idiom. – Marko Topolnik Jul 26 '15 at 15:46
2

@MarkoTopolnik: Although as far as I can tell, the above doesn't actually use it, nor was it explained (e.g., that if it does use a sneaky throw, the `throws` decl. on `send` is essential and the compiler won't force you to add it). – T.J. Crowder Jul 26 '15 at 15:54

Throwing exception from lambda

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