recover

Allow sending of one last element downstream when a failure has happened upstream.

Error handling

Signature

Source.recoverSource.recover Flow.recoverFlow.recover

Description

recover allows you to emit a final element and then complete the stream on an upstream failure. Deciding which exceptions should be recovered is done through a PartialFunction. If an exception does not have a matching case match defined the stream is failed.

Recovering can be useful if you want to gracefully complete a stream on failure while letting downstream know that there was a failure.

Throwing an exception inside recover will be logged on ERROR level automatically.

Reactive Streams semantics

Below example demonstrates how recover gracefully complete a stream on failure.

Scala

copysourceSource(0 to 6)
  .map(n =>
    // assuming `4` and `5` are unexpected values that could throw exception
    if (List(4, 5).contains(n)) throw new RuntimeException(s"Boom! Bad value found: $n")
    else n.toString)
  .recover {
    case e: RuntimeException => e.getMessage
  }
  .runForeach(println)

Java

copysourceSource.from(Arrays.asList(0, 1, 2, 3, 4, 5, 6))
    .map(
        n -> {
          // assuming `4` and `5` are unexpected values that could throw exception
          if (Arrays.asList(4, 5).contains(n))
            throw new RuntimeException(String.format("Boom! Bad value found: %s", n));
          else return n.toString();
        })
    .recover(
        PFBuilder.<Throwable, String>create()
            .match(RuntimeException.class, Throwable::getMessage)
            .build())
    .runForeach(System.out::println, system);

This will output:

Scala

copysource0
1
2
3                         // last element before failure
Boom! Bad value found: 4  // first element on failure

Java

copysource0
1
2
3                         // last element before failure
Boom! Bad value found: 4  // first element on failure

The output in the line before failure denotes the last successful element available from the upstream, and the output in the line on failure denotes the element returns by partial function when upstream is failed.