如何在异常处理中使用CompletableFuture和CompletionStage

Question

I am dealing with some code involving multiple blocking I/O operations to be performed in sequence. The context is a JVM based web application running on Java 8 and Tomcat. I have code that performs three operations in sequence. All are of the blocking nature, and the output of each operation may be utilized in a subsequent operation. Each operation can throw exceptions, either checked or unchecked. I have illustrated with some sample exceptions that can be thrown:

String output1 = callToRestAPI(); // throws MalformedUrlException, ConcurrentModificationException

callToDatabase(output1); // throws SQLException, ConcurrentModificationException

boolean output2 = callToSendEmail(output1); // throws MessagingException

How would I write this using Java 8's promising (no pun intended) CompletionStage and CompletableFuture?

Answer 1

Hope this helps.

import java.util.concurrent.CompletableFuture;

public class HandleException {

  public static void main(String[] args) {
    CompletableFuture.completedFuture(null)
        .thenApply(__ -> callToRestAPI())
        .thenApply(
            output1 -> {
              callToDatabase(output1);
              return output1;
            })
        .thenAccept(output1 -> callToSendEmail(output1))
        .exceptionally(
            t -> {
              System.out.println("Exception occurred: " + t.getMessage());
              return null;
            });
  }
}

thenApply and thenAccept is used when the code is blocking. thenApply is used when we want to return something to next thenXXX method. thenAccept does not return anything. You can also make callToRestAPI,callToDatabase as Async(non blocking code) using a runAsync or supplyAsync