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Asynchronously handle exception and return a transformed future
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import static java.lang.String.format;
public class AsynchronousExceptionsHandlingWithHandle {
public static void main(final String[] args) throws InterruptedException, ExecutionException {
for (final boolean failure : new boolean[]{false, true}) {
CompletableFuture<Integer> x = CompletableFuture.supplyAsync(() -> {
if (failure) {
throw new RuntimeException("Oops, something went wrong");
}
return 42;
});
/**
* Returns a new CompletableFuture that, when this CompletableFuture completes either normally or exceptionally,
* is executed with this stage's result and exception as arguments to the supplied function.
*/
CompletableFuture<HttpResponse> tryX = x.handle((value, ex) -> { // Note that tryX and x are of different type.
if (value != null) {
// We get a chance to transform the result...
return new HttpResponse(200, format("{\"value\": %s}", value));
} else {
// ... or return details on the error using the ExecutionException's message:
return new HttpResponse(500, format("{\"error\": \"%s\"}", ex.getMessage()));
}
});
// Blocks (avoid this in production code!), and either returns the promise's value:
System.out.println(tryX.get());
System.out.println("isCompletedExceptionally = " + tryX.isCompletedExceptionally());
// Output[failure=false]: 200 - {"value": 42}
// Output[failure=false]: isCompletedExceptionally = false
// Output[failure=true]: 500 - {"error": "java.lang.RuntimeException: Oops, something went wrong"}
// Output[failure=true]: isCompletedExceptionally = false
}
}
private static class HttpResponse {
private final int status;
private final String body;
public HttpResponse(final int status, final String body) {
this.status = status;
this.body = body;
}
public String toString() { return status + " - " + body; }
}
}
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