Very Simple CompletableFuture Example and Descriptions
There are a lot of great examples out there when it comes to Java8 CompletableFuture but they all seem to miss some basic stuff for those moving from other ways they have done it. For me it was moving F.Promise to CompletableFuture in the Play Framework.
This example pretty much answers a lot of basic exception handling questions like;
- Why I cannot throw an exception within lambda?
- How can I really interrupt the chain of future operations if I cannot throw an exception?
- What do Exceptionally do, really?
I am sure there will be fancier and more correct way of doing things, but after reading this document and playing with the provided sample code, you should be able to get-by in moving from your current future implementation to the new CompletableFuture framework.
The supplied main.java contains the playground code you can tweak to learn.
The CompletableFuture containing function should have throws InterruptedException in function declaration. Like;
void go(final String v) throws InterruptedException {...}
When we are just cutting and pasting the code to move F.Promise to CompatibleFuture we would run into situations where the compiler warns of Unhandled Exception.
CompletableFuture<String> cf = CompletableFuture.supplyAsync(() -> {
try {
if (v == "barf") {
throw new Exception("Barfing as `barf` is the parameter. ");
}
return "Did not barf : Given: " + v;
} catch (Exception ex) {
// If you re-throw the exception as RunTimException, you can throw an exception in
// a lambda
throw new RuntimeException(ex);
}
}
);
This can be fixed by re-throwing a caught exception with RunTimeException(). This is the key number 1.
If the function "barfs" with an exception, and if .exceptionally is not supplied, it will trigger the ExecutionException and the completed future never returns a thing.
String res = "never done";
try {
res = cf
.thenApply(v2 -> {
System.out.print(v2);
return v2;
})
.get();
} catch (ExecutionException ex) {
System.out.printf("CF execution exception %s", ex.toString());
}
Here is an example of .exceptionally. Also note that cf.isCompletedExceptionally() can tell if it has encountered an issue in the chain. Note that in .exceptionally part I recover from the issue by returning a string.
try {
res = cf
.thenApply(v2 -> {
System.out.print(v2);
return v2;
})
.exceptionally(ex -> {
System.out.print("Exceptionally " + ex.toString());
return "Exceptionally handled.";
})
.get();
} catch (ExecutionException ex) {
System.out.printf("CF execution exception %s", ex.toString());
}
if (cf.isCompletedExceptionally()) {
System.out.print("\nIt was CompletedExceptionally");
}