How to handle exception in completableFuture in the for loop - exception

Question

I am new to completableFuture, what I am trying to do is I have this logic below in spring boot project which I am trying to convert it to the parallel processing approach using completableFuture.
#Transaction
void serviceMethod{
for(Object obj : objList) //objlist can have 10000 objects and its a multi level composite objects
{
//Get corresponding entity obj from the database, if not found throw user exception
//Process i.e. change some fields
}
}
In the above logic sinnce the method is annotated with #Transaction I am not calling JPA save explicitly to save the entity.
Now, I am trying to do parallel processing with the above logic.
#Transaction
void serviceMethod{
for(Object obj : objList) //objlist can have 10000 objects and its a multi level composite objects
{
//Get corresponding entity obj from the database, if not found throw user exception
CompletableFuture<Optional<Obj>> optionalObjFuture = CompletableFuture.supplyAsync( () -> {//get the object from repository})
CompletableFuture<Obj> objFuture = optionalObjFuture.thenApply( optionalObj -> {
if(obj.isPresent()){
return obj.get();
}
else{
throw user exception;
}
})
////Process i.e. change some fields
}
}
Now the question is
what is the approach I have to follow to break the for loop when there is an exception?
How to handle transaction in this scenario. Is there any way to handle transaction without the need to call saveAll on the processed objects stored in datastructure?

Answer 1

For (1), you could split those 10_000 calls into much smaller batches. For example:
public static void main(String[] args) throws Exception {
int innerIterations = 5;
ExecutorService service = Executors.newFixedThreadPool(innerIterations);
for(int i=0;i<10;++i) {
List<CompletableFuture<String>> list = new ArrayList<>();
int j = 0;
for(;j<innerIterations;++j) {
CompletableFuture<Optional<String>> cf = CompletableFuture.supplyAsync(() -> Optional.of("a"), service);
CompletableFuture<String> cf2 = cf.thenApplyAsync(x -> {
if(x.isPresent()) {
return x.get();
} else {
throw new RuntimeException("test");
}
}, Executors.newFixedThreadPool(2));
list.add(cf2);
}
i += j;
CompletableFuture<Void> all5 = CompletableFuture.allOf(list.toArray(new CompletableFuture[0]));
if(all5.isCompletedExceptionally()) {
// log exception, whatever
break;
} else {
List<String> set = list.stream().map(CompletableFuture::join).collect(Collectors.toList());
System.out.println(set);
}
}
}
This does a couple of things:
splits the initial 10 into two batches of 5.
call CompletableFuture::allOf on those 5 futures. Read the documentation to see that if at least one future fails from those 5, then all5 failed also.
if all5 did not fail (that is all5.isCompletedExceptionally() did not pass), call CompletableFuture::join to get all of the results. There will be no "joining" really, since the previous allOff already waited for them to be completed.
For the second question - you can't. You will need to create a transaction manually, but Spring makes it rather easy.