private CompletableFuture<AggregatedMultipart> aggregate0(@Nullable EventExecutor executor,
                                                              @Nullable ByteBufAllocator alloc) {
        final BodyPartAggregator aggregator = new BodyPartAggregator(alloc);
        if (executor == null) {
            parts.subscribe(aggregator);
        } else {
            parts.subscribe(aggregator, executor);
        }
        return UnmodifiableFuture.wrap(
                aggregator.completionFuture.thenApply(parts -> AggregatedMultipart.of(boundary, parts)));
    }
Returns a random boundary used for encoding multipart messages.
core/src/main/java/com/linecorp/armeria/common/multipart/DefaultMultipart.java