FlowableEmitter fails to emit the items - rx-java2

Question

I have a program which polls 2 directories - dir1 and dir2 and emit the file as soon as the file arrives in any of the 2 directories. But when the files arrive at same time in both directories, some files from either of the directory is failed to get emitted.
The code is as follows:
Flowable.create((FlowableEmitter<Path> em) -> pollDirectory(Arrays.asList(dir1,dir2), em),
BackpressureStrategy.BUFFER).subscribe();
The method pollDirectory has the below code:
for (Path directory: pathList) {
FileAlterationObserver fao = new FileAlterationObserver(directory);
fao.addListener(new FileAlterationListenerImpl(emitter));
final FileAlterationMonitor monitor = new FileAlterationMonitor(5000);
monitor.addObserver(fao);
try {
monitor.start();
} catch (Exception e) {
handleException(e);
}
}
The FileAlterationListenerImpl class emits the path on file creation and code is as follows:
public class FileAlterationListenerImpl implements FileAlterationListener {
FlowableEmitter<Path> source;
public FileAlterationListenerImpl(FlowableEmitter<Path> emitter) {
super();
this.source = emitter;
}
#Override
public void onFileCreate(final File file) {
source.onNext(file.toPath());
}
}
Is there any way in RxJava to handle this scenario so that emitter emits files from both directories even if the files are arrived at same time?

Answer 1

FileAlterationObserver creates a thread that will trigger its emission thus if you have two or more, that will result in concurrent onNext invocation.
The javadoc indicates this is not safe and you need to use serialize() to ensure thread safety.
Flowable.create((FlowableEmitter<Path> em) ->
pollDirectory(Arrays.asList(dir1,dir2), em.serialize()),
BackpressureStrategy.BUFFER
)
.subscribe();