Vertx merge contents of multiple files in single file - vert.x

Question

What is the best way to append the contents of multiple files into single file in vertx? I have tried vertx filesystem and asyncFile but both do not have a option to append file or I did not know of any. Is there any alternative approach to merge or append files in vertx asynchronously.
The only solution I could find is to make buffer list and write content on the end of each previous buffer length using loop.

Answer 1

Indeed, as of Vert.x 3.4, there is no helper method on FileSystem to append a file to another file.
You could do it with AsyncFile and Pump as follows.
First create a utility method to open files:
Future<AsyncFile> openFile(FileSystem fileSystem, String path, OpenOptions openOptions) {
Future<AsyncFile> future = Future.future();
fileSystem.open(path, openOptions, future);
return future;
}
Then another one to append a file to another file with a Pump:
Future<AsyncFile> append(AsyncFile source, AsyncFile destination) {
Future<AsyncFile> future = Future.future();
Pump pump = Pump.pump(source, destination);
source.exceptionHandler(future::fail);
destination.exceptionHandler(future::fail);
source.endHandler(v -> future.complete(destination));
pump.start();
return future;
}
Now you can combine those with sequential composition:
void merge(FileSystem fileSystem, String output, List<String> sources) {
openFile(fileSystem, output, new OpenOptions().setCreate(true).setTruncateExisting(true).setWrite(true)).compose(outFile -> {
Future<AsyncFile> mergeFuture = null;
for (String source : sources) {
if (mergeFuture == null) {
mergeFuture = openFile(fileSystem, source, new OpenOptions()).compose(sourceFile -> {
return append(sourceFile, outFile);
});
} else {
mergeFuture = mergeFuture.compose(v -> {
return openFile(fileSystem, source, new OpenOptions()).compose(sourceFile -> {
return append(sourceFile, outFile);
});
});
}
}
return mergeFuture;
}).setHandler(ar -> {
System.out.println("Done");
});
}