Read large file using vertx - vert.x

Question

I am new to using vertx and I am using vertx filesystem api to read file of large size.
vertx.fileSystem().readFile("target/classes/readme.txt", result -> {
if (result.succeeded()) {
System.out.println(result.result());
} else {
System.err.println("Oh oh ..." + result.cause());
}
});
But the RAM is all consumed while reading and the resource is not even flushed after use. The vertx filesystem api also suggest
Do not use this method to read very large files or you risk running out of available RAM.
Is there any alternative to this?

Answer 1

To read large file you should open an AsyncFile:
OpenOptions options = new OpenOptions();
fileSystem.open("myfile.txt", options, res -> {
if (res.succeeded()) {
AsyncFile file = res.result();
} else {
// Something went wrong!
}
});
Then an AsyncFile is a ReadStream so you can use it together with a Pump to copy the bits to a WriteStream:
Pump.pump(file, output).start();
file.endHandler((r) -> {
System.out.println("Copy done");
});
There are different kind of WriteStream, like AsyncFile, net sockets, HTTP server responses, ...etc.

Answer 2

To read/process a large file in chunks you need to use the open() method which will return an AsyncFile on success. On this AsyncFile you setReadBufferSize() (or not, the default is 8192), and attach a handler() which will be passed a Buffer of at most the size of the read buffer you just set.
In the example below I have also attached an endHandler() to print a final newline to stay in line with the sample code you provided in the question:
vertx.fileSystem().open("target/classes/readme.txt", new OpenOptions().setWrite(false).setCreate(false), result -> {
if (result.succeeded()) {
result.result().setReadBufferSize(READ_BUFFER_SIZE).handler(data -> System.out.print(data.toString()))
.endHandler(v -> System.out.println());
} else {
System.err.println("Oh oh ..." + result.cause());
}
});
You need to define READ_BUFFER_SIZE somewhere of course.

Answer 3

The reason for that is that internally .readFile calls to Files.readAllBytes.
What you should do instead is create a stream out of your file, and pass it to Vertx handler:
try (InputStream steam = new FileInputStream("target/classes/readme.txt")) {
// Your handling here
}