FileOutputStream: Does the "close" method calls also "flush"?

Question

I'm really confused about flush and close method.In my code I always close my FileOutputStream object. But I want to know that if I have to use flush method here, and where can I use it?

I will write a project that download 4 or 5 files repeatedly. I will write a method(for download files) and my method will be in a loop and download files repeatedly.My method will have a code like this.

Does the close method calls flush, or do I have to use flush before closing?

try {
    InputStream inputStream = con.getInputStream();
    FileOutputStream outputStream = new FileOutputStream("C:\\programs\\TRYFILE.csv");

    int bytesRead = -1;
    byte[] buffer = new byte[4096];
    while ((bytesRead = inputStream.read(buffer)) != -1) {
    outputStream.write(buffer, 0, bytesRead);
}

} catch(Exception e) {
    //
} finally {
    outputStream.close();
    inputStream.close();
}    

Note that the code works well: it download the file successfully. But I'm not sure about using flush.

Smew

Documentation serves for a purpose. – Ridinger

Closely related: you need not call flush(), but you must call `close() – Bumper

possible duplicate of Using flush() before close() – Bumper

Answer 1

The method flush is used to "flush" bytes retained in a buffer. FileOutputStream doesn't use any buffer, so flush method is empty. Calling it or not doesn't change the result of your code.

With buffered writers the method close call explicitly flush.

So you need to call flush when you like to write the data before closing the stream and before the buffer is full (when the buffer is full the writer starts writing without waiting a flush call).

The source code of class FileOutputStream hasn't a custom version of method flush. So the flush method used is the version of its super class OutputStream. The code of flush in OutputStream is the following

public void flush() throws IOException {
}

As you see this is an empty method doing nothing, so calling it or not is the same.

Polygenesis

So,you say for my code; there is no difference between using flush or not? Are you absolutely sure about that? – Smew

@Smew But when accessing the filesystem it's actually recommended to use buffered reader/ writers. But then the BufferedOutputStream calls flush on close automatically. – Polymorphonuclear

I looked at the source codes of Fileoutputstream.Yes you are right but there is an finalize method in that class and it has flush() method.But that flush() method comes from superclass and again it is empty and do nothing.Am ı right and do I understand the approach truely? – Smew

Answer 2

I will write a project that download 4 or 5 files repeatedly. I will write a method(for download files) and my method will be in a loop and download files repeatedly.My method will have a code like this.

Does the close method calls flush, or do I have to use flush before closing?

I recommend to use the NIO.2 API and the try-with-resources statement. This will reduce the amount of code and takes care of flushing and closing the streams:

try (InputStream inputStream = con.getInputStream()){
    Files.copy(inputStream, Paths.get("C:\\programs\\TRYFILE.csv"));
}

The topic is a bit confusing since OutputStream.close does indeed not require an automatic flush, but subclasses might specify that. They might also provide a flush method which does nothing (e.g. as the one inherited from OutputStream, which is the case for FileOutputStream). In this case it has no effect to call the flush method, of course, so you can omit it.

If in doubt (if you don't know which subclass you're working with) I guess it's better to call the flush manually.

But again, using the code above this is taken care for you.

Polymorphonuclear

This does not answer (directly) the question, that's why I down-voted. – Ridinger

@Ridinger it solves the problem without rising the question about flushing/ closing, so I think the answer is valid. – Polymorphonuclear

@Polymorphonuclear there is no problem here, just a simple question – Kerseymere

No, this is simply an alternative to the way the OP is doing, it does not answer the question, even if it might solve the problem, I don't know to be honest, but I will remove my down-vote because I don't like to be [__] – Ridinger

@Kerseymere It's not just a sample, but a concrete use case. – Polymorphonuclear