flush in java.io.FileWriter

Question

I have a question in my mind that, while writing into the file, before closing is done, should we include flush()??. If so what it will do exactly? dont streams auto flush??

Answer 1

You don't need to do a flush because close() will do it for you.

From the javadoc:

"Close the stream, flushing it first. Once a stream has been closed, further write() or flush() invocations will cause an IOException to be thrown. Closing a previously-closed stream, however, has no effect."

Answer 2

FileWriter is an evil class as it picks up whatever character set happens to be there, rather than taking an explicit charset. Even if you do want the default, be explicit about it.

The usual solution is OutputStreamWriter and FileOutputStream. It is possible for the decorator to throw an exception. Therefore you need to be able to close the stream even if the writer was never constructed. If you are going to do that, you only need to flush the writer (in the happy case) and always close the stream. (Just to be confusing, some decorators, for instance for handling zips, have resources that do require closing.)

Answer 3

To answer your question as to what flush actually does, it makes sure that anything you have written to the stream - a file in your case - does actually get written to the file there and then.

Java can perform buffering which means that it will hold onto data written in memory until it has a certain amount, and then write it all to the file in one go which is more efficient. The downside of this is that the file is not necessarily up-to-date at any given time. Flush is a way of saying "make the file up-to-date.

Close calls flush first to ensure that after closing the file has what you would expect to see in it, hence as others have pointed out, no need to flush before closing.