Java class BufferedReader size

Question

I have a dubt about to class BufferedReader. This my test. I create the simple txt file: "test.txt" with 1024 lines. Each line contains the line number and the file name (example: "1 - test.txt"):

Test file creation:

$ for i in {0..1023}; do echo "$i - test.txt" >> test.txt; done
$ ls -la test.txt
-rw-r--r--    1 osb      osb           12288 Oct 26 12:01 test.txt

The Java class for my test (f is a variable of File class):

...
while (f.exists())
    {
        BufferedReader br = null;

        try
        {

            br = new BufferedReader( new FileReader( f ), 10);

            while ((line = br.readLine())!=null)
            {
                System.out.println (line);
                Thread.sleep(1000);
            }

        }

        ...
        catch excepion
        ...

        finally
        {
            try
            {
                br.close();
            }
            catch (IOException e)
            {
                System.out.println ("\nError closing Buffer reader!");
                e.printStackTrace();
            }
            br = null;
            line = null;
        }

    }
...

When the application starts to write the lines I execute this unix command:

gzip test.txt

I expect an exception because the buffer size is very little ( 10 byte ) less than un line size, but the application reads all the 1024 lines and only when it reaches the end of the file it raises the exception ( file not exist! ). Does the BufferedReader has a buffersize greater than 10? Is there a minimum buffer size?

Thanks

GhostCat · Accepted Answer · 2016-10-26 12:22:52Z

1

Your problem is basically that your assumptions do not at all match up reality.

You think that your Java code directly interacts "with that file" somewhere on some drive.

But in reality, you have several layers between "your java code" and the "thing" out there. First and foremost: the Linux kernel. Even when you are using a BufferedReader, it is very much likely that the Linux kernel might have read the complete content of your little text file as soon as your program starts running. Thus your "reading" code doesn't notice at all that the underlying file was renamed in the meantime!

Just in the very end, when you close the buffer, the JVM notices that things are askew.

answered Oct 26, 2016 at 12:22

GhostCat

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1 Comment

alessio.zatti Over a year ago

Thanks GhostCat for your quick reply, I understand the scenario

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