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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3770) Create a utility class for skipping data in an InputStream
Date Wed, 27 Aug 2008 13:03:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3770?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12626097#action_12626097

Knut Anders Hatlen commented on DERBY-3770:

Thanks for the new patch. It basically looks good. A couple of small issues:

1) Package and class name in the header of InputStreamUtilTest should be updated.

2) I think Dan's point with skipFully (now skipUntilEOF) was that you don't necessarily have
to call skipPersistent until it returns 0. It is OK to stop calling it once it returns less
bytes than requested. So to reduce the number of times skipPersistent is called, skipUntilEOF
could do something like this:

long bytes = 0;
while (true) {
    long skipped = skipPersistent(is, SKIP_FRAGMENT_SIZE);
    bytes += skipped;
    if (skipped < SKIP_FRAGMENT_SIZE) {
        return bytes;

3) I noticed that SKIP_FRAGMENT_SIZE had been lowered from 1024*1024 to 512*1024 in this patch.
I don't think there's any reason to keep this constant small. There shouldn't be any disadvantages
with having a higher value, so it might be better to set it to a very high value, for instance

> Create a utility class for skipping data in an InputStream
> ----------------------------------------------------------
>                 Key: DERBY-3770
>                 URL: https://issues.apache.org/jira/browse/DERBY-3770
>             Project: Derby
>          Issue Type: Improvement
>          Components: Miscellaneous
>    Affects Versions:
>            Reporter: Kristian Waagan
>            Assignee: Junjie Peng
>            Priority: Minor
>         Attachments: derby-3770-1.patch, derby-3770-1.stat, derby-3770-2.patch, derby-3770-2.stat,
derby-3770-3.patch, derby-3770-3.stat, derby-3770-4.patch, derby-3770-4.patch, derby-3770-4.stat,
> The contract of InputStream.skip is somewhat difficult, some would even say broken.
> See http://java.sun.com/javase/6/docs/api/java/io/InputStream.html#skip(long))
> A utility class should be created to ensure that we use the same skip procedure throughout
the Derby code base.
> Suggested functionality:
>  - long skipFully(InputStream) : skips until EOF, returns number of bytes skipped
>  - void skipFully(InputStream,long) : skips requested number of bytes, throws EOFException
if there is too few bytes in the stream
> I know of two different approaches, both skipping in a loop:
>  a) Verify EOF with a read call when skip returns zero.
>  b) Throw EOFException if skip returns zero before requested number of bytes have been
> There's related code in iapi.util.UTF8Util. Maybe this class, say StreamUtil, could be
put in the same package?

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