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From "Borut Hadžialić" <>
Subject Did anyone had problems with JspWriter in Tomcat 5.5.17?
Date Mon, 16 Jul 2007 08:33:13 GMT
Hi list,

I have spent last 15 working hours trying to figure out the cause of a
strange bug. Home page on one of our web sites sometimes gets
generated with no content - as a blank page, the response buffer
having size 0. We use caching filter (modified CachingFilter from
Ehcache 1.3.0) which uses a response wrapper to capture responses to
make caching possible. The response wrapper internaly uses standard
classes ByteArrayOutputStream and PrintWriter from the Java API.

The page cache time-to-live is set to 1 day. When the page is
generated first time by jsps and tiles it is put into cache, and
served for next 24 hours from the cache. After 24 hours  it gets
generated again.

The manifestation of the bug is that randomly, the homepage gets
generated as a blank page, about once in every 3 cache refreshes. When
it occurs, the ByteArrayOutputStream has size 0, after the generation
of the page has completed.

Generating the homepage involves pulling some data from the database,
which changes rarely, about once every week. I put cache time to live
300 seconds to eliminate the posibility that corrupt data from
database causes errors in page generation. With 300 second time to
live the problem was still there. The data in database does not change
between good and faulty page generations. Home page is relatively
simple - it does not involve pulling data from other data sources. It
just reads some data the database and uses that data to make some
html, using jsp.

After turning on logging on database calls, all filters, and using a
PrintWriter decorator inside our response wraper that echoes all print
statements to the log,  I found out something strange -

A correct page generation logs some database calls (by the database
logger), then a chunk of html (by the logging PrintWriter decorator,
as a result of flushing JspWriter after enough out.write calls have
been made), then some more database calls, then some more html.. and
the page gets generated.

** A faulty generation (the one that produces a blank page) logs
exactly same database calls as a correct page generation, but the log
entries produced by the echoing PrintWriter are missing! **, like as
jsps are executing normally, but something is wrong with the JspWriter
out object.

I am suspecting that somehow JspWriter writes somewhere else, not
where it should. It should write into response wrapper's buffer,
though the echoing/logging PrintWriter. But there is no PrintWriter
echo and no bytes in response wrapper buffer... more exactly,
*sometimes* there is no PrintWriter echo and no bytes in response
wrapper buffer.

I have read all the code that makes up the life cycle of PageContext
and JspWriter on the docjar site (not sure for what version of
Tomcat), and couldn't find anything that would help me solve this.

After adding some more logging statements I found out that 1 instance
of JspWritter and 1 instance of PageContext are used shared across all
page generations (site has low traffic and rarely 2 requests in same
time). Same JspWritter and  PageContext instances sometimes
participate in good page and faulty page generations.

I was hoping that someone on this list might know something that could
help me find out why is this happening. Please help.

Because YES!

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