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From "Borut Hadžialić" <borut.hadzia...@gmail.com>
Subject Re: Did anyone had problems with JspWriter in Tomcat 5.5.17?
Date Mon, 16 Jul 2007 09:03:31 GMT
The caching filter uses a BlockingCache. A miss (get that returns
null) locks that entry/key, and other threads that try to get the page
from the cache using the same key, must wait until the thread that
locked the entry puts something into the cache and releases the lock.
Lock are released implicitly when cache put method is called, by the
thread that locked it. All this is implemented inside Ehcache. We
havent observed any thread deadlock problems (using JMX to monitor
servers)

Lock implementation is net.sf.ehcache.constructs.concurrent.Mutex
which is a copy of Doug Lea's mutex class from standard Java API,
copied and imported into Ehcache.



On 7/16/07, rpr_listas <rpr_listas@telefonica.net> wrote:
> Hello, Borut.
>
> Are you sure that there are no other  thread writing in the cache at
> this moment? How do you prevent this case?
>
> Best regards.
> Ricardo
>
>
> Borut Hadžialić escribió:
> > 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.
> >
> > http://www.docjar.com/html/api/org/apache/jasper/runtime/JspWriterImpl.java.html
> >
> > http://www.docjar.com/html/api/org/apache/jasper/runtime/PageContextImpl.java.html
> >
> > http://www.docjar.com/html/api/org/apache/jasper/runtime/JspFactoryImpl.java.html
> >
> >
> > 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.
> >
> >
>
>
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>


-- 
Why?
Because YES!
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