Re: Detecting multiple class loaders

Matt Humphrey wrote:
"Chris" <spam_me_not@xxxxxxxxxx> wrote in message news:47e12e73$0$29056$9a6e19ea@xxxxxxxxxxxxxxxxxxxxxxx

Is there any way to detect when a class has been loaded by two different class loaders?

Our app has a class that reads and writes a file on disk. The file gets corrupt if more than one process gets at it at once. We can detect multiple processes by locking the file on disk and throwing an error; no problem. We can control access by multiple threads using synchronization. So far, so good.

The problem is Websphere. When this class gets used in a webapp, Websphere, like most other app servers, creates a classloader just for that webapp. But it does more than that; it creates multiple classloaders

Have the writing code allocate a server socket on a rarely used port. If successful, the file write can proceed. Otherwise, someone else has the socket and is writing. Afer writing, release the socket. Note that the socket is never actually used. Sockets are a global resource independent of classloaders and allocation qualifies as atomic test-and-set.

Alternatively, I have never tried using it, but from the javadocs
it would appear that java.nio.channels.FileChannel's lock() could
maybe do the locking in a global way. That method returns a
FileLock, and the documentation for FileLock says,

This file-locking API is intended to map directly to
the native locking facility of the underlying operating
system. Thus the locks held on a file should be visible
to all programs that have access to the file, regardless
of the language in which those programs are written.

That would certainly imply the scope of the lock goes beyond
the classloader and beyond the JVM as well.

The advantages of this over the socket method is that (a) you don't
grab a serversocket port that somebody might actually *need*, and
(b) you don't have to worry about maintaining some mapping between
pathnames and port numbers. The disadvantage is that the behavior
is platform-specific and not exactly completely guaranteed to do
what you want, maybe.

- Logan
.

Relevant Pages

  • Re: Detecting multiple class loaders
    ... The file gets corrupt if more than one process gets at it at once. ... We can detect multiple processes by locking the file on disk and throwing an error; ... When this class gets used in a webapp, Websphere, like most other app servers, creates a classloader just for that webapp. ...
    (comp.lang.java.programmer)
  • Re: Detecting multiple class loaders
    ... Our app has a class that reads and writes a file on disk. ... corrupt if more than one process gets at it at once. ... we can use to ensure that only one classloader within a JVM ... Have the writing code allocate a server socket on a rarely used port. ...
    (comp.lang.java.programmer)
  • Re: Inter-process communication - does data hit disk?
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    (comp.os.linux.development.apps)
  • Re: Inter-process communication - does data hit disk?
    ... > You should probably investigate the socket API further. ... > It certainly will hit disk if you write enough data to require the ... >> the complicated business of sending commands to the hard disk, ... > is going to decide to write some of your data out in order to free RAM ...
    (comp.os.linux.development.apps)
  • Re: Async Socket IO Question
    ... I think you're mostly bumping into Socket Timeout issues. ... If you kill the process of your client app, the TCP session isn't cleanly ... When your server sends to the client app, that send happens just fine (the ...
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