Logging API

[Andrew Evers <ae...@redwood.nl>]

Hi guys,

I've just been doing some debugging of one of our packages that uses
XML-RPC to perform some of its communication. One thing we needed to
do is get accurate information about the methods and parameters sent
in RPC's. Having omitted to put this information into our own code,
I tried to use the XmlRpc debug facilities.

I noticed a few things that I would like to fix:
1. There is no System property to set to enable debugging, this makes
   enabling debugging in the field difficult. I think there should be
   a System.getProperty in a static initializer in the XmlRpc class
   that enables debugging if the 'org.apache.xmlrpc.XmlRpc.debug'
   system property is set to anything.

2. The current debugging goes to a mix of System.out and System.err,
   probably for testing vs debugging purposes. This really needs to
   be logged via a dedicated debugging/tracing facility that can log
   to various locations (especially application log files).

   There are four obvious logging facilities available:
   a. System.out/err - not feasible in large or multi-threaded apps.
   b. Java 1.4 Logging API - I have no great love for this, and it
      is only available in 1.4. It should be an option though.
   c. Log4J - my preferred logging library, but not everyone's.

   d. Jakarta Commons Logging API - Jakarta wrapper for any or
      all of the above, which should be possible to use at the
      cost of importing one class (NoOpLog) and one interface
      (Log). Or, even one interface if lots of nullity checking
      is done.

The Jakarta Commons Logging API is probably the best of the lot, and
with a little work the impact of using it can be kept to a minimum. The
applet can be distributed with only the Log interface and NoOpLog (and
maybe SimpleLog).

In order to keep the packaging as light as possible, the logging API
would only be enabled if a System property 'org.apache.xmlrpc.XmlRpc.Log'
is specified. This would then trigger a little reflection trickeryto create an instance of the specified class. This constructor would be
responsible for using the LogFactory in the Jakarta Commons Logging API
to create a Log instance, or setting up a Log instance in some other
manner.

My implementation (not yet done, but 1 night's work) would be to add
a protected static Log field called log to the XmlRpc class and change
the System.out/err statements to:

if (log.isXXXEnabled())
{
  log.XXX(expression);
}

And, add some extra code to log XmlRpcClientException's to the error
facility (these exceptions are thrown when something serious occurs
processing an RPC that is not the fault of the caller).

The facilities available are:
(the least serious)
trace - xml parsing and timing information
debug - request/response XML documents
info  - requests/responses and parameters
warn  - XmlRpcException (?)
error - XmlRpcClientException
fatal
(the most serious)

Does anyone have any other ideas on this, or am I duplicating
anyone's work? I am planning on doing this work within the next
few days. So, pipe up soon ;)

Andrew.