Need help implementing Java modules

Jochen Theodorou blackdrag at
Tue May 9 19:50:36 UTC 2017

On 09.05.2017 20:51, Ralph Goers wrote:
> I am attempting to modularize Log4j and am running into some trouble
> understanding how this can work.
> Log4j supports many Appenders, such as the FlumeAppender,
> KafkaAppender, CassandraAppender, and JPAAppender. Each of these use
> their respective libraries to cause the log events to be delivered to
> their destinations. All of these frameworks perform logging and so
> would have a dependency on a logging framework. For example, the
> FlumeAppender passes the LogEvent to an embedded Flume agent that
> then sends the event to a remote Flume node. The embedded agent
> performs logging using the Log4j 1 API and also uses other libraries
> that perform logging. So if Flume was modularized it would require
> some logging module. That module would almost certainly route logging
> calls back to Log4j 2 and so would require Log4j 2 and result in a
> circularity.  In the same manner, the KafkaAppender uses the SLF4J
> API. When the implementation of that is log4j-slf4j-impl a
> circularity will occur. Since the Java module system forbids
> circularities I don’t understand how Log4j (or Logback) can
> realistically be modularized.

I will try to explain and depend on others to correct me if I am wrong..

Assuming you have your log4j-base module, which will also provide an 
interface to communicate with a implementation of log4j. A FlumeAppender 
would then for example depends on that interface, but log4j-base would 
not depend on the Flume Appender. So the question is how log4j-base can 
then get the FlumeAppender... which is what SPI is for. So every 
Appender would then be a service provider.

> We also noticed that System.LoggerFinder [1] now provides a new way
> to integrate an external logging framework with Java. If Log4j
> creates the service provider for that how will that not cause a
> runtime circularity since java.base now is bound to log4j-api and/or
> log4j-core and both of those require java.base.
> Ralph
> 1.
> <>

SPI again. Since Log4j depends on java.base for compilation, but 
java.base does still compile without log4j-api or -core it is not a 
circular dependency.

bye Jochen

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