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.
http://docs.jboss.org/hibernate/orm/4.3/topical/html/logging/Logging.html
<http://docs.jboss.org/hibernate/orm/4.3/topical/html/logging/Logging.html>
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
