Need help implementing Java modules

[Alex Buckley]

On 5/9/2017 11:51 AM, 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.

I don't know how Log4j is currently discovering Appenders, but I guess 
it's through reflective means (Class.forName, classpath scanning, etc). 
Services are a better approach because then the JDK does the discovery 
while you focus on the type-safe protocol between Log4j and Appenders. 
Consider this:

- The Log4j module should not require the FlumeAppender module et al.

- The Log4j module should export a service interface AppenderIntf.

- The FlumeAppender module should i) require the Log4j module to gain 
access to AppenderIntf, and ii) provide its Flume-specific 
implementation of the service interface ('provides AppenderIntf with 
FlumeAppenderImpl;'). If the FlumeAppender module requires helper 
modules, and they in turn require the Log4j module, that's fine.

- The Log4j module should use its own service interface ('uses 
AppenderIntf;') and discover appender implementations via ServiceLoader.

Alex