Writing a Logstash GEM gives an error: undefined l

Trying to build a gem to use it for Logstash as a filter. I'm using classes and methods from a .jar file.

The following is my Jruby code so far:

# encoding: utf-8
require "logstash/filters/base"
require "logstash/namespace"
require 'java'
require 'Processing.jar'

java_import 'process.Process'

class LogStash::Filters::Process < LogStash::Filters::Base

  config_name "process"

  StringPath = "/path/to/file/"
  ML = JavaUtilities.get_proxy_class('processing.Processing')
  @ML = ML.new(StringPath)

  public
  def register
  end

  public
  def filter(event)    
     event["result"] = @ML.Process("any string.")
     filter_matched(event)
  end

end

However, it gave me the following error:

    ←[31mException in filterworker, the pipeline stopped processing new events, plea
se check your filter configuration and restart Logstash. {"exception"=>#<NoMetho
dError: undefined method 'Process' for nil:NilClass>, "backtrace"=>[
"/path/to/logstash-2.0.0/vendor/local_gems
/dda91dc0/logstash-filter-Process-0.5.0/lib/logstash/filters/processing.rb:54:in 'filter'", "/path/to/log
stash-2.0.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/f
ilters/base.rb:152:in 'multi_filter'", "org/jruby/RubyArray.java:1613:in 'each'"
, "/path/to/logstash-2.0.0/vendor/bundle/j
ruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/filters/base.rb:149:in 'mult
i_filter'", "(eval):233:in 'filter_func'", "/path/to/logstash-2.0.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/
lib/logstash/pipeline.rb:219:in 'filterworker'", "path/to/log/logstash-2.0.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0
-java/lib/logstash/pipeline.rb:154:in 'start_filters'"], :level=>:error}←[0m
NoMethodError: undefined method 'Process' for nil:NilClass
         filter at /path/to/logstash-2.0.0
/vendor/local_gems/dda91dc0/logstash-filter-Process-0.5.0/lib/logstash
/filters/Process.rb:54
   multi_filter at /path/to/logstash-2.0.0
/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/filters/base
.rb:152
           each at org/jruby/RubyArray.java:1613
   multi_filter at /path/to/logstash-2.0.0
/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/filters/base
.rb:149
    filter_func at (eval):233
   filterworker at /path/to/logstash-2.0.0
/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/pipeline.rb:
219
  start_filters at /path/to/logstash-2.0.0
/vendor/bundle/jruby/1.9/gems/logstash-core-2.0.0-java/lib/logstash/pipeline.rb:
154

I tried the following:

  .
  .
  .
  public
  def filter(event)
     StringPath = "/path/to/file/"
     ML = JavaUtilities.get_proxy_class('processing.Processing')
     @ML = ML.new(StringPath)    
     event["result"] = @ML.Process("any string.")
     filter_matched(event)
  end   
end

But it gave me the error:

SyntaxError: dynamic constant assignment error.

I tried this in a normal jruby file and everything worked with me. Using Logstash's filter however is what seems to be giving me an issue here.

It seems you are struggling with local and instance variables in Ruby. Please see this blog post for more information.

When you call event["result"] = ML.Process(event["text"]) ruby tries to find a local variable named ML which doesn't exist. Therefore you get undefined local variable or method ML.Process(event["text"]). So, messing around with your variables might solve your problem.

@ML is an instance variable and is available in all methods calling it using @ML or self.ML. Please try this:

public
def filter(event)
   event["result"] = @ML.Process(event["text"])
   filter_matched(event)
end

Things I stumbled upon:

Where do MLVectorPath and MLStopWordPath come from?
What is StringPath? It is assigned but never used.
Do you have a field text? Otherwise change event["text"] into event["message"].

However, I know neither your Java code nor your logstash configuration so these things may be okay. Please let me know if this helps.