Plugin loader of Java plugins

[frsyuki]

Embulk can't load java-based plugins (jar files) dynamically. Users need to add the jar files to classpath. Inconvenient...

[frsyuki]

An idea of the API design:

• a) Java-based plugins include jar files at classpath/* directory
• b) Even java-based plugins include lib/embulk/input_xyz.rb (or output_xyz.rb) file. Its code is as like this:

loader = Embulk::JavaPluginClassLoader.new(File.dirname(__FILE__)+"../../../classpath")
Plugin.register_output("csv", loader.get("org.embulk.output.CsvOutput.class"))

PluginClassLoader loads a class if it's found at the parent class loader. Otherwise, it loads the class from itself. This is how JRubyClassLoader works to not cause phantom definition. (In other words, we might be able to use JRubyClassLoader).

[frsyuki]

#56 implemented java plugin loader.

[muga]

Great Job! I wanted the feature!