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[java] Dynamically loadable classes?

Started by Shadow Mint Aug 8, 2002 at 8:55 PM 3 replies 250+ views

Original Post

Aug 8, 2002 at 8:55 PM

Is there a java equivalent to DLLs? I''ve been trawling google for the last 20 minutes and I''ve found absolutely nothing on the topic. Given a path I want to be able to open that class / archive and instanciate a class from it, but be able to pick the name of the class dynamically...or something. Basically, I want to be able to write a java application that uses plugins, written in java, specified by command line options. Any suggestions? Cheers.

TangentZ

Aug 8, 2002 at 10:13 PM

Look into java.lang.ClassLoader and the the java.lang.reflect
packages. Be afraid. Be very afraid.

Kami no Itte ga ore ni zettai naru!

WayfarerX

Aug 8, 2002 at 11:59 PM

java.lang.Class.forName(String className) & java.lang.ClassLoader (for dynamicly loading classes)

Using JAR-related APIs Introduced in 1.2 (How to dynamicly mount jar files)

[edited by - wayfarerx on August 9, 2002 1:01:24 AM]

Shadow Mint

Author

Shadow Mint

Aug 10, 2002 at 7:40 PM

*sighs* This is probably gonna sound stupid to you ppl who know what you''re talking about.

OK: I wrote a new class loader. yay. It loads class data from a file (and yes, I''ll have to modify that so it works from a jar archive too).

Problem: What do I do with the Class object I get back. I cant understand the JDK docs; an I sposed to somehow use the Class to create instances of type Class? Do I do something different (use the class loader to do that)? Is this Class actually AN instance of said class; in which case, how the heck to a cast it to the appropriate class type...

Cheers!

c_wraith

Aug 10, 2002 at 8:43 PM

Ack. Wrote your own classloader? Why?

Unless you''re doing something really freaky, like generating (or modifying) classes on the fly using BCEL or something, you can just use the URLClassLoader. It''s a nice versatile classloader that can load from the local filesystem (file:// URL''s), jar files (jar:// URL''s), and remote filesystems (http:// URL''s).

As far as how to use the Class object... The best approach is to specify an interface that all of your plugins will implement. Then create a no-arg constructor for your class and use newInstance on the class object to get an instance of it.

Something like this (very incomplete example):

  
Class pluginClass = loader.loadClass(pluginClassName);

Plugin p = (Plugin)pluginClass.newInstance();

p.whatever();

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