Dynamic loading of modules in Java

By : Domchi
Source: Stackoverflow.com
Question!

In Java, I can dynamically add stuff to classpath and load classes ("dynamically" meaning without restarting my application). Is there a known framework/library which deals with dynamic loading/unloading of modules without restart?

The usual setup, especially for web-apps, is load balancer, several instances of application, and gradual deployment and restart of new version. I'm looking for something else - application with several services/plugins, possibly single-instance desktop application, where disabling single service is cheap, but bringing down or restarting complete application is not feasible.

I'm thinking about typical plugin infrastructure, where plugins can be upgraded or installed without restarting application. Do I have to program that from scratch, or is something already available? Spring-compatible and opensource is a plus, but not a requirement.

By : Domchi


Answers

Perhaps the simplest approach is to load each plugin with it's own class loader. Then discard the class loader and create a new one to reload the plugin. You will want init() and destroy() methods in the plugin API to allow a chance for startup/shutdown type functionality.

This also has the advantage of isolating the plugins from each other.

A URLClassLoader is your starting point for this. The general idea is that you provide a XxxPlugin superclass that any plugin subclasses. Consider the example of Applet, which is essentially a GUI plugin (or Midlet, etc).



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