mwnorman / Simple-Java-Plugin-Manager

simple implementation of a plugin system for Java - in a single file!

Simple Java Plugin Manager

This is a very simple implementation of a plugin system for Java - it only has one class!

The 'trick' is to leverage the little-used Java artifact package-info.class. This gives us a consistent pattern to search for while scanning the classpath (both directories and jar files).

Once a package-info.class file is found, the java.lang.Package for it can be loaded which in turn forces the resolution of any imports and annotations in the package-info class. In the following example, three different 'helper' classes are declared to be plugins:

@Plugin({Helper1.class, Helper2.class, Helper3.class})
package simple.test;

import org.simple.pluginspi.PluginManager.Plugin;

import simple.test.Helper1;
import simple.test.Helper2;
import simple.test.Helper3;

Each helper class implements a common Helper interface:

public interface Helper {
    public void help();
}

import javax.annotation.PostConstruct;

public class Helper1 implements Helper {

    public Helper1() {
        //public default constructor required
    }
    
    @PostConstruct
    protected void setUp() {
        System.out.println("Helper1 setUp");
    }
    
    public void help() {
        System.out.println("with a little help from my plugin friends");
    }
}

import javax.annotation.Resource;

public class Helper2 implements Helper {

    @Resource(description="some random unique resource name", type = Integer.class)
    int someValue;
    
    public Helper2() {
        //public default constructor required
    }
    
    public void help() {
        System.out.printf("(some value=%d)some more help\n", someValue);
    }
}

public class Helper3 implements Helper {

    public Helper3() {
        //public default constructor required
    }
    
    public void help() {
        System.out.println("that's the last bit of help yer gonna get outa me!");
    }
}

Now it is possible to ask the PluginManager to find all Helper's without having to know exactly where they are on the classpath - or even what package they are in:

//javase imports
import java.util.List;

//simple plugin imports
import org.simple.pluginspi.PluginManager;

//domain imports
import simple.test.Helper;

public class Main {

    public static void main(String ...args) {
        PluginManager pluginManager = PluginManager.getPluginManager();
        pluginManager.addResource("some random unique resource name", Integer.class, 3);
        List<Helper> helpers = pluginManager.findPlugins(Helper.class);
        for (Helper h : helpers) {
            h.help();
        }
    }
}

Helper1 setUp
with a little help from my plugin friends
(some value=3)some more help
that's the last bit of help yer gonna get outa me!