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!