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Java URLClassLoader example 

Dr. Xi
Joined:
04/09/2007
Posts:
727

June 16, 2011 14:23:44    Last update: June 16, 2011 14:25:04
This example shows how a service implementation can be loaded with a URLClassLoader.
  1. The files.
    • HelloService.java: 
      public interface HelloService {
    public void sayHello();
}

    • HelloServiceImpl.java: 
      public class HelloServiceImpl implements HelloService {
    @Override
    public void sayHello() {
	System.out.println("Hello!");
    }
}

    • ServiceFactory.java uses URLClassLoader to load the service implementation: 
      import java.io.*;
import java.net.*;

public class ServiceFactory {
    private static URL[] classLoaderUrls;

    public static void setURLs(URL[] urls) {
	classLoaderUrls = urls;
    }

    public static HelloService getHelloService() throws Exception {
	URLClassLoader loader = new URLClassLoader(classLoaderUrls);
	return (HelloService) Class.forName("HelloServiceImpl", true, loader).newInstance();
    }
}

    • Test.java: 
      import java.io.*;
import java.net.*;

public class Test {
    public static void main(String[] args) throws Exception {
	ServiceFactory.setURLs(new URL[] { 
	    new URL("file:service.jar")
	});

	HelloService h = ServiceFactory.getHelloService();
	System.out.println("Class Loader: " + h.getClass().getClassLoader());
	h.sayHello();
    }
}


  2. Create a runnable client jar: 
    C:\>jar -cfe client.jar Test Test.class ServiceFactory.class HelloService.class

  3. Create a service jar: 
    jar -cf service.jar HelloServiceImpl.class

  4. Test: 
    C:\>java -jar client.jar
Class Loader: java.net.URLClassLoader@addbf1
Hello!


Extra Note:: JDK7 added a method URLClassLoader.close(), so that a URLClassLoader can be discarded, and a new one created to load a new implementation.
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