source: src/main/java/geniusweb/partiesserver/jarClassLoader1.java@ 26

package geniusweb.partiesserver;

import java.io.File;
import java.io.IOException;
import java.net.JarURLConnection;
import java.net.MalformedURLException;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.Attributes;

/**
 * Classloader that ensures loaded code is kept separate from other loaded code.
 * This is to prevent dependency conflicts between parties that we load.
 *
 */
class JarClassLoader1 extends URLClassLoader {

        private URL u;

        /**
         * @param url    The URL is like
         *               jar:file:/Web%20Servers/../partiesserver-1.4.1/partiesrepo/agentgg-1.5.5.jar!/
         * 
         * @param parent the parent classloader.
         */
        public JarClassLoader1(URL url, ClassLoader parent) {
                super(new URL[] { url }, parent);
                this.u = url;

                // we need to get to the actual file. This is very tricky,
                // most solutions on the web don't work, throw, don't remove %20,etc
                // url.toURI throws for multi-protocol URLs like "jar:file://...".

                // https://docs.oracle.com/javase/7/docs/api/java/net/JarURLConnection.html
                // the syntax of a JAR URL is: jar:<url>!/{entry}

                String filename = url.getPath(); // file:/bla%20bla/file.jar!/
                filename = filename.substring(0, filename.indexOf("!/"));
                File jarfile;
                try {
                        jarfile = new File(new URL(filename).toURI());
                } catch (MalformedURLException | URISyntaxException e) {
                        throw new IllegalArgumentException(
                                        "file " + filename + " seems malformed", e);
                }

                if (jarfile == null || !jarfile.exists()) {
                        throw new IllegalArgumentException(
                                        "file does not exist:" + jarfile);
                }
                if (!jarfile.canRead()) {
                        throw new IllegalArgumentException(
                                        "File " + jarfile + " has no read permissions");
                }
                if (!jarfile.isFile() || !jarfile.getName().endsWith(".jar")) {
                        throw new IllegalArgumentException("File " + jarfile
                                        + " must be a file and have extension .jar");

                }

        }

        public String getMainClassName() throws IOException {
                JarURLConnection uc = (JarURLConnection) u.openConnection();
                Attributes attr = uc.getMainAttributes();
                return attr != null ? attr.getValue(Attributes.Name.MAIN_CLASS) : null;
        }
}