source: trunk/JFCMonitor/src/de/ugoe/cs/eventbench/jfcmonitor/JarLauncher.java @ 265

package de.ugoe.cs.eventbench.jfcmonitor;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.JarInputStream;
import java.util.jar.Manifest;

/**
 * <p>
 * Class that launches an executable Jar-file in the same thread and VM where
 * the JarLauncher instance is created. The requirements on the Jar file are:
 * <li>Must contain a MANIFEST.</li>
 * <li>The MANIFEST must define the main-class of the application ("Main-Class"
 * entry).</li>
 * <li>The MANIFEST must define the classpath of the application ("Class-Path"
 * entry).</li>
 * </p>
 * 
 * @author Steffen Herbold
 * @version 1.0
 */
public class JarLauncher {

        /**
         * <p>
         * Name of the Jar file to be executed.
         * </p>
         */
        private String jarfile;

        /**
         * <p>
         * Arguments for launching the Jar file.
         * </p>
         */
        private String[] args;

        /**
         * <p>
         * Helper variable with the path to the working directory.
         * </p>
         */
        final String workingDir = System.getProperty("user.dir") + "/";

        /**
         * <p>
         * Internal variable used to store the classpath extracted from the Jar
         * file's MANIFEST.
         * </p>
         */
        private String[] classPath = new String[] {};

        /**
         * <p>
         * Internal variable used to store the name (including package information)
         * of the Jar file's main function extracted from the Jar file's MANIFEST.
         * </p>
         */
        private String mainClassName = "";

        /**
         * <p>
         * Inner class that defines an exception that is thrown if launching the
         * application in the Jar file fails.
         * </p>
         * 
         * @author Steffen Herbold
         * @version 1.0
         */
        private class JarLaunchException extends Exception {

                /**
                 * <p>
                 * Id for object serialization.
                 * </p>
                 */
                private static final long serialVersionUID = 1L;

                /**
                 * <p>
                 * Constructor. Creates a new JarLaunchException.
                 * </p>
                 * 
                 * @param string
                 *            error message of the exception
                 */
                public JarLaunchException(String string) {
                        super(string);
                }

                /**
                 * <p>
                 * Constructor. Creates a new JarLaunchException as a copy of an
                 * existing exception.
                 * </p>
                 * 
                 * @param e
                 *            exception that is copied
                 */
                public JarLaunchException(Exception e) {
                        super(e);
                }

        }

        /**
         * <p>
         * Constructor. Creates a new JarLauncher.
         * </p>
         * 
         * @param jarfileJjar
         *            file to be launched; must not be complete path but in relation
         *            to the current working directory
         * @param args
         *            arguments with which the main function of the Jar file is
         *            called
         */
        public JarLauncher(String jarfile, String[] args) {
                this.jarfile = jarfile;
                this.args = args;
        }

        /**
         * <p>
         * Executes the main function of the Jar file associated with this launcher.
         * </p>
         */
        public void exec() {
                try {
                        getInfoFromJar();
                        initClassLoader();
                        runMain();
                } catch (JarLaunchException e) {
                        System.err.println("Failure to launch application.");
                        System.err.println(e.getMessage());
                }
        }

        /**
         * <p>
         * Retrieves the classpath and main function from the Jar file's MANIFEST.
         * </p>
         * 
         * @throws JarLaunchException
         *             thrown if reading of Jar file or MANIFEST fails
         */
        private void getInfoFromJar() throws JarLaunchException {
                JarInputStream jarInputStream;
                try {
                        jarInputStream = new JarInputStream(new FileInputStream(workingDir
                                        + jarfile));
                } catch (FileNotFoundException e) {
                        throw new JarLaunchException(e);
                } catch (IOException e) {
                        throw new JarLaunchException(e);
                }
                Manifest manifest = jarInputStream.getManifest();
                mainClassName = manifest.getMainAttributes().getValue("Main-Class");
                String jarClassPath = manifest.getMainAttributes().getValue(
                                "Class-Path");
                String[] jarClassPathElements = jarClassPath.split(" ");
                classPath = new String[jarClassPathElements.length];
                for (int i = 0; i < jarClassPathElements.length; i++) {
                        classPath[i] = "file:" + workingDir + jarClassPathElements[i];
                }
        }

        /**
         * <p>
         * Modifies the {@link ClassLoader} of the current VM such that it includes
         * the class path defined in the Jar file's MANIFEST.
         * </p>
         * 
         * @throws JarLaunchException
         *             thrown if modification of {@link ClassLoader} fails.
         */
        private void initClassLoader() throws JarLaunchException {
                URLClassLoader classLoader = (URLClassLoader) ClassLoader
                                .getSystemClassLoader();
                Method method;
                try {
                        method = URLClassLoader.class.getDeclaredMethod("addURL",
                                        new Class[] { URL.class });
                } catch (SecurityException e) {
                        throw new JarLaunchException(
                                        "addURL method of URLClassLoader not accessible via reflection.");
                } catch (NoSuchMethodException e) {
                        throw new JarLaunchException(
                                        "URLClassLoader does not have addURL method. Should be impossible!!");
                }
                method.setAccessible(true);

                try {
                        method.invoke(classLoader, new Object[] { new URL("file:"
                                        + workingDir + jarfile) });
                        for (String element : classPath) {
                                method.invoke(classLoader, new Object[] { new URL(element) });
                        }
                } catch (IllegalArgumentException e) {
                        throw new JarLaunchException(
                                        "Illegal arguments for addURL method. Should be impossible!!");
                } catch (MalformedURLException e) {
                        throw new JarLaunchException(e);
                } catch (IllegalAccessException e) {
                        throw new JarLaunchException(
                                        "addURL method of URLClassLoader not accessible via reflection.");
                } catch (InvocationTargetException e) {
                        e.printStackTrace();
                }
        }

        /**
         * <p>
         * Executes the main function.
         * </p>
         * 
         * @throws JarLaunchException
         *             thrown if execution of main function fails or the main
         *             function itself throws an exception
         */
        private void runMain() throws JarLaunchException {
                Class<?> mainClass;
                try {
                        mainClass = Class.forName(mainClassName);
                } catch (ClassNotFoundException e) {
                        throw new JarLaunchException("Main class not found: "
                                        + mainClassName);
                }
                Method mainMethod;
                try {
                        mainMethod = mainClass.getMethod("main",
                                        new Class[] { String[].class });
                } catch (SecurityException e) {
                        throw new JarLaunchException("Main method not accessible.");
                } catch (NoSuchMethodException e) {
                        throw new JarLaunchException("Main method not found.");
                }
                try {
                        mainMethod.invoke(null, new Object[] { args });
                } catch (IllegalArgumentException e) {
                        throw new JarLaunchException(
                                        "Illegal arguments for main method. Should be impossible!!");
                } catch (IllegalAccessException e) {
                        throw new JarLaunchException("Main method not accessible.");
                } catch (InvocationTargetException e) {
                        throw new JarLaunchException(e);
                }
        }
}