MainClass.java Source code

Introduction

Here is the source code for MainClass.java

Source

import java.applet.Applet;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.Hashtable;

import javax.swing.JFrame;

public class MainClass {

    public static void main(String args[]) {
        String name = "http://urlWithClassName";
        try {
            if (!name.endsWith(".class")) {
                System.err.println("That doesn't look like a byte code file!");
                return;
            }
            URL u = new URL(name);
            URLClassLoader ucl = new URLClassLoader(u);

            // parse out the name of the class from the URL
            String s = u.getFile();
            String classname = s.substring(s.lastIndexOf('/'), s.lastIndexOf(".class"));
            Class AppletClass = ucl.loadClass(classname, true);
            Applet apl = (Applet) AppletClass.newInstance();
            JFrame f = new JFrame();
            f.setSize(200, 200);
            f.add("Center", apl);
            apl.init();
            apl.start();
            f.setVisible(true);
        } catch (Exception e) {
            System.err.println(e);
        }
    }
}

class URLClassLoader extends ClassLoader {
    Hashtable cache = new Hashtable();
    URL url;

    public URLClassLoader(URL u) {
        this.url = u;
    }

    public synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        Class cls = (Class) cache.get(name);
        if (cls == null) {
            try {
                cls = findSystemClass(name);
            } catch (ClassNotFoundException e) {
            }
        }
        if (cls == null) {
            byte classData[] = loadClassData(name);
            cls = defineClass(classData, 0, classData.length);
            cache.put(name, cls);
        }
        if (resolve) {
            resolveClass(cls);
        }
        return cls;
    }

    private byte[] loadClassData(String name) throws ClassNotFoundException {
        byte[] buffer;
        InputStream theClassInputStream = null;
        int bufferLength = 128;
        try {
            URL classURL = new URL(url, name + ".class");
            URLConnection uc = classURL.openConnection();
            uc.setAllowUserInteraction(false);

            try {
                theClassInputStream = uc.getInputStream();
            } catch (NullPointerException e) {
                System.err.println(e);
                throw new ClassNotFoundException(name + " input stream problem");
            }
            int contentLength = uc.getContentLength();

            // A lot of web servers don't send content-lengths
            // for .class files
            if (contentLength == -1) {
                buffer = new byte[bufferLength * 16];
            } else {
                buffer = new byte[contentLength];
            }

            int bytesRead = 0;
            int offset = 0;

            while (bytesRead >= 0) {
                bytesRead = theClassInputStream.read(buffer, offset, bufferLength);
                if (bytesRead == -1)
                    break;
                offset += bytesRead;
                if (contentLength == -1 && offset == buffer.length) { // grow the array
                    byte temp[] = new byte[offset * 2];
                    System.arraycopy(buffer, 0, temp, 0, offset);
                    buffer = temp;
                } else if (offset > buffer.length) {
                    throw new ClassNotFoundException(name + " error reading data into the array");
                }
            }

            if (offset < buffer.length) { // shrink the array
                byte temp[] = new byte[offset];
                System.arraycopy(buffer, 0, temp, 0, offset);
                buffer = temp;
            }

            // Make sure all the bytes were received
            if (contentLength != -1 && offset != contentLength) {
                throw new ClassNotFoundException("Only " + offset + " bytes received for " + name + "\n Expected "
                        + contentLength + " bytes");
            }
        } catch (Exception e) {
            throw new ClassNotFoundException(name + " " + e);
        } finally {
            try {
                if (theClassInputStream != null)
                    theClassInputStream.close();
            } catch (IOException e) {
            }
        }
        return buffer;
    }
}