Re: Java ByteCode

"hiwa" <>
22 Nov 2006 19:10:30 -0800
andrewzzz wrote:

Hi guys,
I have some questions about java bytecode..
1-java bytecode is cointained in the .Class file,that is autogenerated
by Ecliplse(the ide I'm using) or by javac.
I need to extract the bytecode from a .class file,and actually I'm
using this piece of code :

        //leggo il codice dal file java e lo memorizzo come byte
        file=new File(path);
        InputStream is;
        try {

            is = new FileInputStream(file);
            long length = file.length();// Get the size of the file
            // You cannot create an array using a long type.
            // It needs to be an int type.
            // Before converting to an int type, check
            // to ensure that file is not larger than Integer.MAX_VALUE.
            if (length > Integer.MAX_VALUE) {
                // File is too large
            // Create the byte array to hold the data
            dati = new byte[(int)length];
            // Read in the bytes
            int offset = 0;
            int numRead = 0;
            while (offset < dati.length
                    && (, offset,dati.length-offset))>=0) {
                offset += numRead;

            // Ensure all the bytes have been read in
            if (offset < dati.length) {
                throw new IOException("Could not completely read file

            // Close the input stream and return bytes

Is it right?(I think so..)

2-Then my program will serialize and send through TCP socket the bytes
loaded from the .class.The receiver need to create an instance of the
.class do I do this...(note that I know the name of the

Thanks a lot guys...BYE!--JAVA ROCKS

For this example program, use a simple single-class
application class with main() method:
import java.lang.reflect.*;

public class ClassFileRead{

  public static void main(String[] args){
    String cname = "Rex"; // class name
    String path;
    InputStream is;
    byte[] dati;
    File file;
    ThrowAwayClassLoader cder;
    Class clas;
    int flen;

    dati = null;

    if (args.length > 0){
      cname = args[0];
    path = cname + ".class";
    try {
      file = new File(path);
      is = new FileInputStream(path);

      long length = file.length();
      if (length > Integer.MAX_VALUE) {
        System.out.println("File too large, exitting...");
      flen = (int)length;
      dati = new byte[flen];
      int numRead =;
      if (numRead != flen){
        System.out.println("read failed, exitting...");
    catch (Exception e){

      cder = new ThrowAwayClassLoader(cname, dati);
      clas = cder.getTheClass();
      Object ob = clas.newInstance();
      Method meth = clas.getMethod("main", String[].class);
      meth.invoke(ob, (Object)null);
    catch (Exception e){


class ThrowAwayClassLoader extends ClassLoader{
  String className;
  byte[] classData;

  public ThrowAwayClassLoader(String nam, byte[] ba){
    className = nam;
    classData = ba;

  public Class getTheClass() {
    return defineClass(className, classData, 0, classData.length);

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