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Java Forum / General / November 2006

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Java ByteCode

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andrewzzz - 22 Nov 2006 21:17 GMT
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
                    && (numRead=is.read(dati, 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
"+file.getName());
            }

            // Close the input stream and return bytes
            is.close();

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 received...how do I do this...(note that I know the name of the
class)?

Thanks a lot guys...BYE!--JAVA ROCKS
hiwa - 23 Nov 2006 03:10 GMT
> Hi guys,
> I have some questions about java bytecode..
[quoted text clipped - 44 lines]
>
> Thanks a lot guys...BYE!--JAVA ROCKS
For this example program, use a simple single-class
application class with main() method:
--------------------------------------------------------------------------------------------
import java.io.*;
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...");
       System.exit(1);
     }
     flen = (int)length;
     dati = new byte[flen];
     int numRead = is.read(dati);
     if (numRead != flen){
       System.out.println("read failed, exitting...");
       System.exit(1);
     }
     is.close();
   }
   catch (Exception e){
     e.printStackTrace();
   }

   try{
     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){
     e.printStackTrace();
   }

 }
}

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);
 }
}
------------------------------------------
EJP - 23 Nov 2006 03:32 GMT
> 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 received...how do I do this...(note that I know the name of the
> class)?

Why not just load the class at your end and serialize it to the other
end as is?


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