How can update my class instance at runtime?

Question

What would happen if at runtime I update a class file?
Will the JVM know to use that instead?
How can I use the updated class at runtime?
Who can give me an example?
thanks all!

Answer 1

yes,I write it like these:
//MyClassLoader.java
import java.io.*;
public class MyClassLoader extends ClassLoader
     public Class loadClass(String name,boolean resolve)
          throws ClassNotFoundException{
          //return super.loadClass(name,resolve);
          Class c=findClass(name);
          resolveClass(c);
          return c;
     public Class loadClass(String name)
          throws ClassNotFoundException{
          return loadClass(name,true);
public Class findClass(String name) {
byte[] b = loadClassData(name+".class");
//return defineClass(name, b, 0, b.length);
          Class c=null;
          System.out.println("load--class--data--success!");
          c=defineClass(name,b,0,b.length);
          System.out.println("define class success!");
          return c;
private byte[] loadClassData(String name) {
// load the class data from the connection
     byte[] b=null;
     try{
          //DataInputStream in =
//new DataInputStream(
          BufferedInputStream in=
new BufferedInputStream(
new FileInputStream(name));
          int len=in.available();
          b=new byte[len];
          in.read(b,0,len);
          System.out.println("load "+name+" is success!\n"+name+"'s length is"+len);
     }catch(Exception e){System.out.println("load--"+name+"--data error:"+e);}
     return b;
//Client.java
import java.io.*;
public class Client
     public static void main(String[] args)
     try{
          System.out.println("����������Load��������������");
          DataInputStream input=new DataInputStream(System.in);//��������������Load��������������
          MyClassLoader loader=new MyClassLoader();
          Load load=(Load)loader.loadClass(input.readLine()).newInstance();
          while(true){
               load.print(); //print a sentence here! I will modify it when Client is runing!
               input=new DataInputStream(System.in);
               String inStr=input.readLine();
               if(inStr!=null && inStr.trim()!=null)
                    load = (Load)loader.loadClass(inStr).newInstance();
     }catch(Exception e){System.out.println("main error:"+e);}
I skip the Load interface and it's sub class,it's easy.
MyClassLoader can load my class from the disk!
why?