Creating classes dynamically with Java

There are many different ways to achieve this (e.g proxies, ASM), but the simplest approach, one that you can start with when prototyping is:

import java.io.*;
import java.util.*;
import java.lang.reflect.*;

public class MakeTodayClass {
  Date today = new Date();
  String todayMillis = Long.toString(today.getTime());
  String todayClass = "z_" + todayMillis;
  String todaySource = todayClass + ".java";

  public static void main (String args[]){
    MakeTodayClass mtc = new MakeTodayClass();
    mtc.createIt();
    if (mtc.compileIt()) {
       System.out.println("Running " + mtc.todayClass + ":\n\n");
       mtc.runIt();
       }
    else
       System.out.println(mtc.todaySource + " is bad.");
    }

  public void createIt() {
    try {
      FileWriter aWriter = new FileWriter(todaySource, true);
      aWriter.write("public class "+ todayClass + "{");
      aWriter.write(" public void doit() {");
      aWriter.write(" System.out.println(\""+todayMillis+"\");");
      aWriter.write(" }}\n");
      aWriter.flush();      
      aWriter.close();
      }
    catch(Exception e){
      e.printStackTrace();
      }
    }

  public boolean compileIt() {
    String [] source = { new String(todaySource)};
    ByteArrayOutputStream baos= new ByteArrayOutputStream();

    new sun.tools.javac.Main(baos,source[0]).compile(source);
    // if using JDK >= 1.3 then use
    //   public static int com.sun.tools.javac.Main.compile(source);    
    return (baos.toString().indexOf("error")==-1);
    }

  public void runIt() {
    try {
      Class params[] = {};
      Object paramsObj[] = {};
      Class thisClass = Class.forName(todayClass);
      Object iClass = thisClass.newInstance();
      Method thisMethod = thisClass.getDeclaredMethod("doit", params);
      thisMethod.invoke(iClass, paramsObj);
      }
    catch (Exception e) {
      e.printStackTrace();
      }
    }
}