Subclass on the fly

[ewoo]

I have a based class within my Java application. I would like to add new classes on the fly based on my based class. Is there any way to do it while running the application.

For example in my program, I have

class BasedClass {
private String name;
BasedClass(String name){ this.name = name;}

}

Thanks for any opinion to achieve this.

[ramper]

If I understand the question correct, one way to do this can be by extending the BaseClass into an abstract class and dynamically creating the abstract class' implementation / instance.

eg :

public abstract class ExtendedAbstract extends BaseClass {

public abstract void abstractMethod();

}


And...

//create dynamically anywhere in your code.

ExtendedAbstract dynClass = new ExtendedAbstract(){
public void abstractMethod(){
//implementation
...
...
}
};


.. well if this is what you want.

[sjalle]

If this could be done at all, then how would the already running code be able to use the new classes and invoke their methods ?

[ewoo]

I am using the base class to call the corresponding method or use the reflective tool to do it.

Ideally, I would like to be able to construct a class into a string and have some kind of Java object that will compile the class into byte stream. Once the class is in ByteCode I can then use classloader to load it in and run it.

[Zodoz]

AhHaHaHa!!! I found out how to do it. This question was so inspirational that I had to find a solution, and here it is:

//file: RealTimeClassCreation.java
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLClassLoader;
/*
 * ClassCreationMain.java
 *
 * Created on July 27, 2005, 6:49 PM
 */

/**
 * @author Zodoz
 */
public class ClassCreationMain {
    
    BufferedWriter out;
    
    public ClassCreationMain() throws Exception {
	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
	System.out.println("What do you want to say?");
	String sayWhat = in.readLine();
        //create the source file
	File f = new File ("SayClass.java");
	out = new BufferedWriter(new FileWriter(f));
	write("public class SayClass extends TestSay {");
	write("\tpublic String getText() {");
	write("\t\treturn \""+sayWhat+"\";");
	write("\t}");
	write("}");
	out.close();
        /*****************************
         *java source file is created
         *****************************/
	File f2 = new File ("");
	URL url = new URL("file:"+f2.getAbsolutePath()+"\\");
	URLClassLoader loader = new URLClassLoader(new URL[] { url }); //url is location
                             //that the classes can be found
	Process p = Runtime.getRuntime().exec("javac -classpath build/classes SayClass.java");  //make sure your system path is setup to run javac
	/*******************************************
         * use if you want to see any errors:
	 *
	 *  BufferedReader pIn = new BufferedReader(new InputStreamReader(p.getErrorStream()));
	 *  String line = "";
	 *  while((line = pIn.readLine())!=null)
	 *      System.out.println(line);
	 ***********************************/
	p.waitFor(); //wait to finish the process, this line is a must
	Class c = loader.loadClass("SayClass");  //load the class
	Object o = c.newInstance();  //create an instance of that class
	TestSay ts = (TestSay)o;  //typecast it to the real class that it is
	System.out.println(ts.getText());  //use it
    }
    
    public void write(String write) throws Exception {
	out.write(write);
	out.newLine();
	out.flush();
    }
    
    public static void main(String args[]) throws Exception {
	new ClassCreationMain();
    }
}

A simple class to test extensions:

/*
 * TestSay.java
 *
 * Created on July 27, 2005, 8:23 PM
 */

/**
 *
 * @author Zodoz
 */
public abstract class TestSay {
    public abstract String getText();
}

Please tell me if this works for you, and please fell free to ask any questions.