Java - creating a subclass dynamically

Java Proxies may be able to do what you require - they essentially allow you to dynamically layer functionality on top of an object, as you can intercept any method calls to that object, and either handle them yourself or dispatch the method calls to the underlying class. Depending on what you are looking to do, it may be that you can get the same result as you would by creating a sub-class dynamically

Oracle has a decent introduction on their website (the URL references Java version 1.4.2, but I don't think the behavior of this has changed in more recent versions). Here is a more concise example that gives a good flavor for what proxy code looks like.

It is also possible to do things using direct byte code manipulation (as supported by the ASM framework) however I imagine using proxies would be a simpler approach.

One library that I'm particularly fond of may be used here; Bytebuddy.

Example taken directly from the landing page:

Class<?> dynamicType = new ByteBuddy()
  .subclass(Object.class)
  .method(ElementMatchers.named("toString"))
  .intercept(FixedValue.value("Hello World!"))
  .make()
  .load(getClass().getClassLoader(), ClassLoadingStrategy.Default.WRAPPER)
  .getLoaded();

It's incredibly flexible and definitely worth checking out if you'd like to keep your hair, I personally find heavy usage of javassist can get quite ugly and messy at times, bytebuddy feels like a well needed breath of fresh air!

Rafael Winterhalter is also active on StackOverflow which makes finding out anything you're unsure of a breeze.

Edit: my apology for necroposting. Landed here when a friend linked the question and forgot to check the date.

It's quite easy with Javassist:

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.NotFoundException;

static Class<? extends DefinitionBasedMigrator> createClass( String fullName )
        throws NotFoundException, CannotCompileException
{
    ClassPool pool = ClassPool.getDefault();

    // Create the class.
    CtClass subClass = pool.makeClass( fullName );
    final CtClass superClass = pool.get( DefinitionBasedMigrator.class.getName() );
    subClass.setSuperclass( superClass );
    subClass.setModifiers( Modifier.PUBLIC );

    // Add a constructor which will call super( ... );
    CtClass[] params = new CtClass[]{
        pool.get( MigratorDefinition.class.getName() ),
        pool.get( GlobalConfiguration.class.getName()) 
    };
    final CtConstructor ctor = CtNewConstructor.make( params, null, CtNewConstructor.PASS_PARAMS, null, null, subClass );
    subClass.addConstructor( ctor );

    return subClass.toClass();
}

Maven dependency:

<!-- https://mvnrepository.com/artifact/org.javassist/javassist -->
<dependency>
    <groupId>org.javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.22.0-GA</version>
</dependency>