Java - creating a subclass dynamically

Question

I'd like to create a subclass programatically. I guess I have few options - Javassist, CGLib, BCEL, or ASM.

The use case is that one app's internals are class-oriented, and extensions are class-based. Therefore I can't have a single class as a base for multiple extensions driven by externalized scripts.

Now - how would I do that? I've found examples with intercepting method calls, field access, initialization etc. But nothing about subclassing.

I'd like to end up with a class which:

• has a name which I want.
• is a (direct, at best) subclass of a given class
• copies the constructor(s) from the parent class (or calls super(...))
• eventually, I'd like to give it some annotations.

I know it's possible because various dynamic languages integrations, like GroovyClassLoader, can do that.

Answer 1

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>