How do I get and use a class type from a Java class that I've dynamically created at runtime using ASM?

Question

I'd like extend the class, get a class literal, use it as a type parameter, or cast to it.

I already have a workaround, but I'm still curious.

Currently I can define a class at runtime with ASM, create an instance with Object type and invoke its methods with reflection, but I don't know how to use it as a class type.

Granted, the compiler doesn't know about classes defined at runtime, but in this case I know about it, and I think that I see it hidden in the ASMified .java file (example below). How can convince the compiler that it actually does know the class type at compile time?

Thanks for taking a look,

-Julian

Example of an ASMified class B: bytecode-generating code that spoofs class B{String hello = "Hello B!"}:

import java.util.*;
import org.objectweb.asm.*;
//import org.objectweb.asm.attrs.*;
public class BDump implements Opcodes {

public static byte[] dump () throws Exception {

ClassWriter cw = new ClassWriter(0);
FieldVisitor fv;
MethodVisitor mv;
AnnotationVisitor av0;

cw.visit(V1_6, ACC_SUPER, "B", null, "java/lang/Object", null);

{
fv = cw.visitField(0, "hello", "Ljava/lang/String;", null, null);
fv.visitEnd();
}
{
mv = cw.visitMethod(0, "<init>", "()V", null, null);
mv.visitCode();
mv.visitVarInsn(ALOAD, 0);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
mv.visitVarInsn(ALOAD, 0);
mv.visitLdcInsn("Hello B!");
mv.visitFieldInsn(PUTFIELD, "B", "hello", "Ljava/lang/String;");
mv.visitInsn(RETURN);
mv.visitMaxs(2, 1);
mv.visitEnd();
}
cw.visitEnd();

return cw.toByteArray();
}
}

Peter Lawrey · Accepted Answer · 2013-04-21 21:12:42Z

4

You can't use a class at compile time which doesn't exist at compile time. What you can do is

use a class or interface which exists at compile time but is replaced at runtime with the desired implementation.
use an interface which is available at compile time but is only implemented at runtime.
use reflection to call the constructor/methods at runtime.

answered Apr 21, 2013 at 21:12

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Julian Peeters Over a year ago

Thanks for the helpful response. It begs more questions:) Where is the breakdown, exactly? Since there is nothing I don't know about the class at compile time, I naively expect to be able to provide the compiler with a spoofed version of whatever it should expect, but I couldn't find a "Java Compiler For Dummies" on the web.

Peter Lawrey Over a year ago

Generating byte code is something which few developers with ten years experience have done whereas you should understand the basics of how to use compiler after a few months. I am a little confused as to your familiarity with Java.

Julian Peeters Over a year ago

Noob status here, learning Java through some fun projects. I know some of the basics about compilers, e.g. lexer/scanner-parser-analyzer phases, but I can't seem to find out how exactly ASTs are used and what exactly happens in each phase so that I might be able to circumvent whatever check is throwing the error.

Collectives™ on Stack Overflow

How do I get and use a class type from a Java class that I've dynamically created at runtime using ASM?

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related