ASM create method with parameter name w/ ClassWriter

Question

Kotlin Code using org.objectweb.asm library

import org.objectweb.asm.ClassWriter
import org.objectweb.asm.Opcodes.*

fun main(args: Array<String>) {
    val classWriter = ClassWriter(ClassWriter.COMPUTE_MAXS)

    classWriter.visit(
            V1_8,
            ACC_PUBLIC,
            "com/github/patrick/learnasm/build/HelloWorld",
            null,
            "java/lang/Object",
            null
    )

    val methodVisitor = classWriter.visitMethod(
            ACC_PUBLIC + ACC_STATIC,
            "main",
            "([Ljava/lang/String;)V",
            null,
            null
    )

    methodVisitor.visitFieldInsn(
            GETSTATIC,
            "java/lang/System",
            "out",
            "Ljava/io/PrintStream;"
    )

    methodVisitor.visitLdcInsn(
            "Hello World!"
    )

    methodVisitor.visitMethodInsn(
            INVOKEVIRTUAL,
            "java/io/PrintStream",
            "println",
            "(Ljava/lang/String;)V",
            false
    )

    methodVisitor.visitInsn(
            RETURN
    )

    methodVisitor.visitMaxs(
            0,
            0
    )

    methodVisitor.visitEnd()

    val bytes = classWriter.toByteArray()
    val clazz = defineClass(
            "com.github.patrick.learnasm.build.HelloWorld",
            bytes,
            0,
            bytes.count()
    )

    clazz.getDeclaredMethod("main", Array<String>::class.java).invoke(null, null)
}

I am trying to learn asm, and I'm curious whether creating a method with named parameter is possible.

For example in this case, the created class has no parameter name (defaults to var0 in decompiler), and I wish I could save parameter names like "args". Would it be possible?

actually i used methodVisitor.visitLocalVariable, which worked very well. I'll edit and provide some example below. — PatrickKR
You should use both: One for the local variable, the other to get the parameter name using reflection. — Johannes Kuhn

PatrickKR PatrickKR · Accepted Answer · 2020-11-11T07:35:11

Found an answer by looking into compiled class (by including debugging information)

The simple way is to just mock the compiled class

All I had to do is put some label on it, and visit local variable with name.

            methodVisitor = classWriter.visitMethod(
                    Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC,
                    "main",
                    "([Ljava/lang/String;)V",
                    null,
                    null
            )

            methodVisitor.visitParameterAnnotation(
                    0,
                    "Lorg.jetbrains.annotations.NotNull;",
                    false
            )

            val start = Label()
            methodVisitor.visitLabel(start)

            methodVisitor.visitFieldInsn(
                    Opcodes.GETSTATIC,
                    "java/lang/System",
                    "out",
                    "Ljava/io/PrintStream;"
            )

            methodVisitor.visitLdcInsn("Hello World!")

            methodVisitor.visitMethodInsn(
                    Opcodes.INVOKEVIRTUAL,
                    "java/io/PrintStream",
                    "println",
                    "(Ljava/lang/String;)V",
                    false
            )

            methodVisitor.visitInsn(Opcodes.RETURN)

            val end = Label()
            methodVisitor.visitLabel(end)

            methodVisitor.visitLocalVariable(
                    "args",
                    "[Ljava/lang/String;",
                    null,
                    start,
                    end,
                    0
            )

            methodVisitor.visitMaxs(0, 0)
            methodVisitor.visitEnd()

ASM create method with parameter name w/ ClassWriter

1 Answers