HelloWorldTranslator.java

import java.io.IOException;
import java.lang.classfile.ClassFile;
import java.lang.classfile.ClassModel;
import java.lang.classfile.instruction.ConstantInstruction.LoadConstantInstruction;
import java.lang.constant.ClassDesc;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Arrays;

import static java.lang.classfile.ClassFile.ConstantPoolSharingOption;
import static java.lang.classfile.ClassTransform.transformingMethodBodies;

public class HelloWorldTranslator {
    public static void main(String[] args) throws IOException {
        if (args.length != 2) throw new RuntimeException("Expected input and translation.");
    
        var input = Path.of(args[0]);
        var translation = args[1];
    
        var cf = ClassFile.of();
        var classModel = cf.parse(input);
    
        byte[] newBytes = cf
            .withOptions(ConstantPoolSharingOption.NEW_POOL)
            .transformClass(classModel, transformingMethodBodies(
                (codeBuilder, codeElement) -> {
                    if (codeElement instanceof LoadConstantInstruction ldc &&
                        ldc.constantValue().equals("Hello World")) {
                        codeBuilder.ldc(translation);
                    } else {
                        codeBuilder.with(codeElement);
                    }
                }));
    
        Files.write(input, newBytes);
    }
}