无法反编译JAR文件

Question

I found a really strange JAR file. It contains two classes. I tried every single decompiler I know, even IntelliJ's Fernflower. All of them gave me this result: Screenshot of IntelliJ's decompiler When I try to view the bytecode of the class, I can see some weird stuff aswell: Screenshot of ASM Bytecode Viewer in IntelliJ It says that the class is "synthetic". I looked up what it means in google and I really have no idea how such a class can be considered synthetic. Any ideas how this is made and how it's possible to replicate such a thing?

Answer 1

In the most cases you are not allowed to decompile software. In this case I'm pretty sure you are not allowed, because the class name was obfuscated . But the strange @thingys are properly some type of Lombok or custom Annotation from a libary

Edit: It is a Annotation of minecraftforge. Here is some information on it: https://mcforge.readthedocs.io/en/latest/gettingstarted/structuring/#what-is-mod

Please do not decompile sourcecode you dont own!

Answer 2

The synthetic flag is used to mark classes/fields/methods/... that have been generated by the compiler and did therefore not exist in source code. (see JVM-spec 4.1-B )

Since you've added the java-bytecode-asm i guess you want to know how to achieve this using the objectweb asm library. It is done by setting the ACC_SYNTETHIC flag for the class.

Here is a class visitor that does it for the class and all fields and methods:

public class MakeSyntheticVisitor extends ClassVisitor {
    public MakeSyntheticVisitor(int api, ClassVisitor classVisitor) {
        super(api, classVisitor);
    }

    @Override
    public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
        super.visit(version, access | Opcodes.ACC_SYNTHETIC, name, signature, superName, interfaces);
    }

    @Override
    public FieldVisitor visitField(int access, String name, String descriptor, String signature, Object value) {
        return super.visitField(access | Opcodes.ACC_SYNTHETIC, name, descriptor, signature, value);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
        return super.visitMethod(access | Opcodes.ACC_SYNTHETIC, name, descriptor, signature, exceptions);
    }
}

Let's use our visitor to transform a class:

public static void main() throws Exception {
    InputStream in = new FileInputStream(new File("in/Test.class"));

    ClassReader cr = new ClassReader(in);
    ClassWriter cw = new ClassWriter(0);

    cr.accept(new MakeSyntheticVisitor(Opcodes.ASM7, cw), 0);

    FileOutputStream out = new FileOutputStream(new File("obfuscated/Test.class"));
    out.write(cw.toByteArray());
    out.close();
}