FAQ - Frequently Asked Questions

General Questions

What is bytekin?

bytekin is a lightweight Java bytecode transformation framework built on ASM. It allows you to modify Java classes at the bytecode level without touching the source code.

Why would I need bytecode transformation?

Common use cases include:

Adding logging without modifying source code
Implementing cross-cutting concerns
Testing and mocking
Performance profiling
Security enhancements

How does bytekin compare to other tools?

Tool	Size	Complexity	Use Case
bytekin	Small	Simple	Direct bytecode manipulation
Spring AOP	Large	Complex	Enterprise framework
Mockito	Medium	Medium	Testing/Mocking
Aspect	Medium	Complex	Aspect-oriented programming

Is bytekin production-ready?

Yes, bytekin is designed for production use. It has minimal dependencies (only ASM) and has been tested thoroughly.

Technical Questions

What Java versions does bytekin support?

bytekin requires Java 8 or higher.

Can I use bytekin with Spring Boot?

Yes! bytekin can work alongside Spring Boot. You would typically apply transformations during a custom ClassLoader setup or at build time.

Does bytekin work with obfuscated code?

Yes, with mappings! Use the mapping system to handle obfuscated class and method names.

Can I combine multiple transformations?

Yes! You can use multiple @Inject, @Invoke, and other annotations on the same class. They all get applied.

Usage Questions

How do I find the method descriptor for a method?

Use javap:

javap -c MyClass.class

Look at the method signature and convert it to JVM descriptor format:

int add(int a, int b) → (II)I
String process(String s) → (Ljava/lang/String;)Ljava/lang/String;

What's the difference between Inject and Invoke?

Inject: Insert your code at a specific point in a method
Invoke: Intercept a method call within a method and possibly modify arguments

Can I cancel a method execution?

Yes, set ci.cancelled = true in your hook method. However, this only works for certain transformation types.

How do I modify method arguments?

Use CallbackInfo.modifyArgs:

ci.modifyArgs = new Object[]{ modifiedArg1, modifiedArg2 };

Can I access static fields from hook methods?

Yes, you can reference static fields from your hook class:

@Inject(...)
public static CallbackInfo hook() {
    // Access static fields
    if (cacheEnabled) {
        // ...
    }
}

Performance Questions

What's the overhead of using bytekin?

Transformation time: Minimal, happens once at class load
Runtime overhead: Zero! Transformed bytecode runs at same speed as hand-written code

Should I rebuild transformers for each transform?

No! Build once and reuse:

// Good
BytekinTransformer transformer = new BytekinTransformer.Builder(MyHooks.class).build();
for (String className : classNames) {
    byte[] transformed = transformer.transform(className, bytecode);
}

// Bad
for (String className : classNames) {
    BytekinTransformer transformer = new BytekinTransformer.Builder(MyHooks.class).build();
    byte[] transformed = transformer.transform(className, bytecode);
}

Wrong class name: Check the @ModifyClass value matches exactly
Wrong method descriptor: Verify the methodDesc parameter
Class not loaded: Ensure the class is loaded before transformation