Core Concepts

This section explains the fundamental concepts behind bytekin and bytecode transformation.

What is Bytecode?

Java source code is compiled into bytecode - a platform-independent intermediate representation that runs on the Java Virtual Machine (JVM). Unlike source code, bytecode is not human-readable but is standardized and can be manipulated programmatically.

Bytecode Structure

Java bytecode consists of:

• Constant Pool: String literals, method references, field references
• Methods: Compiled methods with instruction sequences
• Fields: Class properties
• Attributes: Metadata like line numbers, local variables, exceptions

What is Bytecode Transformation?

Bytecode transformation is the process of reading, analyzing, and modifying bytecode after compilation but before class loading. This allows you to alter class behavior without modifying source code.

Use Cases

1. Runtime Monitoring: Add logging without changing source
2. Cross-Cutting Concerns: Apply behavior to multiple classes
3. Testing: Mock or stub methods
4. Security: Inject security checks
5. Performance: Add profiling instrumentation

How bytekin Fits In

bytekin simplifies bytecode transformation by:

1. Abstracting ASM Complexity: Provides a clean API over raw ASM
2. Annotation-Based Configuration: Use Java annotations to define transformations
3. Multiple Strategies: Support for injections, method interception, and redirects
4. Mapping Support: Handle obfuscated or renamed classes
5. Flexible Builder Pattern: Programmatically compose transformations

The Transformation Pipeline

Target Class Bytecode
        ↓
   BytekinTransformer
        ↓
  Scan Hook Classes
        ↓
  Apply Transformations
        ↓
   Modified Bytecode

Key Components

Hook Classes

Classes annotated with @ModifyClass that define how to transform target classes. They contain methods with transformation annotations.

Transformers

Objects that apply transformations to bytecode. bytekin provides BytekinTransformer which handles the entire transformation process.

Annotations

Special markers in Java code that define transformation behavior:

• @ModifyClass: Mark the target class
• @Inject: Insert code at specific points
• @Invoke: Intercept method calls
• And more...

CallbackInfo

A data structure that controls transformation behavior:

• cancelled: Whether to skip original code
• returnValue: Custom return value
• modifyArgs: Modified method arguments

Important Concepts

Method Descriptors

Method descriptors describe method signatures in JVM format:

(ParameterTypes)ReturnType

Examples:

• (II)I - Takes two ints, returns int
• (Ljava/lang/String;)V - Takes String, returns void
• ()Ljava/lang/String; - Takes nothing, returns String

Class Names

In bytekin, class names use dot notation:

• Java notation: java.lang.String
• Internal notation: java/lang/String
• bytekin uses Java notation

Next Steps

• Learn about How bytekin Works
• Explore All Features