Features Overview

bytekin provides several powerful transformation features to manipulate Java bytecode. This section provides an overview of each feature.

Available Features

1. Inject - Code Insertion

Insert custom code at specific points in methods without modifying source code.

Use Cases:

• Add logging statements
• Implement cross-cutting concerns
• Add security checks
• Validate parameters

Example:

@Inject(methodName = "calculate", methodDesc = "(II)I", at = At.HEAD)
public static CallbackInfo logStart(int a, int b) {
    System.out.println("Starting calculation");
    return CallbackInfo.empty();
}

Learn More: Inject Transformation

2. Invoke - Method Call Interception

Intercept method calls and optionally modify arguments or return values.

Use Cases:

• Intercept specific method calls
• Modify method arguments
• Mock or stub methods
• Add pre/post processing

Example:

@Invoke(
    targetMethodName = "process",
    targetMethodDesc = "(Ljava/lang/String;)V",
    invokeMethodName = "validate",
    invokeMethodDesc = "(Ljava/lang/String;)V",
    shift = Shift.BEFORE
)
public static CallbackInfo validateBefore(String input) {
    return new CallbackInfo(false, null, new Object[]{input.trim()});
}

Learn More: Invoke Transformation

3. Redirect - Method Call Redirection

Change which method is called at runtime.

Use Cases:

• Redirect calls to alternative implementations
• Mock method behavior
• Implement method forwarding
• Change behavior based on conditions

Example:

@Redirect(
    targetMethodName = "oldMethod",
    targetMethodDesc = "(I)V",
    redirectMethodName = "newMethod",
    redirectMethodDesc = "(I)V"
)
public static void redirectCall(int value) {
    System.out.println("Redirected to new method: " + value);
}

Learn More: Redirect Transformation

4. Constant Modification

Modify constant values embedded in bytecode.

Use Cases:

• Change hardcoded configuration values
• Modify string literals
• Change numeric constants
• Patch constants at runtime

Example:

@ModifyConstant(
    methodName = "getVersion",
    oldValue = "1.0",
    newValue = "2.0"
)
public static CallbackInfo updateVersion() {
    return CallbackInfo.empty();
}

Learn More: Constant Modification

5. Variable Modification

Modify local variable values within methods.

Use Cases:

• Sanitize inputs
• Transform data
• Debug variable values
• Implement custom logic

Example:

@ModifyVariable(
    methodName = "process",
    variableIndex = 1
)
public static void transformVariable(int original) {
    // Transformation logic
}

Learn More: Variable Modification

Combining Features

You can use multiple features together for complex transformations:

@ModifyClass("com.example.Service")
public class ServiceHooks {
    
    // Inject logging
    @Inject(methodName = "handle", methodDesc = "(Ljava/lang/String;)V", at = At.HEAD)
    public static CallbackInfo logStart(String input) {
        System.out.println("Processing: " + input);
        return CallbackInfo.empty();
    }
    
    // Intercept internal calls
    @Invoke(
        targetMethodName = "handle",
        targetMethodDesc = "(Ljava/lang/String;)V",
        invokeMethodName = "validate",
        invokeMethodDesc = "(Ljava/lang/String;)V",
        shift = Shift.BEFORE
    )
    public static CallbackInfo validateInput(String input) {
        return new CallbackInfo(false, null, new Object[]{sanitize(input)});
    }
    
    private static String sanitize(String input) {
        return input.trim().toLowerCase();
    }
}

Choosing the Right Feature

Next Steps

• Learn about Inject transformation
• Explore Invoke interception
• Check Examples