CASM

The CASM annotation can be used to inject into a class or method using direct ASM.
You get direct access to the ClassNode or MethodNode and can modify it however you want.
The position of CASM in the handler chain is at the top or bottom, depending on the shift value: @CASM(shift = Shift.BOTTOM).

The code inside the transformer method is executed isolated from the rest of the transformer. Isolated does not mean the code is sandboxed!
This means that you can't access any fields from the transformer class. Methods are copied over to the isolated class and can be accessed from there.
Due to some issues with the LambdaMetaFactory, the use of lambdas is not supported.

Transforming a class

To transform a class, you need to keep the value of the annotation empty.

@CASM
public static void transform(final ClassNode classNode) {
    //Do something with the class
}

The transformer method has to be static and needs to have a single parameter of type ClassNode and must return void.

Transforming a method

To transform a method, you need to specify the method pattern in the value of the annotation (see Targets for more information).
The value field is an array of strings, so you can specify multiple methods to transform.

@CASM("methodName")
public static void transform(final MethodNode methodNode) {
    //Do something with the method
}

The transformer method has to be static and needs to have a single parameter of type MethodNode and must return void.