sk1erllc/hookinjection

Lightweight utility for injecting the instructions of hooks into MethodNodes

Hook Injection

Lightweight utility for injecting the instructions of hooks into MethodNodes

Getting your hook's instructions:

getInstructions {

    // Reference your hook method. This can be static but it doesn't matter
    // [from] can also be used in-place of [of]
    of(MyHookClass::hookMethod)
    
    // Specify the [MethodNode] into which you are injecting
    target(methodNode)
    
    // The [AbstractInsnNode] before which you will inject your code; you can also use [after] instead
    // If neither is used, the hook's instructions will be appended to the target method's instructions
    before(methodNode.instructions.first)

    // If your hook takes parameters, you must then specify what should be passed as each one
    // There are a number of different options which can be used interchangeably:

    param(3) // The local variable with index 3 will be passed as the first parameter to the hook

    params(2, 4) // The local variables with the indexes 2 and 4 will be passed as the second and third parameters of the hook respectively
    
    param {
        // A set of instructions to load a value onto the stack, which will be treated as the fourth parameter of the hook
        // Koffee:
        iload_1
        iconst_2
        imul
    }

    param(LdcInsnNode(10f)) // An instruction to load a value onto the stack, which will be treated as the fifth parameter of the hook
    
    keepReturns // If you return directly after calling your hook, add this and don't add a return instruction
}

getInstructions returns an InsnList, with which you can then do what you please.

Your hook can also be injected directly into the target node, using injectInstructions instead of getInstructions.

In such a case, target can be replaced with into for clarity, but they are functionally identical.

If your hook contains any try-catch blocks, you should use getInstructionsWithTryCatchNodes (which returns Pair<InsnList, List<TryCatchBlockNode>>), or injectInstructionsWithTryCatchNodes.

Dealing with IllegalAccessExceptions:

Due to the nature of this utility, you are likely to run into some of these. Have a look at what is being accessed, and if it something you have defined yourself, make sure it's public. If it is a synthetic function, e.g. a lambda, then you should call HookInjectionUtils.makeMethodsPublic(classNode) in a transformer for your hook class(es).

General Tips

• It isn't recommended that you use HookInjectionUtils directly (other than for making synthetic methods public), but if you have to, e.g. you want to write your transformer in Java, everything should be (relatively?) well-documented inside the class. I will probably make some form of Java Builder in the future.
• If you run into any (unknown) problems, either submit an Issue, make a pull-request to fix said issue, or DM me on Discord (LlamaLad7#7444)

Known Issues