Java Community | Help. Code. Learn.•2y ago

Bytecode manipulation

I'm interested in some bytecode manipulation and want to develop a simple library which would allow you to inject annotated code into a different method(similiar to Spongepowered Mixins). The thing is that I don't know how should I approach the bytecode at first. I can:

Create an agent, which would be loaded and would transform the bytecode
- disadvantage is that the user would have to specify the agent to be loaded, the idea was that it should've been done automatically
Use custom gradle plugin to transform the bytecode while the jar is making
- disadvantage is that it would only support code inside the jar, so any method in classpath wouldn't work(the idea was it could be injected into any method in classpath)
  Maybe there is a different approach I could use, heard fabric uses some sort of custom classloaders but I don't really know how it works

JavaBot•2/8/24, 11:57 AM

⌛

⌛

This post has been reserved for your question.

Hey @Koblížkáč! Please use
/close
/close
or the
Close Post
Close Post
button above when your problem is solved. Please remember to follow the help guidelines. This post will be automatically closed after 300 minutes of inactivity.

TIP: Narrow down your issue to simple and precise questions to maximize the chance that others will reply in here.

KoblížkáčOP•2/8/24, 11:57 AM

note that I know how to transform the bytecode, i just need help with the obtaining and applying the bytecode

0x150•2/8/24, 12:10 PM

fabric has a custom class loader which minecraft and mods get passed through, that applies the mixin when the class is loaded

0x150•2/8/24, 12:11 PM

when you make a classloader and ask it for a class, the class loader needs to find, load the bytes of the class and define the class using those bytes, then gives you the newly defiend Class instance

0x150•2/8/24, 12:11 PM

you can make your own class loader that transforms those original class bytes before defining the class, which is what fabric does

0x150•2/8/24, 12:12 PM

agents have a similar story but their transforming capabilities are more limited, since the class the bytes belong to is already loaded, and new changes need to conform to a certain limited set of modifications you can make to that already existing class

0x150•2/8/24, 12:12 PM

eg you cant add, remove or change any field descriptors or method descriptors belonging to the class, you can only change method bodies

0x150•2/8/24, 12:13 PM

with the classloader approach, those limitations go away since the class doesnt even exist yet. you can manipulate the raw class bytes in (almost) any way you'd like before defining the class

00x150 with the classloader approach, those limitations go away since the class doesnt ...

KoblížkáčOP•2/8/24, 12:19 PM

but you would somehow need to set the custom classloader as the default one, right? Since every class is loaded by appclassloader by default, isnt it?

0x150•2/8/24, 12:20 PM

all classes you load with that class loader will have that class loader as parent class loader, and ergo use that class loader by default

00x150 all classes you load with that class loader will have that class loader as paren...

KoblížkáčOP•2/8/24, 12:38 PM

why does this then prints:

KoblížkáčOP•2/8/24, 12:38 PM

jdk.internal.loader.ClassLoaders$AppClassLoader@5cb0d902

jdk.internal.loader.ClassLoaders$AppClassLoader@5cb0d902

0x150•2/8/24, 12:39 PM

might be because its an inner class

KoblížkáčOP•2/8/24, 12:39 PM

i see

00x150 might be because its an inner class

KoblížkáčOP•2/8/24, 12:43 PM

okay its not that

KoblížkáčOP•2/8/24, 12:44 PM

public static void main(String[] args) throws ClassNotFoundException {
        CustomClassLoader customClassLoader = new CustomClassLoader();
        Class<?> clazz = customClassLoader.loadClass(Main.class.getName());
        System.out.println(clazz.getClassLoader());
    }

public static void main(String[] args) throws ClassNotFoundException {
        CustomClassLoader customClassLoader = new CustomClassLoader();
        Class<?> clazz = customClassLoader.loadClass(Main.class.getName());
        System.out.println(clazz.getClassLoader());
    }

prints: jdk.internal.loader.ClassLoaders$AppClassLoader@5cb0d902jdk.internal.loader.ClassLoaders$AppClassLoader@5cb0d902

0x150•2/8/24, 12:45 PM

is your class loader actually loading the class or are you passing it to another class loader down the line

0x150•2/8/24, 12:45 PM

like is it your class loader that is defining the class

KoblížkáčOP•2/8/24, 12:52 PM

yeah youre right i was just calling super

KoblížkáčOP•2/8/24, 12:52 PM

but in case like this:
System.out.println(Main.class.getClassLoader());System.out.println(Main.class.getClassLoader());
it would still print the AppClassLoader

KoblížkáčOP•2/8/24, 12:53 PM

is it possible to have it use the custom classloader?

0x150•2/8/24, 12:53 PM

well you load the class via it

0x150•2/8/24, 12:53 PM

that should do it

KoblížkáčOP•2/8/24, 12:53 PM

well

0x150•2/8/24, 12:54 PM

Main.class is a direct request to the parent class loader

0x150•2/8/24, 12:54 PM

loading it via the parent sets the class loader for that Class instance

0x150•2/8/24, 12:54 PM

if you'd invoke anything on that Class, it would use your class loader as class loader

00x150 Main.class is a direct request to the parent class loader

KoblížkáčOP•2/8/24, 1:23 PM

and can i somehow load it so i can use the class normally and have the bytecode changed?

KKoblížkáč and can i somehow load it so i can use the class normally and have the bytecode ...

0x150•2/8/24, 1:24 PM

you load the entry class from the class loader and call a method on it

0x150•2/8/24, 1:24 PM

that method then uses your class loader to load its classes

0x150•2/8/24, 1:24 PM

and that continues recursively

00x150 you load the entry class from the class loader and call a method on it

KoblížkáčOP•2/8/24, 1:25 PM

what is the entry class?

KKoblížkáč what is the entry class?

0x150•2/8/24, 1:25 PM

whatever class you define to be the entry point to the other parts of the program you want to modify

0x150•2/8/24, 1:26 PM

for fabric, this would be minecraft's main class

00x150 whatever class you define to be the entry point to the other parts of the progra...

KoblížkáčOP•2/8/24, 5:42 PM

Well but lets say id want every class to be loaded with my classloder, is it possible

KKoblížkáč Well but lets say id want every class to be loaded with my classloder, is it pos...

0x150•2/8/24, 6:09 PM

yes

0x150•2/8/24, 6:09 PM

just make the class calling those classes loaded by your class loader

0x150•2/8/24, 6:11 PM

class Init {
  public static void start() {
    // SomeOtherClass is loaded by your class loader
    SomeOtherClass.bruh();  
  }
}
class SomeOtherClass {
  public static void bruh() {
    // will also load with your class loader
  }
}

class Main {
  public static void main(String[] a) {
    CustomClassLoader cl = new CustomClassLoader();
    cl.loadClass("Init").getMethod("bruh").invoke();
  }
}

class Init {
  public static void start() {
    // SomeOtherClass is loaded by your class loader
    SomeOtherClass.bruh();  
  }
}
class SomeOtherClass {
  public static void bruh() {
    // will also load with your class loader
  }
}

class Main {
  public static void main(String[] a) {
    CustomClassLoader cl = new CustomClassLoader();
    cl.loadClass("Init").getMethod("bruh").invoke();
  }
}

00x150 ```java class Init { public static void start() { // SomeOtherClass is loa...

KoblížkáčOP•2/8/24, 6:28 PM

but no way to make it be done automatically without need to use reflection?

KKoblížkáč but no way to make it be done automatically without need to use reflection?

0x150•2/8/24, 7:47 PM

the only part where you need to do anything is in the reflection part

0x150•2/8/24, 7:48 PM

that is you calling the init method

0x150•2/8/24, 7:48 PM

all methods called by that init method, all classes referenced will be loaded via your class loader

JavaBot•2/9/24, 12:50 AM

💤

💤

Post marked as dormant

This post has been inactive for over 300 minutes, thus, it has been archived.
If your question was not answered yet, feel free to re-open this post or create a new one.
In case your post is not getting any attention, you can try to use
/help ping
/help ping
.
Warning: abusing this will result in moderative actions taken against you.

Bytecode manipulation

Similar Threads

Similar Threads

Similar Threads