About

This plugin is a small POC to add an event that is triggered when primed TNT is spawned.

How it works

This plugin does a few things:

1. Attach a javaagent to the JVM running the server

   1. If it is running Java 8 or lower, all is fine. Just attach.
   2. If it is running Java 9 or newer and the system property switch to allow self-attachment is set, all is fine.
   3. If however, the switch is not set and it is running Java 9 or newer, the attaching process must be in a different JVM. This is achieved by packaging a small two class launcher jar that is then called and executed.

2. Use that agent to request a redefinition of the NMS World class, as that is where entities are added

3. Modify the bytecode of that class using javassist and inject some code. To understand what exactly, have a look at what the method in question does normally:

   public boolean addEntity(Entity entity, SpawnReason spawnReason) {
     if (entity == null) {
       return false;
     } else {
       Cancellable event = null;
       boolean flag;
       if (entity instanceof EntityLiving) {}
       if (entity instanceof EntityItem) {}
       // and so on
   
       if (event == null || !event.isCancelled()) {
         // add entity
       }
    }

   It now injects a single new if after the Cancellabl event line, that checks for EntityTNTPrimed and, if it finds it, calls the custom event. This requires some trickery, as the classloader used for the Event and plugin differs from the one running the NMS code. This means that two versions of the Event class could be instantiated, which means the handlerlist could differ. Unfortunately that means no event listeners will be registered either, so it effectively useless. To circumvent that we let Bukkit load the class and fetch the plugin for us — using that we get a class from our plugin class loader back. How neat!

4. So now our event is properly injected, called, actually distributed to listeners and handled in the same way as any other event. Life's good.

Caveats

• libattach.so needs to be present. It is bundled with the JDK, but a linux version is also found inside the jar. For other OSs I simply had no file to bundle, so it is not included.