fixed.java

  ClassReader reader = new ClassReader(bungeecordJar.getInputStream(jarEntry));
  ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS);

  ClassNode node = new ClassNode(); {
      reader.accept(node, 0);
  }


  for (int i = 0; i < node.fields.size(); i++) {
      // prevent duplicates if the jar is already patched
      if (node.fields.get(i).name.equals("lcServer")) {
          node.fields.remove(node.fields.get(i--));
      }
  }
  node.fields.add(new FieldNode(ACC_PRIVATE, "lcServer", "Ljava/lang/String;", null, null));

  for (MethodNode mn: node.methods) {

      if (mn.name.equalsIgnoreCase("<init>")) {


          InsnList insnList = new InsnList(); {
              // this.lcServer = "Server Name";
              insnList.add(new VarInsnNode(ALOAD, 0));
              insnList.add(new LdcInsnNode(server));
              insnList.add(new FieldInsnNode(PUTFIELD, "net/md_5/bungee/api/ServerPing", "lcServer", "Ljava/lang/String;"));
          }

          for (int i = 0; i < mn.instructions.size(); i++) {
              // insert at the very end
              if (mn.instructions.get(i).getOpcode() == INVOKESPECIAL) {
                  if (mn.instructions.get(i) instanceof MethodInsnNode) {
                      if (!((MethodInsnNode) mn.instructions.get(i)).desc.equalsIgnoreCase("()V")) {
                          // likely an constructor similar to this(x, y, z);
                          // we want to skip these.
                          break;
                      }
                  }
              }

              if (mn.instructions.get(i).getOpcode() == PUTFIELD) {
                  if (((FieldInsnNode) mn.instructions.get(i)).name.equalsIgnoreCase("lcServer")) {
                      // We want to prevent possible duplicates if the jar is already patched
                      for (int j = 0; j < 3; j++) {
                          mn.instructions.remove(mn.instructions.get(i--));
                      }
                  }
              }

              // Inject right before the RETURN statement
              // We don't need to worry about anything like IRETURN. This is a void function
              if (mn.instructions.get(i).getOpcode() == RETURN) {
                  mn.instructions.insertBefore(mn.instructions.get(i), insnList);
                  break;
              }
          }
      }
  }
  node.accept(classWriter);