Vim plugins declared in ftplugin do not work - vim

Question

Gvim is behaving weird and I can't find the reason. I use Vundle, and all the plugins declared in my .vimrc are working fine. I declared some additional settings and plugins in .vim/after/ftplugin/java.vim.
The mappings work fine, but the plugins do not work. If I choose a different file in my current gvim session, I get those error messages:
Error detected while processing function vundle#config#bundle[2]..<SNR>14_check_bundle_name:
line 2:
Vundle error: Name collision for Plugin Raimondi/delimitMate. Plugin Raimondi/delimitMate previously used the name "delimitMate". Skipping Plugin Raimondi/delimitMate.
Vundle error: Name collision for Plugin artur-shaik/vim-javacomplete2...
[comment: same error message for all plugins declared in the ftplugin]
I noticed, that if I run :VundleInstall the plugins are suddenly working (the error messages stay when I change the file, no plugins are installed when I use the command).
Here is the beginning of my .vimrc:
syntax on
set guifont=Inconsolata\ Medium\ 12
set nocompatible
set t_Co=256
filetype off
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()
" let Vundle manage Vundle, required
Plugin 'VundleVim/Vundle.vim'
"[comment: all plugins I use for every filetype]
call vundle#end() " required
filetype plugin indent on
and this is my java.vim file:
filetype off
"to automatically close brackets
Plugin 'Raimondi/delimitMate'
"omni-complete for Java
Plugin 'artur-shaik/vim-javacomplete2'
"use tab to navigate through insert completion
Plugin 'ervandew/supertab'
filetype plugin indent on
"needed to make javacomplete2 working properly
autocmd FileType java setlocal omnifunc=javacomplete#Complete
My OS is Ubuntu 16.04.

Answer 1

You're mistaken regarding what ftplugins are doing and what they should contain.
Ftplugins are loaded once per buffer, every time a new buffer is created/opened.
They are meant to contain buffer local definitions:
:map <buffer> keybinding action
:iab <buffer> keybinding expanded sequence
:setlocal option[=value]
:command -b CommandName :Action
:let b:option = value
set the localleader (but be certain it's done before any other ftplugin for the same filetype) (EDIT: localleader is actually a global setting, my mistake)
They could then load other things that work the same way with :runtime or :so. They could contain functions, but it's best to define them into autoload plugins since Vim7. They may contain buffer local menu definitions, but this requires a plugin as this is not standard.
They are definitively not meant to contain global definition like the ones you have defined. It's not really the place to load global plugins that'll stay activated afterwards.
I know that some plugins manager load plugins on the fly depending on the type of the file we work on. I've never shared this need when we are using properly defined ftplugins, and lightweight plugins that only define a few mappings and keep their functions into autoload plugins.
Last thing, ftplugins are supposed to contain anti reinclusion guards. On a typical scenario this is not that useful. Many use b:did_ftplugin for that purpose, but I avoid this variable as I prefer to have as many ftplugins (for a same filetype) as themes (one that specializes the brackets pairs, one that defines the mapping to automatically expand a switch statement from the type of a variable, one that defines abbreviations for control statements, and so on). As a consequence I cannot use the same guard for all files.

Answer 2

All your :Plugin commands are supposed to be between these two lines:
call vundle#begin()
" :Plugin commands go here
call vundle#end()
Try another plugin manager if you absolutely need lazy loading.