Troubleshoot hook_library_alter() not being called for the jQuery Update module I have the jQuery Update's development shapshot version 7.x-2.x-dev installed and enabled. The version jQuery configured in UI is not being imported. As result, it is importing version 1.4.4 instead of the version I specified. Debugging, I noticed that jquery_update_library_alter() is not being called. In meanwhile, my custom hook_library_alter() is called, and I use that as temporary solution until I figure out why the hook implemented by the jQuery Update module is not called.

function MODULE-NAME_library_alter(&amp;$javascript, $module) {
  module_load_include('module', 'jquery_update');
  jquery_update_library_alter($javascript, $module);
}


This is first time I encounter a situation where hooks are not called. Where are hooks attached to and where are they called from? How would you approach to troubleshoot hooks? Where can I check all registered hooks?
I had this problem as well and I tracked it down to a custom module calling drupal_add_js() right at the top of the module file. This meant that drupal_get_library(system, jquery) and in turn module_implements(library_alter) were getting called before the jquery_update.module file had a chance to get loaded and hence jquery_update_library_alter wasn't defined yet.

To find this I put the following lines into drupal/include/module.inc and cleared the cache.

...
function module_implements($hook, $sort = FALSE, $reset = FALSE) {
    if ($hook == 'library_alter') { debug_print_backtrace(); exit; }
...


You might have some other problem, but a call to drupal_add_js to early in execution was my issue.