Bukkit

Debug Output Levels: Below indicates what is output for a given debug level.
It also shows the indentation that will happen in the logs.

  level     What is displayed at given level and in what order
    0       no debug reporting
      2       Notification that cache entry for given hopper (location) was cleared.
      2       Notification that cache entry for all hoppers were cleared.
      2       Cache for a given hopper (location) if it exists in cache
      2       Notification that cache entry is being created for a given hopper (location) if it does not exist
       3        Notification that we are looking for item frames
        4         Notification that it found an attached item frame
        4         Information about the item in the itemFrame if one was found
       3        Notification that we are looking for chests
        4         Notification as to what direction the hopper is facing (output direction to avoid looking for a chest there)
        4         Notification that it found a chest N, S, E, or W of the hopper that is not being pushed into by the hopper
        4         Information about each item in the chest if any are found
      2       Displays generated cache
     1      Notification of what item in in what filter (location) is compared against what cache
     1      Notification that an itemFrame was placed on a hopper
     1      Notification that an itemFrame on a hopper was broken
     1      Notification that an itemFrame on a hopper was altered
     1      Notification that a chest was placed next to a hopper
     1      Notification that a chest was broken next to a hopper
     1      Notification that a chest next to hopper had its contents altered
     1      Notification that a hopper with a filter was broken