When a Dbi is created using a custom comparator it only works whilst a full GC has not run. This can be show by adding a System.gc() between the construction of the map and the put invocation in org.lmdbjava.DbiTest#customComparator.

On executing put it throws:
java.lang.NullPointerException: callable is null at jnr.ffi.provider.jffi.NativeClosureProxy.getCallable(NativeClosureProxy.java:57) at jnr.ffi.provider.jffi.NativeClosureProxy$$impl$$0.invoke(Unknown Source) at com.kenai.jffi.Foreign.invokeL5(Native Method) at com.kenai.jffi.Invoker.invokeL5(Invoker.java:438) at org.lmdbjava.Library$Lmdb$jnr$ffi$0.mdb_put(Unknown Source) at org.lmdbjava.Dbi.put(Dbi.java:421)

This happens since NativeClosureFactory.ClosureReference is a WeakReference and nothing else holds a reference to theComparatorCallback.

I've tested that converting ComparatorCallback ccb into a field fixes this, since it ensures the callback remains alive whilst the Dbi object is accessible.