When allocating off-heap memory in Java (through direct buffers, or JNI native code for example), will the allocated memory be backed by huge pages if the JVM is using -XX:+UseLargePages ?
No, HotSpot JVM uses a plain libc malloc call to allocate memory for a direct ByteBuffer.
However, if you replace the standard system allocator with, for example, jemalloc - you'll be able to configure malloc to use huge pages when available.
Another option to use huge pages for direct ByteBuffers is to create a file on a hugetlbfs filesystem and then map it in Java as a MappedByteBuffer.