Dozer supports mappings done on proxy objects. This is typically the case when using persistence framework, which supports sophisticated features like lazy-loading. In this case application is working with fake objects, containing the real objects encapsulated. Implementation of proxies is dependant on the technology you use. Generally speaking, there are two popular libraries for creating Java proxies (Cglib and Javassist). However, how particular framework makes uses of them could also vary. Dozer offers by default a generic way to handle simple proxy scenarios, both Javassist and Cglib. However it is strongly recommended to tune proxy handling behavior for your particular scenario.
Proxy implementation is set-up by modifying configuration file. Currently, besides of default behavior, Hibernate and No-Proxy modes are supported. For the full list of the implementations, see the list of org.dozer.util.DozerProxyResolver interface implementations. The list could be retrieved from JavaDocs.
In case you do not map proxied objects - use NoProxy resolver, which imposes minimum performance overhead.