The methods getSubjectDN() and getIssuerDN() in X509Certificate and getIssuerDN() in X509CRL are problematic. They return an unspecified class implementing the java.security.Principal interface, which has a very loose specification.
Because no additional specification is present in the getSubjectDN() and getIssuerDN() methods, it is permissible for implementations to return an arbitrary, implementation specific class. Real world experience has shown that this is the case resulting in non-portability or unreliability of the code. For compatibility reasons, the specifications for those methods cannot be changed and they must be considered unsalvageable.
Replacement methods getSubjectX500Principal() & co that return an instance of the well-defined X500Principal class were added in JDK 1.4. The implementations of those methods have been designed to avoid all problems of this sort. However, the new methods suffer from underexposure and programmers continue to use the familiar and more intuitively named getSubjectDN() & co methods.
To change this, the old getSubjectDN() and getIssuerDN() methods should be deprecated. That will ensure that developers who use this methods receive a compile time warning.