As a result of introducing the replacement SunRSASign provider in JDK 5.0, the following classes found in earlier (i.e. 1.4.2 and earlier) were removed from the JDK.

com/sun/net/ssl/internal/ssl/JSA_RSAPrivateKey
com/sun/net/ssl/internal/ssl/JSA_RSAnonCRTPrivateKey
com/sun/net/ssl/internal/ssl/JSA_RSAPublicKey

Serialized instances of these classes will not deserialize on JDK 1.5 and above.