Generation of DHKeyPairs with bit lengths > 1024 is not supported in JDK 6u, 7u at moment. Proposal to increase the limit up to 2048 bits similar to that found in JDK 8. This functionality probably means that the SHA224 hash function support needs to be backported along with loosening of checks in the keygenerator classes.
java.lang.RuntimeException: Could not generate DH keypair and
java.security.InvalidAlgorithmParameterException: Prime size must be multiple of 64, and can only range from 512 to 1024 (inclusive).