The definitive JNA reference (including an overview and usage details) is in the JavaDoc. Please read the overview. Questions, comments, or exploratory conversations should begin on the mailing list, ...
Recommended hash pairings (not enforced by JPGPJ; Bouncy Castle fails at runtime if invalid): RSA / DSA: SHA-256 or SHA-512 ECDSA (NIST P-256): SHA-256; P-384: SHA-384; P-521: SHA-512 Ed25519: SHA-512 ...