Date now = new Date(System.currentTimeMillis());
claimsSet.setIssueTime(now);
claimsSet.setNotBeforeTime(now);
SignedJWT jwt = new SignedJWT(new JWSHeader(alg), claimsSet);
signer.signJwt(jwt, alg);
form.add("client_assertion_type", "urn:ietf:params:oauth:client-assertion-type:jwt-bearer");
form.add("client_assertion", jwt.serialize());