java - need to import private key from jks(java keystore)

I have a stored my certificate in Java KeyStore which is required for accessing Restful API. To access this API I need to create JWT (Json Web Token) which I am implementing in Java. For generating JWT I need to sign the payload with given private key. So, what I am doing is, importing private key from .jks file into my code

FileInputStream is = new FileInputStream("src/main/resources/file.jks");
            KeyStore keystore = KeyStore.getInstance(KeyStore.getDefaultType());
            keystore.load(is, "password".toCharArray());  
PrivateKey privateKey = (PrivateKey) keystore.getKey("key", "".toCharArray());
JWTClaimsSet claimsSetOne = new JWTClaimsSet.Builder()
                .subject("alias")
                .issueTime(new Date(123000L))
                .issuer("https://issuer")
                .audience("https://audience")
                .build();

JWSSigner signer = new RSASSASigner(privateKey);
SignedJWT signedJWT = new SignedJWT (new JWSHeader(JWSAlgorithm.RS256), claimsSetOne);
signedJWT.sign(signer);
String orderOne = signedJWT.serialize();

Now, with this piece of code, It returns private key as "null" value. which results into failure of my code. So, what could be my code so that I can read private key to sign the payload to generate JWT. Or any other alternative to achieve same. Thanks in advance.
Edit : Then I tried this command,

File file = new File("src/main/resources/sho1.jks");
InputStream   is = new FileInputStream(file);
KeyStore keystore = KeyStore.getInstance(KeyStore.getDefaultType());
String password = "sho123";
keystore.load(is, password.toCharArray());

KeyStore.PrivateKeyEntry keyEnt = 
   (KeyStore.PrivateKeyEntry) keystore.getEntry("sho",new KeyStore.PasswordProtection(password.toCharArray()));
PrivateKey privateKey = keyEnt.getPrivateKey();

It gives me error as Invalid_Signature.