openssl pkcs12 -in ./file.pfx -out file.crt -nokeys -clcerts
openssl x509 -inform pem -in ./file.crt -outform der -out ./file_der.crt

byte[] key = FileUtils.readFileToByteArray(new File("file_der.crt"));

KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(key);
RSAPrivateKey priv_key = (RSAPrivateKey) keyFactory.generatePrivate(keySpec);

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException : DER input, Integer tag error

openssl req -new -x509 -days 3600 -newkey rsa:1024 -keyout partner.pem -out partner.crt -subj "/C=UA/ST=Kyiv region/L=Kiev/O=Partner/OU=IT/CN=www.partner.com.ua" 
Generating a 1024 bit RSA private key
...................++++++
......++++++
writing new private key to 'partner.pem'
Enter PEM pass phrase: ******
Verifying - Enter PEM pass phrase: ******
-----

openssl pkcs12 -export -out partner.pfx -inkey partner.pem -in partner.crt

            byte[] key = FileUtils.readFileToByteArray(new File("5\\partner.pfx"));

            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(key);
            RSAPrivateKey privKey = (RSAPrivateKey) keyFactory.generatePrivate(keySpec);

            signature = Signature.getInstance("SHA1withRSA");
            signature.initSign(privKey);

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException : version mismatch: (supported:     00, parsed:     03