How to use keystore in Java to store private key?

NOTE: This code is for demonstration purposes only. Private keys must be encrypted when you store them on disk. Do not use it as is.

You can do something like this:

 KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
 kpg.initialize(2048);

 KeyPair kp = kpg.genKeyPair();

 KeyFactory fact = KeyFactory.getInstance("RSA");

 RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(),
        RSAPublicKeySpec.class);
 saveToFile(PUBLIC_KEY_FILE, 
        pub.getModulus(), pub.getPublicExponent());

 RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(),
        RSAPrivateKeySpec.class);
 saveToFile(PRIVATE_KEY_FILE, 
         priv.getModulus(), priv.getPrivateExponent());

The save function:

private static void saveToFile(String fileName,
                               BigInteger mod, BigInteger exp) 
    throws SomeException {
    ObjectOutputStream oout = new ObjectOutputStream(
            new BufferedOutputStream(new FileOutputStream(fileName)));
    try {
        oout.writeObject(mod);
        oout.writeObject(exp);
    } catch (Exception e) {
        throw new SomeException(e);
    } finally {
        oout.close();
    }
}

And read the same way back:

private static PublicKey readPublicKey() throws SomeException {
    InputStream in = new FileInputStream(PUBLIC_KEY_FILE);
    ObjectInputStream oin =
            new ObjectInputStream(new BufferedInputStream(in));
    try {
        BigInteger m = (BigInteger) oin.readObject();
        BigInteger e = (BigInteger) oin.readObject();
        RSAPublicKeySpec keySpec = new RSAPublicKeySpec(m, e);
        KeyFactory fact = KeyFactory.getInstance("RSA");
        PublicKey pubKey = fact.generatePublic(keySpec);
        return pubKey;
    } catch (Exception e) {
        throw new SomeException(e);
    } finally {
        oin.close();
    }
}

Reading private key is similar.