code: public static RSAPrivateKey getPrivateKey(String modulus, String exponent) {
  		try {
  			BigInteger b1 = new BigInteger(modulus);
  			BigInteger b2 = new BigInteger(exponent);
  			KeyFactory keyFactory = KeyFactory.getInstance(KEY_ALGORITHM);
  			RSAPrivateKeySpec keySpec = new RSAPrivateKeySpec(b1, b2);
  			return (RSAPrivateKey) keyFactory.generatePrivate(keySpec);
  		} catch (Exception e) {
  			e.printStackTrace();
  			return null;
  		}
  	}
docs: 使用模和指数生成RSA私钥
  注意：【此代码用了默认补位方式，为RSA/None/PKCS1Padding，不同JDK默认的补位方式可能不同，如Android默认是RSA
  /None/NoPadding】
  
  @param modulus
  模
  @param exponent
  指数
  @return