code: public byte[] sign(byte[] content,PrivateKey privateKey)  
      {  
          try   
          {  
              java.security.Signature signature = java.security.Signature.getInstance(SIGN_ALGORITHMS);  
              signature.initSign(privateKey);  
              signature.update(content);  
              return signature.sign();  
          }catch (Exception e)   
          {  
              log.error(e.getMessage(),e);
          }  
          return null;  
      }
docs: RSA签名
  @param content 待签名数据
  @param privateKey 商户私钥
  @return 签名值