code: public String writePolicyToString(Policy policy) {
  
          if(!isNotNull(policy))
              throw new IllegalArgumentException("Policy cannot be null");
  
          try {
              return jsonStringOf(policy);
          } catch (Exception e) {
              String message = "Unable to serialize policy to JSON string: "
                      + e.getMessage();
              throw new IllegalArgumentException(message, e);
          } finally {
              try { writer.close(); } catch (Exception e) { }
          }
      }
docs: Converts the specified AWS policy object to a JSON string, suitable for
  passing to an AWS service.
  
  @param policy
  The AWS policy object to convert to a JSON string.
  
  @return The JSON string representation of the specified policy object.
  
  @throws IllegalArgumentException
  If the specified policy is null or invalid and cannot be
  serialized to a JSON string.