public String build() {
        if (!root.containsKey("mdm")) {
            insertCustomAlert();
            root.put("aps", aps);
        }
        try {
            return mapper.writeValueAsString(root);
        } catch (final Exception e) {
            throw new RuntimeException(e);
        }
    }
Returns the JSON String representation of the payload
according to Apple APNS specification

@return  the String representation as expected by Apple
src/main/java/com/notnoop/apns/PayloadBuilder.java