某個ASP.NET MVC Action需要頻繁傳回大型數字陣列，數字大部分是整數，但部分帶有1-2位小數，故陣列採double[]或decimal[]。經Json.NET轉換後有個小問題: 即便是整數，轉換結果也會帶有".0"字尾，例如: double d = 2，Json.NET轉成"2.0"，而decimal有個有趣特性，小數尾端的零會被原原本本保存，例如: decimal d = 1.200M，d.ToString()為"1.200"，JSON結果也是"1.200"。
本來不是什麼大不了的事，但是當陣列元素一多，原本個位數字1被轉成"1.0"，傳輸內容便多兩個Byte，乘上陣列元素個數，佔用頻寬也算可觀。即便IIS有GZip壓縮，但網站效能調校就是這些細節優化所累積出來的。
由於JavaScript不像C#採用強型別，JSON傳回"1"或"1.0"不影響處理結果，我想在Json.NET轉換過程動手腳，當decimal或double是整數時，去除JSON".0"字尾降低傳輸量；若deicmal產出"1.200"這種帶小數零字尾字串，也去除字尾零，輸出"1.2"就好。
準備一個測試ASP.NET MVC Action如下，傳回用亂數產生的1萬筆double陣列，約75%為整數，25%為1位小數: (關於JsonNetResult類別請參考舊文)
staticdouble[] numArray = null;
staticdouble[] GetNumArray()
{if (numArray == null)
{ Random rnd = new Random(32767);List<double> lst = new List<double>();
for (int i = 0; i < 10000; i++)
{var n = rnd.NextDouble() * 10;
if (rnd.Next() % 4 != 0) n = Math.Floor(n);else n = Math.Round(n, 1);lst.Add(n);
}
numArray = lst.ToArray();
}
return numArray;}
public ActionResult LotOfData() {returnnew JsonNetResult()
{Data = GetNumArray()
};
}
執行結果可以看到5.0, 0.0, 8.0...，一大堆帶有".0"的整數:
Json.NET提供了很棒的擴充性，可自訂JsonConverter針對特殊型別執行指定序列化邏輯。於是我寫了一顆MinifiedNumArrayConveter，繼承JsonConverter，實做CanConvert()，遇到型別為double[]或decimal[]時傳回true，代表支援這兩種型別的轉換；之後Json.NET在遇到double[]或decimal[]時就會呼叫MinifiedNumArrayConveter.WriteJson()。要去除字尾零，我的做法是將double、decimal用.ToString()轉成字串，若出現".0"字尾表示為整數直接去除".0"；若字串包含小數點時則用TrimEnd()去掉字尾"0"；其餘狀況則直接輸出ToString()。為求效率，我直接呼叫JsonWriter.WriteRawValue輸出處理好的字串，省去讓Json.NET再做一次數字轉字串的程序。另外，去尾零修正只適用WirteJson()，CanRead()一律傳回false代表不處理JSON讀取，並補上一個空的ReadJson()。
using Newtonsoft.Json;using System;using System.Collections.Generic;using System.Linq;using System.Web;namespace Web.Models{publicclass MinifiedNumArrayConverter : JsonConverter
{privatevoid dumpNumArray<T>(JsonWriter writer, T[] array)
{foreach (T n in array)
{var s = n.ToString();
if (s.EndsWith(".0"))
writer.WriteRawValue(s.Substring(0, s.Length - 2));
elseif (s.Contains("."))
writer.WriteRawValue(s.TrimEnd('0'));elsewriter.WriteRawValue(s);
}
}
publicoverridevoid WriteJson(JsonWriter writer, objectvalue,
JsonSerializer serializer)
{writer.WriteStartArray();
Type t = value.GetType();if (t == dblArrayType)dumpNumArray<double>(writer, (double[])value);
elseif (t == decArrayType)
dumpNumArray<decimal>(writer, (decimal[])value);
elsethrownew NotImplementedException();
writer.WriteEndArray();
}
private Type dblArrayType = typeof(double[]);
private Type decArrayType = typeof(decimal[]);
publicoverridebool CanConvert(Type objectType)
{if (objectType == dblArrayType || objectType == decArrayType)returntrue;
returnfalse;
}
publicoverridebool CanRead
{ get { returnfalse; }}
publicoverrideobject ReadJson(JsonReader reader, Type objectType,
object existingValue, JsonSerializer serializer) {thrownew NotImplementedException();
}
}
}
使用時很簡單，JsonConvert.SerializeObject()時要多傳SerializerSettings參數，用SerializerSettings.Converters.Add()掛上MinifiedNumArrayConverter物件即可。
public ActionResult LotOfFixedData() { var res = new JsonNetResult() {Data = GetNumArray()
};
res.SerializerSettings.Converters.Add(new MinifiedNumArrayConverter());return res;}
如此，JSON裡囉嗦的".0"通通消失了!
實際比較，測試樣本(1萬個數字的陣列，1/4為1位小數，3/4為整數)經MinifiedNumArrayConverter處理，Response大小由40352降到24880，減少38%!
這個技術如果要應用在WebAPI上，要將MinifiedNumArrayConverter加進GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.Converters，App_Start/WebApiConfig.cs可修改如下:
using System;using System.Collections.Generic;using System.Linq;using System.Web.Http;using Web.Models;namespace Web{publicstaticclass WebApiConfig
{publicstaticvoid Register(HttpConfiguration config)
{// Web API configuration and services// Web API routesconfig.MapHttpAttributeRoutes();
config.Routes.MapHttpRoute(
name: "DefaultApi", routeTemplate: "api/{controller}/{id}", defaults: new { id = RouteParameter.Optional });
//強制GET時也傳回JSON，不要傳回XMLGlobalConfiguration.Configuration.Formatters
.XmlFormatter.SupportedMediaTypes.Clear();
//加入自訂序列化轉換邏輯GlobalConfiguration.Configuration.Formatters
.JsonFormatter.SerializerSettings.Converters.Add(
new MinifiedNumArrayConverter());}
}
}
如此，所有傳回decimal[]或double[]的WebAPI方法，都會套用MinifiedNumArrayConverter，達到省略小數字尾零的效果。
最後，還有很重要的一點: 加入自訂序列化邏輯是否會嚴重損耗效能呢? 我做了以下實測，比較加入MinifiedNumArrayConverter前後的差異。
public ActionResult Test() { StringBuilder sb = new StringBuilder(); sb.AppendLine("<pre>");var array = GetNumArray();
int times = 200; JsonSerializerSettings settings = new JsonSerializerSettings(); settings.Converters.Add(new MinifiedNumArrayConverter()); Stopwatch sw = new Stopwatch();for (int run = 0; run < 5; run++)
{string res = null;
sw.Reset();
sw.Start();
for (int i = 0; i < times; i++)
{res = JsonConvert.SerializeObject(array);
}
sw.Stop();
sb.AppendFormat("\nStd JSON: {0:n0}ms \n {1}",sw.ElapsedMilliseconds, res.Substring(0, 64));
sw.Reset();
sw.Start();
for (int i = 0; i < times; i++)
{res = JsonConvert.SerializeObject(array, settings);
}
sw.Stop();
sb.AppendFormat("\nMinified JSON: {0:n0}ms \n {1}",sw.ElapsedMilliseconds, res.Substring(0, 64));
}
string test = "[1.0,2.5,3.0]";
double[] test1 = JsonConvert.DeserializeObject<double[]>(test, settings);
decimal[] test2 = JsonConvert.DeserializeObject<decimal[]>(test, settings);
sb.AppendFormat("\n Deserialization Test: double[] {0}, decimal[] {1}",test == JsonConvert.SerializeObject(test1) ? "PASS" : "FAIL",
test == JsonConvert.SerializeObject(test2) ? "PASS" : "FAIL"
);
sb.AppendLine("</pre>");return Content(sb.ToString());}
public ActionResult Test() { StringBuilder sb = new StringBuilder(); sb.AppendLine("<pre>");var array = GetNumArray();
int times = 200; JsonSerializerSettings settings = new JsonSerializerSettings(); settings.Converters.Add(new MinifiedNumArrayConverter()); Stopwatch sw = new Stopwatch();for (int run = 0; run < 5; run++)
{string res = null;
sw.Reset();
sw.Start();
for (int i = 0; i < times; i++)
{res = JsonConvert.SerializeObject(array);
}
sw.Stop();
sb.AppendFormat("\nStd JSON: {0:n0}ms \n {1}",sw.ElapsedMilliseconds, res.Substring(0, 64));
sw.Reset();
sw.Start();
for (int i = 0; i < times; i++)
{res = JsonConvert.SerializeObject(array, settings);
}
sw.Stop();
sb.AppendFormat("\nMinified JSON: {0:n0}ms \n {1}",sw.ElapsedMilliseconds, res.Substring(0, 64));
}
string test = "[1.0,2.5,3.0]";
double[] test1 = JsonConvert.DeserializeObject<double[]>(test, settings);
decimal[] test2 = JsonConvert.DeserializeObject<decimal[]>(test, settings);
sb.AppendFormat("\n Deserialization Test: double[] {0}, decimal[] {1}",test == JsonConvert.SerializeObject(test1) ? "PASS" : "FAIL",
test == JsonConvert.SerializeObject(test2) ? "PASS" : "FAIL"
);
sb.AppendLine("</pre>");return Content(sb.ToString());}
程式共跑5次，每次各執行200次1萬個double數字的陣列JSON轉換，比較套用MinifiedNumArrayConverter與否的執行時間，最後順便測試DesrializeObject()在套用MinifiedNumArrayConverter後是否正常。測試結果如下:
Std JSON: 2,385ms [0.4,5.0,0.0,3.8,8.0,7.0,3.0,9.0,7.7,6.0,5.0,7.8,0.5,8.0,2.0,0.0 Minified JSON: 1,974ms [0.4,5,0,3.8,8,7,3,9,7.7,6,5,7.8,0.5,8,2,0,6.9,1,5.5,7,5,6.5,8,3 Std JSON: 1,615ms [0.4,5.0,0.0,3.8,8.0,7.0,3.0,9.0,7.7,6.0,5.0,7.8,0.5,8.0,2.0,0.0 Minified JSON: 1,720ms [0.4,5,0,3.8,8,7,3,9,7.7,6,5,7.8,0.5,8,2,0,6.9,1,5.5,7,5,6.5,8,3 Std JSON: 1,316ms [0.4,5.0,0.0,3.8,8.0,7.0,3.0,9.0,7.7,6.0,5.0,7.8,0.5,8.0,2.0,0.0 Minified JSON: 2,107ms [0.4,5,0,3.8,8,7,3,9,7.7,6,5,7.8,0.5,8,2,0,6.9,1,5.5,7,5,6.5,8,3 Std JSON: 1,767ms [0.4,5.0,0.0,3.8,8.0,7.0,3.0,9.0,7.7,6.0,5.0,7.8,0.5,8.0,2.0,0.0 Minified JSON: 1,989ms [0.4,5,0,3.8,8,7,3,9,7.7,6,5,7.8,0.5,8,2,0,6.9,1,5.5,7,5,6.5,8,3 Std JSON: 1,591ms [0.4,5.0,0.0,3.8,8.0,7.0,3.0,9.0,7.7,6.0,5.0,7.8,0.5,8.0,2.0,0.0 Minified JSON: 1,786ms [0.4,5,0,3.8,8,7,3,9,7.7,6,5,7.8,0.5,8,2,0,6.9,1,5.5,7,5,6.5,8,3 Deserialization Test: double[] PASS, decimal[] PASS
第一次執行較耗時，依效能測試慣例略過不計，共取四次結果:
1,615ms vs 1,720ms
1,316ms vs 2,107ms
1,767ms vs 1,989ms
1,591ms vs 1,786ms
加入MinifiedNumArrayConverter後速度較慢，但執行兩百萬次的差異約在0.1到0.8秒之間，相較其所節省資料量，評估為划算的投資。