Pool JsonSerializer for Output formatting

src/Microsoft.AspNet.Mvc.Formatters.Json/JsonOutputFormatter.cs

@@ -17,6 +17,10 @@ namespace Microsoft.AspNet.Mvc.Formatters
     {
         private JsonSerializerSettings _serializerSettings;
 
+        // Perf: JsonSerializers are relatively expensive to create, and are thread safe. We cache
+        // the serializer and invalidate it when the settings change.
+        private JsonSerializer _serializer;
+
         public JsonOutputFormatter()
             : this(SerializerSettingsProvider.CreateSerializerSettings())
         {
@@ -40,6 +44,10 @@ namespace Microsoft.AspNet.Mvc.Formatters
         /// <summary>
         /// Gets or sets the <see cref="JsonSerializerSettings"/> used to configure the <see cref="JsonSerializer"/>.
         /// </summary>
+        /// <remarks>
+        /// Any modifications to the <see cref="JsonSerializerSettings"/> object after this
+        /// <see cref="JsonOutputFormatter"/> has been used will have no effect.
+        /// </remarks>
         public JsonSerializerSettings SerializerSettings
         {
             get
@@ -54,6 +62,9 @@ namespace Microsoft.AspNet.Mvc.Formatters
                 }
 
                 _serializerSettings = value;
+
+                // If the settings change, then invalidate the cached serializer.
+                _serializer = null;
             }
         }
 
@@ -95,7 +106,12 @@ namespace Microsoft.AspNet.Mvc.Formatters
         /// <returns>The <see cref="JsonSerializer"/> used during serialization and deserialization.</returns>
         protected virtual JsonSerializer CreateJsonSerializer()
         {
-            return JsonSerializer.Create(SerializerSettings);
+            if (_serializer == null)
+            {
+                _serializer = JsonSerializer.Create(SerializerSettings);
+            }
+
+            return _serializer;
         }
 
         public override Task WriteResponseBodyAsync(OutputFormatterWriteContext context)

test/Microsoft.AspNet.Mvc.Formatters.Json.Test/JsonOutputFormatterTests.cs

@@ -9,6 +9,7 @@ using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Http.Internal;
 using Microsoft.AspNet.Mvc.Abstractions;
+using Microsoft.AspNet.Mvc.Internal;
 using Microsoft.AspNet.Routing;
 using Microsoft.AspNet.Testing;
 using Microsoft.Net.Http.Headers;
@@ -74,6 +75,79 @@ namespace Microsoft.AspNet.Mvc.Formatters
             Assert.Equal(expectedOutput, content);
         }
 
+        [Fact]
+        public async Task ChangesTo_DefaultSerializerSettings_AfterSerialization_NoEffect()
+        {
+            // Arrange
+            var person = new User() { Name = "John", Age = 35 };
+            var expectedOutput = JsonConvert.SerializeObject(
+                person,
+                SerializerSettingsProvider.CreateSerializerSettings());
+
+            var jsonFormatter = new JsonOutputFormatter();
+
+            // This will create a serializer - which gets cached.
+            var outputFormatterContext1 = GetOutputFormatterContext(person, typeof(User));
+            await jsonFormatter.WriteResponseBodyAsync(outputFormatterContext1);
+
+            // These changes should have no effect.
+            jsonFormatter.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
+            jsonFormatter.SerializerSettings.Formatting = Formatting.Indented;
+
+            var outputFormatterContext2 = GetOutputFormatterContext(person, typeof(User));
+
+            // Act
+            await jsonFormatter.WriteResponseBodyAsync(outputFormatterContext2);
+
+            // Assert
+            var body = outputFormatterContext2.HttpContext.Response.Body;
+
+            Assert.NotNull(body);
+            body.Position = 0;
+
+            var content = new StreamReader(body, Encoding.UTF8).ReadToEnd();
+            Assert.Equal(expectedOutput, content);
+        }
+
+        [Fact]
+        public async Task ReplaceSerializerSettings_AfterSerialization_TakesEffect()
+        {
+            // Arrange
+            var person = new User() { Name = "John", Age = 35 };
+            var expectedOutput = JsonConvert.SerializeObject(person, new JsonSerializerSettings()
+            {
+                ContractResolver = new CamelCasePropertyNamesContractResolver(),
+                Formatting = Formatting.Indented
+            });
+
+            var jsonFormatter = new JsonOutputFormatter();
+
+            // This will create a serializer - which gets cached.
+            var outputFormatterContext1 = GetOutputFormatterContext(person, typeof(User));
+            await jsonFormatter.WriteResponseBodyAsync(outputFormatterContext1);
+
+            // This results in a new serializer being created.
+            jsonFormatter.SerializerSettings = new JsonSerializerSettings()
+            {
+                ContractResolver = new CamelCasePropertyNamesContractResolver(),
+                Formatting = Formatting.Indented,
+            };
+
+            var outputFormatterContext2 = GetOutputFormatterContext(person, typeof(User));
+
+            // Act
+            await jsonFormatter.WriteResponseBodyAsync(outputFormatterContext2);
+
+            // Assert
+            var body = outputFormatterContext2.HttpContext.Response.Body;
+
+            Assert.NotNull(body);
+            body.Position = 0;
+
+            var content = new StreamReader(body, Encoding.UTF8).ReadToEnd();
+            Assert.Equal(expectedOutput, content);
+        }
+
         [Fact]
         public async Task CustomSerializerSettingsObject_TakesEffect()
         {