diff --git a/Apewer/ForbiddenException.cs b/Apewer/ForbiddenException.cs
new file mode 100644
index 0000000..df1191e
--- /dev/null
+++ b/Apewer/ForbiddenException.cs
@@ -0,0 +1,24 @@
+using Apewer.Web;
+using System;
+
+namespace Apewer
+{
+
+    /// <summary>表示被禁止访问的错误。</summary>
+    public sealed class ForbiddenException : Exception, IApiException
+    {
+
+        const string DefaultMessage = "禁止访问。";
+
+        /// <summary>表示禁止访问的状态。</summary>
+        public string Status { get => "Forbidden"; }
+
+        /// <summary></summary>
+        public ForbiddenException(string message = DefaultMessage) : base(message.IsEmpty() ? DefaultMessage : message) { }
+
+        /// <summary></summary>
+        public override string ToString() => $"<ForbiddenException> {Message}";
+
+    }
+
+}
diff --git a/Apewer/NotFoundException.cs b/Apewer/NotFoundException.cs
new file mode 100644
index 0000000..3188338
--- /dev/null
+++ b/Apewer/NotFoundException.cs
@@ -0,0 +1,46 @@
+using Apewer.Web;
+using System;
+
+namespace Apewer
+{
+
+    /// <summary>表示目标资源不存在的错误。</summary>
+    public class NotFoundException : Exception, IApiException
+    {
+
+        static string _default = FixMessage(null);
+
+        static string FixMessage(string message)
+        {
+            const string Preset = "Resource not found.";
+            if (message != null)
+            {
+                message = message.Trim();
+                if (!string.IsNullOrEmpty(message)) return message;
+            }
+
+            return Preset;
+        }
+
+        /// <summary>获取或设置默认消息。</summary>
+        public static string DefaultMessage { get => _default; set => _default = FixMessage(value); }
+
+        /// <summary>状态。</summary>
+        /// <value>Unauthorized</value>
+        public virtual string Status { get => "Not Found"; }
+
+        /// <summary>表示目标资源不存在的错误。</summary>
+        /// <remarks>默认消息：Operation is not authorized.</remarks>
+        public NotFoundException() : base(DefaultMessage) { }
+
+        /// <summary>表示目标资源不存在的错误。</summary>
+        /// <remarks>默认消息：Operation is not authorized.</remarks>
+        public NotFoundException(string message) : base(FixMessage(message)) { }
+
+        /// <summary>表示目标资源不存在的错误。</summary>
+        /// <remarks>默认消息：Operation is not authorized.</remarks>
+        public NotFoundException(string message, Exception innerException) : base(FixMessage(message), innerException) { }
+
+    }
+
+}
diff --git a/Apewer/RedundanceException.cs b/Apewer/RedundanceException.cs
index c9c8fb3..d17a04f 100644
--- a/Apewer/RedundanceException.cs
+++ b/Apewer/RedundanceException.cs
@@ -1,9 +1,4 @@
-using Apewer.Internals;
-using System;
-using System.Collections.Generic;
-using System.Runtime.InteropServices;
-using System.Runtime.Serialization;
-using System.Text;
+using System;
 
 namespace Apewer
 {
diff --git a/Apewer/UnauthorizedException.cs b/Apewer/UnauthorizedException.cs
index 6931754..9c0cc28 100644
--- a/Apewer/UnauthorizedException.cs
+++ b/Apewer/UnauthorizedException.cs
@@ -1,11 +1,12 @@
-using System;
+using Apewer.Web;
+using System;
 
 namespace Apewer
 {
 
     /// <summary>表示未授权的错误。</summary>
     /// <remarks>默认消息：Operation is not authorized.</remarks>
-    public class UnauthorizedException : Exception
+    public class UnauthorizedException : Exception, IApiException
     {
 
         static string _default = FixMessage(null);
diff --git a/Apewer/Web/ApiProcessor.cs b/Apewer/Web/ApiProcessor.cs
index 8a70f0a..cf0d26f 100644
--- a/Apewer/Web/ApiProcessor.cs
+++ b/Apewer/Web/ApiProcessor.cs
@@ -1,4 +1,5 @@
-using Apewer.Network;
+using Apewer.Internals;
+using Apewer.Network;
 using System;
 using System.Collections.Generic;
 using System.Net;
@@ -305,7 +306,17 @@ namespace Apewer.Web
                 if (action != null)
                 {
                     _context.ApiAction = action;
-                    InvokeAction(action);
+                    if (action.Unparallel)
+                    {
+                        UnparallelLocker.InLock(GetUnparallelKey(action.MethodInfo), () =>
+                        {
+                            InvokeAction(action);
+                        });
+                    }
+                    else
+                    {
+                        InvokeAction(action);
+                    }
                     return;
                 }
             }
@@ -438,15 +449,27 @@ namespace Apewer.Web
             {
                 // 控制器初始化。
                 var initializer = ApiUtility.GetInitialier(controller);
-                var match = initializer == null ? true : initializer.Invoke(controller);
-                if (!match) return;
+                if (initializer != null)
+                {
+                    var @continue = true;
+                    UnparallelLocker.InLock(GetUnparallelKey(controller.GetType()), () =>
+                    {
+                        @continue = initializer.Invoke(controller);
+                    });
+                    if (!@continue) return;
+                }
+
+                // 此应用独立处理，不受反射控制。
                 if (application.Independent) return;
 
                 if (function != null)
                 {
                     // 调用 API，获取返回值。
                     _context.Controller = controller;
-                    Invoke(_context, function.Method, function.Parameters);
+                    UnparallelLocker.InLock(GetUnparallelKey(function.Method), () =>
+                    {
+                        Invoke(_context, function.Method, function.Parameters);
+                    });
                 }
                 else
                 {
@@ -454,8 +477,11 @@ namespace Apewer.Web
                     var @default = ApiUtility.GetDefault(controller);
                     if (@default != null)
                     {
-                        @default.Invoke(controller);
-                        return;
+                        UnparallelLocker.InLock(GetUnparallelKey(controller.GetType()), () =>
+                        {
+                            @default.Invoke(controller);
+                            return;
+                        });
                     }
 
                     // 没有执行任何 Function，尝试枚举。
@@ -803,6 +829,22 @@ namespace Apewer.Web
 
         #endregion
 
+        #region 不可并行
+
+        static TextLocker UnparallelLocker = new TextLocker();
+
+        static string GetUnparallelKey(MethodInfo method)
+        {
+            return $"{method.DeclaringType.Assembly.ToString()} | {method.DeclaringType.FullName} | {method.ToString()}";
+        }
+
+        static string GetUnparallelKey(Type type)
+        {
+            return $"{type.DeclaringType.Assembly.ToString()}";
+        }
+
+        #endregion
+
     }
 
 }
diff --git a/Apewer/Web/IApiException.cs b/Apewer/Web/IApiException.cs
new file mode 100644
index 0000000..f6ec856
--- /dev/null
+++ b/Apewer/Web/IApiException.cs
@@ -0,0 +1,16 @@
+namespace Apewer.Web
+{
+
+    /// <summary>执行 API 时发生的异常。</summary>
+    public interface IApiException
+    {
+
+        /// <summary>异常状态。</summary>
+        string Status { get; }
+
+        /// <summary>错误消息。</summary>
+        string Message { get; }
+
+    }
+
+}