Singleton¶
- Singleton
- UML class diagram
- Participants
- Structural code in C# .{10}
- Real-world code in C# .{10}
- .NET Optimized code in C# .{10}
Summary: Ensure a class has only one instance
and provide a global point of access to it.
Frequency of use: Medium high
UML class diagram¶
Participants¶
The classes and objects participating in this pattern are:
- Singleton (
LoadBalancer
) - defines an Instance operation that lets clients access its unique instance. Instance is a class operation.
- responsible for creating and maintaining its own unique instance.
Structural code in C# .{10}¶
This structural code demonstrates the Singleton pattern which assures only a single instance (the singleton) of the class can be created.
using System;
namespace DoFactory.GangOfFour.Singleton.Structural
{
/// <summary>
/// MainApp startup class for Structural
/// Singleton Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Constructor is protected -- cannot use new
Singleton s1 = Singleton.Instance();
Singleton s2 = Singleton.Instance();
// Test for same instance
if (s1 == s2)
{
Console.WriteLine("Objects are the same instance");
}
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Singleton' class
/// </summary>
class Singleton
{
private static Singleton _instance;
// Constructor is 'protected'
protected Singleton()
{
}
public static Singleton Instance()
{
// Uses lazy initialization.
// Note: this is not thread safe.
if (_instance == null)
{
_instance = new Singleton();
}
return _instance;
}
}
}
Output¶
Real-world code in C# .{10}¶
This real-world code demonstrates the Singleton pattern as a LoadBalancing object. Only a single instance (the singleton) of the class can be created because servers may dynamically come on- or off-line and every request must go throught the one object that has knowledge about the state of the (web) farm.
using System;
using System.Collections.Generic;
using System.Threading;
namespace DoFactory.GangOfFour.Singleton.RealWorld
{
/// <summary>
/// MainApp startup class for Real-World
/// Singleton Design Pattern.
/// </summary>
class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
LoadBalancer b4 = LoadBalancer.GetLoadBalancer();
// Same instance?
if (b1 == b2 && b2 == b3 && b3 == b4)
{
Console.WriteLine("Same instance\n");
}
// Load balance 15 server requests
LoadBalancer balancer = LoadBalancer.GetLoadBalancer();
for (int i = 0; i < 15; i++)
{
string server = balancer.Server;
Console.WriteLine("Dispatch Request to: " + server);
}
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Singleton' class
/// </summary>
class LoadBalancer
{
private static LoadBalancer _instance;
private List<string> _servers = new List<string>();
private Random _random = new Random();
// Lock synchronization object
private static object syncLock = new object();
// Constructor (protected)
protected LoadBalancer()
{
// List of available servers
_servers.Add("ServerI");
_servers.Add("ServerII");
_servers.Add("ServerIII");
_servers.Add("ServerIV");
_servers.Add("ServerV");
}
public static LoadBalancer GetLoadBalancer()
{
// Support multithreaded applications through
// 'Double checked locking' pattern which (once
// the instance exists) avoids locking each
// time the method is invoked
if (_instance == null)
{
lock (syncLock)
{
if (_instance == null)
{
_instance = new LoadBalancer();
}
}
}
return _instance;
}
// Simple, but effective random load balancer
public string Server
{
get
{
int r = _random.Next(_servers.Count);
return _servers[r].ToString();
}
}
}
}
Output¶
Same instance
ServerIII
ServerII
ServerI
ServerII
ServerI
ServerIII
ServerI
ServerIII
ServerIV
ServerII
ServerII
ServerIII
ServerIV
ServerII
ServerIV
.NET Optimized code in C# .{10}¶
```cs{44,46-50,56-57,70-73} using System; using System.Collections.Generic;
namespace DoFactory.GangOfFour.Singleton.NETOptimized { ///
// Confirm these are the same instance
if (b1 == b2 && b2 == b3 && b3 == b4)
{
Console.WriteLine("Same instance\n");
}
// Next, load balance 15 requests for a server
var balancer = LoadBalancer.GetLoadBalancer();
for (int i = 0; i < 15; i++)
{
string serverName = balancer.NextServer.Name;
Console.WriteLine("Dispatch request to: " + serverName);
}
// Wait for user
Console.ReadKey();
}
}
/// <summary>
/// The 'Singleton' class
/// </summary>
sealed class LoadBalancer
{
// Static members are 'eagerly initialized', that is,
// immediately when class is loaded for the first time.
// .NET guarantees thread safety for static initialization
private static readonly LoadBalancer _instance =
new LoadBalancer();
// Type-safe generic list of servers
private List<Server> _servers;
private Random _random = new Random();
// Note: constructor is 'private'
private LoadBalancer()
{
// Load list of available servers
_servers = new List<Server>
{
new Server{ Name = "ServerI", IP = "120.14.220.18" },
new Server{ Name = "ServerII", IP = "120.14.220.19" },
new Server{ Name = "ServerIII", IP = "120.14.220.20" },
new Server{ Name = "ServerIV", IP = "120.14.220.21" },
new Server{ Name = "ServerV", IP = "120.14.220.22" },
};
}
public static LoadBalancer GetLoadBalancer()
{
return _instance;
}
// Simple, but effective load balancer
public Server NextServer
{
get
{
int r = _random.Next(_servers.Count);
return _servers[r];
}
}
}
/// <summary>
/// Represents a server machine
/// </summary>
class Server
{
// Gets or sets server name
public string Name { get; set; }
// Gets or sets server IP address
public string IP { get; set; }
}
} ```