GOF pattern - Singleton - 毒菇求Buy

GOF pattern - Singleton

Definition

Ensure a class has only one instance and provide a global point of access to it.

Frequency of use:

high

UML class diagram

Participants

The classes and/or objects participating in the Singleton 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.

Sample code

This structural code demonstrates the Singleton pattern which assures only a single instance (the singleton) of the class can be created.

// Singleton pattern -- Structural example

//--------------------------------------------------------
// Copyright (C) 2001 - 2002, Data & Object Factory
// All rights reserved. www.dofactory.com
//
// You are free to use this source code in your
// applications as long as the original copyright
// notice is included.
//
// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT
// WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF
// MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
//--------------------------------------------------------

using System;

// "Singleton"
class Singleton {
  // Fields
  private static Singleton instance;

  // Constructor
  protected Singleton() {}

  // Methods
  public static Singleton Instance() {
    // Uses "Lazy initialization"
    if( instance == null )
      instance = new Singleton();

    return instance;
  }
}

/// <summary>
///  Client test
/// </summary>
public class Client {
  public static void Main() {
    // Constructor is protected -- cannot use new
    Singleton s1 = Singleton.Instance();
    Singleton s2 = Singleton.Instance();

    if( s1 == s2 )
      Console.WriteLine( "The same instance" );

    Console.Read();
  }
}

A .NET specificed implementation:

sealed class Singleton {
private Singleton();

public static readonly Singleton Instance = new Singleton();
}

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.

// Singleton pattern

//--------------------------------------------------
// Copyright (C) 2001, Data & Object Factory
// All rights reserved. www.dofactory.com
//
// You are free to use this source code in your
// applications, although it is intended only for
// educational purposes.
//
// THIS CODE AND INFORMATION IS PROVIDED "AS IS"
// WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR
// FITNESS FOR A PARTICULAR PURPOSE.
//--------------------------------------------------

using System;
using System.Collections;
using System.Threading;

// "Singleton"
class LoadBalancer {
  // Fields
  private static LoadBalancer balancer;
  private ArrayList servers = new ArrayList();
  private Random random = new Random();

  // Constructors (protected)
  protected LoadBalancer() {
    // List of available servers
    servers.Add( "ServerI" );
    servers.Add( "ServerII" );
    servers.Add( "ServerIII" );
    servers.Add( "ServerIV" );
    servers.Add( "ServerV" );
  }

  // Methods
  public static LoadBalancer GetLoadBalancer() {
    // Support multithreaded applications through
    // "Double checked locking" pattern which avoids
    // locking every time the method is invoked
    if( balancer == null ) {
      // Only one thread can obtain a mutex
      Mutex mutex = new Mutex();
      mutex.WaitOne();

      if( balancer == null )
        balancer = new LoadBalancer();

      mutex.Close();
    }

    return balancer;
  }

  // Properties
  public string Server {
    get
    {
      // Simple, but effective random load balancer
      int r = random.Next( servers.Count );
      return servers[ r ].ToString();
    }
  }
}

/// <summary>
///  SingletonApp test
/// </summary>
///
public class SingletonApp {
  public static void Main( string[] args ) {
    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" );

    // Do the load balancing
    Console.WriteLine( b1.Server );
    Console.WriteLine( b2.Server );
    Console.WriteLine( b3.Server );
    Console.WriteLine( b4.Server );

    Console.Read();
  }
}

posted on 2005-07-07 08:38 毒菇求Buy 阅读(276) 评论(0) 编辑收藏引用所属分类: Design Pattern 、C#

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