享元模式(Flyweight Pattern)

享元模式属于结构型模式,它以共享的方式高效的支持大量的细粒度对象。通过复用内存中已存在的对象,降低系统创建对象实例的性能消耗。 

享元模式尝试重用现有的同类对象,如果未找到匹配的对象,则创建新对象,如果找到对象,则直接返回。

角色:

1、抽象享元(Flyweight)

它是所有具体享元类的抽象基类,为其子类规定出需要实现的公共接口;

2、具体享元(Concrete Flyweight)

具体享元类实现了抽象享元类所规定的接口;

3、享元工厂(FlyweightFactoiy)

享元工厂类负责创建和管理享元对象。

示例:




命名空间FlyweightPattern中包含IConnection接口充当抽象享元,Connection类充当具体享元,ConnectionFactory工厂类充当享元工厂。本案例通过使用享元模式来共享数据库连接。
namespace FlyweightPattern public interface IConnection { void Print(); }
IConnection接口,包含一个打印的方法。
public class Connection : IConnection { private string _connectionString =
null; public Connection(string connectionString) { _connectionString =
connectionString; Thread.Sleep(1000); Console.WriteLine("It took 1 second(s) to
create a connection!"); } public void Print() { Console.WriteLine($"Database
connection is {_connectionString}");
Console.WriteLine("-------------------------------------------------------"); }
}
Connection类,定义数据库连接(演示)。
public class ConnectionFactory { private Dictionary<string, IConnection>
_connections = null; private string _connectionString = null; public
ConnectionFactory() { _connections = new Dictionary<string, IConnection>(); }
public IConnection CreateConnection(string connectionString) { if
(!_connections.ContainsKey(connectionString)) { Console.WriteLine("Creating a
new connection!"); IConnection connection = new Connection(connectionString);
_connections.Add(connectionString, connection); return connection; } else {
Console.WriteLine("Return an exist connection!"); var connection =
_connections[connectionString] as IConnection; return connection; } } }

ConnectionFactory类,数据库连接工厂,内部维持对所有连接的引用,CreateConnection方法在发现连接存在时直接返回,如果不存在,则创建一个新的连接并维持进列表。

注:实际开发过程中应该用HashCode来检索数据库连接是否存在。
public class Program { private static ConnectionFactory _factory = null;
private static List<string> _connections = null; private static IConnection
_connection = null; private static void Print(int index) { if (index >
_connections.Count - 1) { Console.WriteLine("Index Out Of Range Exception!");
return; } _connection = _factory.CreateConnection(_connections[index]);
_connection.Print(); } public static void Main(string[] args) { _connections =
new List<string> { "Server=Aron1;Database=pubs;\n" + "Uid=uid;Pwd=password;",
"Provider=sqloledb;Data Source=Aron1;\n" + "User Id=uid;Password=password;",
"Data Source=192.168.0.1,1433;\n" + "UserID=uid;Password=password;" }; _factory
= new ConnectionFactory(); Print(0); Print(1); Print(2); Print(1); Print(3);
Console.ReadKey(); } }
以上是调用方的代码,以下是这个案例的输出结果:
Creating a new connection! It took 1 second(s) to create a connection!
Database connection is Server=Aron1;Database=pubs; Uid=uid;Pwd=password;
------------------------------------------------------- Creating a new
connection! It took 1 second(s) to create a connection! Database connection is
Provider=sqloledb;Data Source=Aron1; User Id=uid;Password=password;
------------------------------------------------------- Creating a new
connection! It took 1 second(s) to create a connection! Database connection is
Data Source=192.168.0.1,1433; UserID=uid;Password=password;
------------------------------------------------------- Return an exist
connection! Database connection is Provider=sqloledb;Data Source=Aron1; User
Id=uid;Password=password;
------------------------------------------------------- Index Out Of Range
Exception!
优点:

1、降低了系统中对象的数量,从而降低了系统中细粒度对象给内存带来的压力。

缺点:

1、为了使对象可以共享,需要将一些状态外部化,这使得程序的逻辑更复杂,使系统复杂化;
2、享元模式将享元对象的状态外部化,而读取外部状态使得运行时间稍微变长。

使用场景:

1、一个系统中有大量的对象;
2、这些对象耗费大量的内存;
3、这些对象中的状态大部分都可以被外部化;
4、这些对象可以按照内部状态分成很多的组,当把外部对象从对象中剔除时,每一个组都可以仅用一个对象代替;
5、软件系统不依赖这些对象的身份。

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