首先，我们虚拟一个系统环境(e-Shop)，即网上商店的应用程序，其中有一个应用: 获取指定分类下的所有产品信息。我们按照传统的思路来实现。图1展示了系统的设计图。
新建一个类库工程EShop.Service. 然后添加相应的类到工程中。
原始程序 namespace EShop.Service { public class Product { } public class ProductRepository { public IList GetProductsByCategory(int categoryId) { IList products = new List (); //进行数据库操作 return products; } } public class ProductService { private ProductRepository productRepository; public ProductService() { productRepository = new ProductRepository(); } public IList GetProductsByCategory(int categoryId) { IList products; string storageKey = String.Format("products_in_category_id_{0}", categoryId); products = (List )HttpContext.Current.Cache.Get(storageKey); if (products == null) { products = productRepository.GetProductsByCategory(categoryId); HttpContext.Current.Cache.Insert(storageKey, products); } return products; } } } 从以上的程序段是否能发现不合理之处呢？我归纳了一下，大致有以下几点：
1. ProductService依赖于ProductRepository，一旦后者的API发生变化，则前者必然跟着发生变化。
2. 难以测试ProductService的方法，因为ProductRepository并未真正连接到数据库，另外还依赖于HttpContext，两者之间紧耦合。
3. 目前采用HttpContext来进行缓存，如果要更换缓存机制(如：Velocity或Memcached)，ProductService将进行更改。
基于以上几点不合理之处，我们将一一进行重构和优化。
针对1：我们采用依赖倒置原则(Dependency Inversion Principle)——依赖于抽象而不是具体实现来解决。我们加入了接口IProductRepository。
增加接口 public interface IProductRepository { IList GetProductsByCategory(int categoryId); } public class ProductRepository : IProductRepository { //... } public class ProductService { private IProductRepository productRepository; public ProductService() { productRepository = new ProductRepository(); } //... } 针对2：我们采用依赖注入原则(Dependency Injection Principle)——通过将抽象注入到构造函数、方法或属性来解决。我们修改ProductService构造函数。
注入构造函数
public class ProductService { private IProductRepository productRepository; public ProductService(IProductRepository productRepository) { this.productRepository = productRepository; } //... } 针对3：我们采用适配器模式(Adapter Pattern)——转换已有接口与客户期望的目标接口使之兼容来解决。我们添加了接口ICacheStorage和类HttpContextCacheAdapter。
采用适配器 public interface ICacheStorage { void Remove(string key); void Store(string key, object data); T Retrieve (string key); } public class HttpContextCacheAdapter : ICacheStorage public class HttpContextCacheAdapter : ICacheStorage { public void Remove(string key) { HttpContext.Current.Cache.Remove(key); } public void Store(string key, object data) { HttpContext.Current.Cache.Insert(key, data); } public T Retrieve (string key) { T item = (T)HttpContext.Current.Cache.Get(key); if (item == null) { item = default(T); } return item; } } public class ProductService { private IProductRepository productRepository; private ICacheStorage cacheStorage; public ProductService(IProductRepository productRepository, ICacheStorage cacheStorage) { this.productRepository = productRepository; this.cacheStorage = cacheStorage; } public IList GetProductsByCategory(int categoryId) { IList products; string storageKey = String.Format("products_in_category_id_{0}", categoryId); products = cacheStorage.Retrieve >(storageKey); if (products == null) { products = productRepository.GetProductsByCategory(categoryId); cacheStorage.Store(storageKey, products); } return products; } }
综合以上的解决方案，这里贴出重构后完整的代码：
重构后的代码 namespace EShop.Service { public class Product { } public interface IProductRepository { IList GetProductsByCategory(int categoryId); } public class ProductRepository : IProductRepository { public IList GetProductsByCategory(int categoryId) { IList products = new List (); //进行数据库操作 return products; } } public interface ICacheStorage { void Remove(string key); void Store(string key, object data); T Retrieve (string key); } public class HttpContextCacheAdapter : ICacheStorage { public void Remove(string key) { HttpContext.Current.Cache.Remove(key); } public void Store(string key, object data) { HttpContext.Current.Cache.Insert(key, data); } public T Retrieve (string key) { T item = (T)HttpContext.Current.Cache.Get(key); if (item == null) { item = default(T); } return item; } } //有时为方便起见，我们定义空对象(空缓存适配器) public class NullCacheAdapter : ICacheStorage { public void Remove(string key) { } public void Store(string key, object data) { } public T Retrieve (string key) { return default(T); } } public class ProductService { private IProductRepository productRepository; private ICacheStorage cacheStorage; public ProductService(IProductRepository productRepository, ICacheStorage cacheStorage) { this.productRepository = productRepository; this.cacheStorage = cacheStorage; } public IList GetProductsByCategory(int categoryId) { IList products; string storageKey = String.Format("products_in_category_id_{0}", categoryId); products = cacheStorage.Retrieve >(storageKey); if (products == null) { products = productRepository.GetProductsByCategory(categoryId); cacheStorage.Store(storageKey, products); } return products; } } }
附上重构后的设计图：
