什么是好的代码

我希望能够编写优美的代码。

优美的代码就像一篇散文，易懂易读，而且看起来很漂亮。在《代码之美》一书中，收录了Ruby之父松本行宏的一篇文章，名为《把代码当作文章》，大约表达了同样的含义。Thoughtworks的一位工程师在《软件开发沉思录》一书中提出，每个类的方法最好不要超过5行。最初让我感觉很惊诧，继而觉得不可能。虽然这位工程师言之凿凿，提到在自己参与的项目中，所有代码都完全遵循了这一规范，我仍然表示怀疑。最近，阅读了Robert C. Martin的著作《代码整洁之道》（英文版名为Clean Code），看到Uncle Bob演示的代码，真是漂亮极了。仔细一看，这些好的代码在每个方法中大多数都没有超过5行。诀窍在哪里？那就是重构手法中最常用的Extract Method。进一步讲，如果我们能够为每个类与方法以及变量定义出好的名字，代码确实可以变成一篇散文。当然，是英文散文。

今天，我在重温.NET的序列化时，在MSDN上找到一篇演示Xml序列化的示范代码。或许是因为示范代码的缘故，这一段代码写得极其地不优雅，甚至显得有些丑陋：

public class Test {
    public static void Main() {
        // Read and write purchase orders.
        Test t = new Test();
        t.CreatePO("po.xml");
        t.ReadPO("po.xml");
    }

    private void CreatePO(string filename) {
        // Create an instance of the XmlSerializer class;
        // specify the type of object to serialize.
        XmlSerializer serializer =
        new XmlSerializer(typeof(PurchaseOrder));
        TextWriter writer = new StreamWriter(filename);
        PurchaseOrder po = new PurchaseOrder();

        // Create an address to ship and bill to.
        Address billAddress = new Address();
        billAddress.Name = "Teresa Atkinson";
        billAddress.Line1 = "1 Main St.";
        billAddress.City = "AnyTown";
        billAddress.State = "WA";
        billAddress.Zip = "00000";
        // Set ShipTo and BillTo to the same addressee.
        po.ShipTo = billAddress;
        po.OrderDate = System.DateTime.Now.ToLongDateString();

        // Create an OrderedItem object.
        OrderedItem i1 = new OrderedItem();
        i1.ItemName = "Widget S";
        i1.Description = "Small widget";
        i1.UnitPrice = (decimal)5.23;
        i1.Quantity = 3;
        i1.Calculate();

        // Insert the item into the array.
        OrderedItem[] items = { i1 };
        po.OrderedItems = items;
        // Calculate the total cost.
        decimal subTotal = new decimal();
        foreach (OrderedItem oi in items) {
            subTotal += oi.LineTotal;
        }
        po.SubTotal = subTotal;
        po.ShipCost = (decimal)12.51;
        po.TotalCost = po.SubTotal + po.ShipCost;
        // Serialize the purchase order, and close the TextWriter.
        serializer.Serialize(writer, po);
        writer.Close();
    }

    protected void ReadPO(string filename) {
        // Create an instance of the XmlSerializer class;
        // specify the type of object to be deserialized.
        XmlSerializer serializer = new XmlSerializer(typeof(PurchaseOrder));
        /* If the XML document has been altered with unknown
        nodes or attributes, handle them with the
        UnknownNode and UnknownAttribute events.*/
        serializer.UnknownNode += new
        XmlNodeEventHandler(serializer_UnknownNode);
        serializer.UnknownAttribute += new
        XmlAttributeEventHandler(serializer_UnknownAttribute);

        // A FileStream is needed to read the XML document.
        FileStream fs = new FileStream(filename, FileMode.Open);
        // Declare an object variable of the type to be deserialized.
        PurchaseOrder po;
        /* Use the Deserialize method to restore the object's state with
        data from the XML document. */
        po = (PurchaseOrder)serializer.Deserialize(fs);
        // Read the order date.
        Console.WriteLine("OrderDate: " + po.OrderDate);

        // Read the shipping address.
        Address shipTo = po.ShipTo;
        ReadAddress(shipTo, "Ship To:");
        // Read the list of ordered items.
        OrderedItem[] items = po.OrderedItems;
        Console.WriteLine("Items to be shipped:");
        foreach (OrderedItem oi in items) {
            Console.WriteLine("\t" +
            oi.ItemName + "\t" +
            oi.Description + "\t" +
            oi.UnitPrice + "\t" +
            oi.Quantity + "\t" +
            oi.LineTotal);
        }
        // Read the subtotal, shipping cost, and total cost.
        Console.WriteLine("\t\t\t\t\t Subtotal\t" + po.SubTotal);
        Console.WriteLine("\t\t\t\t\t Shipping\t" + po.ShipCost);
        Console.WriteLine("\t\t\t\t\t Total\t\t" + po.TotalCost);
    }

    protected void ReadAddress(Address a, string label) {
        // Read the fields of the Address object.
        Console.WriteLine(label);
        Console.WriteLine("\t" + a.Name);
        Console.WriteLine("\t" + a.Line1);
        Console.WriteLine("\t" + a.City);
        Console.WriteLine("\t" + a.State);
        Console.WriteLine("\t" + a.Zip);
        Console.WriteLine();
    }

    private void serializer_UnknownNode
    (object sender, XmlNodeEventArgs e) {
        Console.WriteLine("Unknown Node:" + e.Name + "\t" + e.Text);
    }

    private void serializer_UnknownAttribute
    (object sender, XmlAttributeEventArgs e) {
        System.Xml.XmlAttribute attr = e.Attr;
        Console.WriteLine("Unknown attribute " +
        attr.Name + "='" + attr.Value + "'");
    }
}

看看CreatePO()和ReadPO()，多么地冗长。虽然这个实现极为简单，但对于代码的阅读者而言，想要一下子抓住该方法的中心思想，仍然比较困难。此外，方法中的注释也显得多余，因为，代码本身就可以给予很好的说明。

下面，是我对这段代码的重构，大家可以对比对比，是否更加容易阅读呢？

    public static class PurchaseOrderHandler {
        public static void CreatePurchaseOrder(string filename) {
            PurchaseOrder po = BuildPurchaseOrder();
            XmlSerializer serializer = new XmlSerializer(typeof(PurchaseOrder));
            using (var writer = new StreamWriter(filename)) {
                serializer.Serialize(writer, po);
            }
        }

        private static PurchaseOrder BuildPurchaseOrder() {
            Address address = CreateAddress();
            OrderedItem i1 = CreateOrderedItem();
            OrderedItem[] items = { i1 };

            PurchaseOrder po = new PurchaseOrder();
            po.ShipTo = address;
            po.OrderDate = System.DateTime.Now.ToLongDateString();
            po.OrderedItems = items;
            po.SubTotal = CalculateSubTotal(items);
            po.ShipCost = (decimal)12.51;
            po.TotalCost = po.SubTotal + po.ShipCost;

            return po;
        }

        private static decimal CalculateSubTotal(OrderedItem[] items) {
            decimal subTotal = new decimal();
            foreach (OrderedItem oi in items) {
                subTotal += oi.LineTotal;
            }

            return subTotal;
        }


        private static OrderedItem CreateOrderedItem() {
            OrderedItem i1 = new OrderedItem();
            i1.ItemName = "Widget S";
            i1.Description = "Small widget";
            i1.UnitPrice = (decimal)5.23;
            i1.Quantity = 3;
            i1.Calculate();
            return i1;
        }

        private static Address CreateAddress() {
            Address billAddress = new Address();
            billAddress.Name = "Bruce Zhang";
            billAddress.Line1 = "1 Main St.";
            billAddress.City = "Chong Qing";
            billAddress.State = "Chong Qing";
            billAddress.Zip = "400000";

            return billAddress;
        }

        public static void ReadPurchaseOrder(string filename) {
            XmlSerializer serializer = new XmlSerializer(typeof(PurchaseOrder));

            serializer.UnknownNode += new XmlNodeEventHandler(serializer_UnknownNode);
            serializer.UnknownAttribute += new XmlAttributeEventHandler(serializer_UnknownAttribute);

            FileStream fs = new FileStream(filename, FileMode.Open);

            PurchaseOrder po;
            po = (PurchaseOrder)serializer.Deserialize(fs);
            PurchaseOrderPrinter.PrintPurchaseOrder(po);
        }


        private static void serializer_UnknownNode
        (object sender, XmlNodeEventArgs e) {
            Console.WriteLine("Unknown Node:" + e.Name + "\t" + e.Text);
        }

        private static void serializer_UnknownAttribute
        (object sender, XmlAttributeEventArgs e) {
            System.Xml.XmlAttribute attr = e.Attr;
            Console.WriteLine("Unknown attribute " +
            attr.Name + "='" + attr.Value + "'");
        }

        private static class PurchaseOrderPrinter {
            public static void PrintPurchaseOrder(PurchaseOrder po) {
                PrintOrderDate(po);
                PrintAddress(po.ShipTo);
                PrintOrderedItem(po.OrderedItems);
                PrintOrderCost(po);
            }

            private static void PrintOrderCost(PurchaseOrder po) {
                Console.WriteLine("\t\t\t\t\t Subtotal\t" + po.SubTotal);
                Console.WriteLine("\t\t\t\t\t Shipping\t" + po.ShipCost);
                Console.WriteLine("\t\t\t\t\t Total\t\t" + po.TotalCost);
            }

            private static void PrintOrderDate(PurchaseOrder po) {
                Console.WriteLine("OrderDate: " + po.OrderDate);
            }

            private static void PrintOrderedItem(OrderedItem[] items) {
                Console.WriteLine("Items to be shipped:");
                foreach (OrderedItem oi in items) {
                    Console.WriteLine("\t" +
                    oi.ItemName + "\t" +
                    oi.Description + "\t" +
                    oi.UnitPrice + "\t" +
                    oi.Quantity + "\t" +
                    oi.LineTotal);
                }
            }

            private static void PrintAddress(Address a) {
                // Read the fields of the Address object.
                Console.WriteLine("Ship To:");
                Console.WriteLine("\t" + a.Name);
                Console.WriteLine("\t" + a.Line1);
                Console.WriteLine("\t" + a.City);
                Console.WriteLine("\t" + a.State);
                Console.WriteLine("\t" + a.Zip);
                Console.WriteLine();
            }
        }
    }

阅读代码时，我们可以先关注最主要的方法，即CreatePurchaseOrder()和ReadPurchaseOrder()方法。如果并不希望了解过多构造PO对象的细节，通过阅读这样简短的方法，可以很容易地抓住这两个方法的实现，那就是通过构建一个PO对象，进行序列化，而在反序列化时，将获得的PO对象信息打印出来。

其实，糟糕的代码不一定就是初学者的“专利”，让我们看看NHibernate中的一段代码：

public SessionFactoryImpl(Configuration cfg, IMapping mapping, Settings settings, EventListeners listeners)
{
    Init();
    log.Info("building session factory");

    properties = new Dictionary<string, string>(cfg.Properties);
    interceptor = cfg.Interceptor;
    this.settings = settings;
    sqlFunctionRegistry = new SQLFunctionRegistry(settings.Dialect, cfg.SqlFunctions);
    eventListeners = listeners;
    filters = new Dictionary<string, FilterDefinition>(cfg.FilterDefinitions);
    if (log.IsDebugEnabled)
    {
        log.Debug("Session factory constructed with filter configurations : " + CollectionPrinter.ToString(filters));
    }

    if (log.IsDebugEnabled)
    {
        log.Debug("instantiating session factory with properties: " + CollectionPrinter.ToString(properties));
    }

    try
    {
        if (settings.IsKeywordsImportEnabled)
        {
            SchemaMetadataUpdater.Update(this);
        }
        if (settings.IsAutoQuoteEnabled)
        {
            SchemaMetadataUpdater.QuoteTableAndColumns(cfg);
        }
    }
    catch (NotSupportedException)
    {
        // Ignore if the Dialect does not provide DataBaseSchema
    }

    #region Caches
    settings.CacheProvider.Start(properties);
    #endregion

    #region Generators
    identifierGenerators = new Dictionary<string, IIdentifierGenerator>();
    foreach (PersistentClass model in cfg.ClassMappings)
    {
        if (!model.IsInherited)
        {
            IIdentifierGenerator generator =
                model.Identifier.CreateIdentifierGenerator(settings.Dialect, settings.DefaultCatalogName,
                                                           settings.DefaultSchemaName, (RootClass) model);

            identifierGenerators[model.EntityName] = generator;
        }
    }
    #endregion

    #region Persisters

    Dictionary<string, ICacheConcurrencyStrategy> caches = new Dictionary<string, ICacheConcurrencyStrategy>();
    entityPersisters = new Dictionary<string, IEntityPersister>();
    implementorToEntityName = new Dictionary<System.Type, string>();

    Dictionary<string, IClassMetadata> classMeta = new Dictionary<string, IClassMetadata>();

    foreach (PersistentClass model in cfg.ClassMappings)
    {
        model.PrepareTemporaryTables(mapping, settings.Dialect);
        string cacheRegion = model.RootClazz.CacheRegionName;
        ICacheConcurrencyStrategy cache;
        if (!caches.TryGetValue(cacheRegion, out cache))
        {
            cache =
                CacheFactory.CreateCache(model.CacheConcurrencyStrategy, cacheRegion, model.IsMutable, settings, properties);
            if (cache != null)
            {
                caches.Add(cacheRegion, cache);
                allCacheRegions.Add(cache.RegionName, cache.Cache);
            }
        }
        IEntityPersister cp = PersisterFactory.CreateClassPersister(model, cache, this, mapping);
        entityPersisters[model.EntityName] = cp;
        classMeta[model.EntityName] = cp.ClassMetadata;

        if (model.HasPocoRepresentation)
        {
            implementorToEntityName[model.MappedClass] = model.EntityName;
        }
    }
    classMetadata = new UnmodifiableDictionary<string, IClassMetadata>(classMeta);

    Dictionary<string, ISet<string>> tmpEntityToCollectionRoleMap = new Dictionary<string, ISet<string>>();
    collectionPersisters = new Dictionary<string, ICollectionPersister>();
    foreach (Mapping.Collection model in cfg.CollectionMappings)
    {
        ICacheConcurrencyStrategy cache =
            CacheFactory.CreateCache(model.CacheConcurrencyStrategy, model.CacheRegionName, model.Owner.IsMutable, settings,
                                     properties);
        if (cache != null)
        {
            allCacheRegions[cache.RegionName] = cache.Cache;
        }
        ICollectionPersister persister = PersisterFactory.CreateCollectionPersister(cfg, model, cache, this);
        collectionPersisters[model.Role] = persister;
        IType indexType = persister.IndexType;
        if (indexType != null && indexType.IsAssociationType && !indexType.IsAnyType)
        {
            string entityName = ((IAssociationType) indexType).GetAssociatedEntityName(this);
            ISet<string> roles;
            if (!tmpEntityToCollectionRoleMap.TryGetValue(entityName, out roles))
            {
                roles = new HashedSet<string>();
                tmpEntityToCollectionRoleMap[entityName] = roles;
            }
            roles.Add(persister.Role);
        }
        IType elementType = persister.ElementType;
        if (elementType.IsAssociationType && !elementType.IsAnyType)
        {
            string entityName = ((IAssociationType) elementType).GetAssociatedEntityName(this);
            ISet<string> roles;
            if (!tmpEntityToCollectionRoleMap.TryGetValue(entityName, out roles))
            {
                roles = new HashedSet<string>();
                tmpEntityToCollectionRoleMap[entityName] = roles;
            }
            roles.Add(persister.Role);
        }
    }
    Dictionary<string, ICollectionMetadata> tmpcollectionMetadata = new Dictionary<string, ICollectionMetadata>(collectionPersisters.Count);
    foreach (KeyValuePair<string, ICollectionPersister> collectionPersister in collectionPersisters)
    {
        tmpcollectionMetadata.Add(collectionPersister.Key, collectionPersister.Value.CollectionMetadata);
    }
    collectionMetadata = new UnmodifiableDictionary<string, ICollectionMetadata>(tmpcollectionMetadata);
    collectionRolesByEntityParticipant = new UnmodifiableDictionary<string, ISet<string>>(tmpEntityToCollectionRoleMap);
    #endregion

    #region Named Queries
    namedQueries = new Dictionary<string, NamedQueryDefinition>(cfg.NamedQueries);
    namedSqlQueries = new Dictionary<string, NamedSQLQueryDefinition>(cfg.NamedSQLQueries);
    sqlResultSetMappings = new Dictionary<string, ResultSetMappingDefinition>(cfg.SqlResultSetMappings);
    #endregion

    imports = new Dictionary<string, string>(cfg.Imports);

    #region after *all* persisters and named queries are registered
    foreach (IEntityPersister persister in entityPersisters.Values)
    {
        persister.PostInstantiate();
    }
    foreach (ICollectionPersister persister in collectionPersisters.Values)
    {
        persister.PostInstantiate();
    }
    #endregion

    #region Serialization info

    name = settings.SessionFactoryName;
    try
    {
        uuid = (string) UuidGenerator.Generate(null, null);
    }
    catch (Exception)
    {
        throw new AssertionFailure("Could not generate UUID");
    }

    SessionFactoryObjectFactory.AddInstance(uuid, name, this, properties);

    #endregion

    log.Debug("Instantiated session factory");

    #region Schema management
    if (settings.IsAutoCreateSchema)
    {
        new SchemaExport(cfg).Create(false, true);
    }

    if ( settings.IsAutoUpdateSchema )
    {
        new SchemaUpdate(cfg).Execute(false, true);
    }
    if (settings.IsAutoValidateSchema)
    {
         new SchemaValidator(cfg, settings).Validate();
    }
    if (settings.IsAutoDropSchema)
    {
        schemaExport = new SchemaExport(cfg);
    }
    #endregion

    #region Obtaining TransactionManager
    // not ported yet
    #endregion

    currentSessionContext = BuildCurrentSessionContext();

    if (settings.IsQueryCacheEnabled)
    {
        updateTimestampsCache = new UpdateTimestampsCache(settings, properties);
        queryCache = settings.QueryCacheFactory.GetQueryCache(null, updateTimestampsCache, settings, properties);
        queryCaches = new ThreadSafeDictionary<string, IQueryCache>(new Dictionary<string, IQueryCache>());
    }
    else
    {
        updateTimestampsCache = null;
        queryCache = null;
        queryCaches = null;
    }

    #region Checking for named queries
    if (settings.IsNamedQueryStartupCheckingEnabled)
    {
        IDictionary<string, HibernateException> errors = CheckNamedQueries();
        if (errors.Count > 0)
        {
            StringBuilder failingQueries = new StringBuilder("Errors in named queries: ");
            foreach (KeyValuePair<string, HibernateException> pair in errors)
            {
                failingQueries.Append('{').Append(pair.Key).Append('}');
                log.Error("Error in named query: " + pair.Key, pair.Value);
            }
            throw new HibernateException(failingQueries.ToString());
        }
    }
    #endregion

    Statistics.IsStatisticsEnabled = settings.IsStatisticsEnabled;

    // EntityNotFoundDelegate
    IEntityNotFoundDelegate enfd = cfg.EntityNotFoundDelegate;
    if (enfd == null)
    {
        enfd = new DefaultEntityNotFoundDelegate();
    }
    entityNotFoundDelegate = enfd;
}

这是类SessionFactoryImpl（它实现了ISessionFactoryImplementor接口）的构造函数，其目的时是通过Configuration以及Setting中的某些值，去初始化SessionFactoryImpl，然后构建该类的对象。坦白说，我从来没有看过如此“浩瀚无垠”的构造函数。幸好，Visual Studio提高了Region，否则，更让人头疼。（我在想，既然代码的编写者已经利用了Region来分割实现，为何不进一步将其分割为小的方法呢？）

看这样的代码，我们能够轻易读懂吗？

拙劣代码可谓遗患无穷。在《程序员修炼之道》一书中，提到了所谓“破窗效应”，即“没修复的破窗，导致更多的窗户被打破”。丑陋的代码如果只有一个小的片段，看似无关紧要，就像一幢大楼的一扇破窗一般容易让人忘记。随着时间的推移，当这些丑陋代码不知不觉蔓延到整个项目中时，我们才发现这一幢大楼已经满目疮痍了。“一屋不扫，何以扫天下”，程序员应该从小处着手，未来才可能写出优雅的代码。