淺談XML壓縮算法

作者：蛙蛙池塘 2009-07-14 11:51:00

在用XML傳輸數據的時候，XML本身的元素名稱，屬性名稱可能比有效的信息量占的地方還要大，本文示例一種簡單實用的算法來進行XML壓縮，主要思路是把XML標簽和屬性用整數來表示以便降低傳輸量。

XML壓縮單元測試代碼

class Program {  
   public static string XML = @"<?xml version=""1.0"" encoding=""utf-16""?> 
    <Customer> 
<CustomerID>ALFKI</CustomerID> 
<PO>9572658</PO> 
<Address AddressType=""work""> 
    <Street>One Main Street</Street> 
    <City>Anywhere</City> 
    <State>NJ</State> 
    <Zip>08080</Zip> 
</Address> 
<Order> 
    <OrderID>10966</OrderID > 
    <LineItem> 
        <ProductID>37</ProductID> 
        <UnitPrice>26.50 </UnitPrice> 
        <Quantity>8</Quantity> 
        <Description>Gravad lax </Description>            　  
    </LineItem> 
    <LineItem> 
        <ProductID>56 </ProductID> 
        <UnitPrice>38.00</UnitPrice> 
        <Quantity>12</Quantity> 
        <Description>Gnocchi di nonna Alice</Description>            　  
    </LineItem> 
</Order>      
</Customer>";  
static void Main(string[] args) {  
    XmlZip zip = new XmlZip();  
 
    byte[] bs = Encoding.UTF8.GetBytes(XML);  
    Console.WriteLine("原始文件長度:{0}", bs.Length);  
    MemoryStream ms = new MemoryStream();  
    DeflateStream compressedzipStream = new DeflateStream(ms, CompressionMode.Compress, true);  
    compressedzipStream.Write(bs, 0, bs.Length);  
    compressedzipStream.Close();  
    Console.WriteLine("Deflate壓縮后長度: {0}", ms.Length);  
 
    zip.Init(XML);  
    bs = zip.XmlToBytes(XML);  
    Console.WriteLine("XML壓縮后長度:{0}", bs.Length);  
    string str = zip.BytesToXml(bs);  
    Console.WriteLine("還原后長度:{0}", Encoding.UTF8.GetByteCount(str));  
    Console.WriteLine(str);  
 
 
    ms = new MemoryStream();  
    compressedzipStream = new DeflateStream(ms, CompressionMode.Compress, true);  
    compressedzipStream.Write(bs, 0, bs.Length);  
    compressedzipStream.Close();  
    Console.WriteLine("先XML壓縮，再Deflate壓縮后的長度：{0}", ms.Length);  
    Console.ReadKey();  
 
}  
}

測試輸出

原始文件長度:740

Deflate壓縮后長度: 438

XML壓縮后長度:295

還原后長度:727

<?xml version="1.0" encoding="utf-16"?> 
<Customer> 
  <CustomerID>ALFKI</CustomerID> 
  <PO>9572658</PO> 
  <Address AddressType="work"> 
    <Street>One Main Street</Street> 
    <City>Anywhere</City> 
    <State>NJ</State> 
    <Zip>08080</Zip> 
  </Address> 
  <Order> 
    <OrderID>10966</OrderID> 
    <LineItem> 
      <ProductID>37</ProductID> 
      <UnitPrice>26.50 </UnitPrice> 
      <Quantity>8</Quantity> 
      <Description>Gravad lax </Description>            　  
    </LineItem> 
    <LineItem> 
      <ProductID>56 </ProductID> 
      <UnitPrice>38.00</UnitPrice> 
      <Quantity>12</Quantity> 
      <Description>Gnocchi di nonna Alice</Description>            　  
    </LineItem> 
  </Order> 
</Customer>

先XML壓縮，再Deflate壓縮后的長度：357

可以看到，XML壓縮后的數據約是原來數據的3分之一，可能沒有其它專有的壓縮算法的壓縮率高，但效果還算是滿意吧，而且我的算法是比較通用的，只要通信雙方知道了XML的Schema，甚至雙方只需要有一段完整的示例代碼，就可以進行壓縮通信，只做了功能測試，沒做性能測試，大家可以先借鑒下思路。

完整代碼

大致原理，就是通信雙方各持有一個XML文檔節點名稱，屬性名稱的一個字典，然后發送方傳輸的時候用ushort代替原有的XML標簽和屬性名，接收方通過字典把ushort再轉換成原始的元素名和屬性名，這樣大量不必要的重復的標簽等就省去了。

代碼只做本文的示例，寫的比較隨意，沒有什么防御性和健壯性。

internal enum ItemType {  
    Element,  
    Attritube  
}  
internal class XmlNodeItem {  
    public string Xpath { get; set; }  
    public string Text { get; set; }  
    public ItemType ItemType { get; set; }  
    public override string ToString() {  
        return Xpath;  
    }  
}  
internal class MyXpath {  
    LinkedList<string> _node = new LinkedList<string>();  
    public void AddElement(string name) {  
        _node.AddLast(string.Format("/{0}", name));  
    }  
    public void AddAttribute(string name) {  
        _node.AddLast(string.Format("/@{0}", name));  
    }  
    public void RemoveLastElement() {  
        _node.RemoveLast();  
    }  
    public override string ToString() {  
        StringBuilder sb = new StringBuilder();  
        LinkedListNode<string> node = _node.First;  
        sb.Append(node.Value);  
        while ((nodenode = node.Next) != null) {  
            sb.Append(node.Value);  
        }  
        return sb.ToString();  
    }  
}  
class XmlZip {  
    Dictionary<ushort, XmlNodeItem> _map = new Dictionary<ushort, XmlNodeItem>();  
    Dictionary<string, ushort> _map2 = new Dictionary<string, ushort>();  
    MyXpath _path = new MyXpath();  
 
    public void Init(string xmlInput) {  
        StringReader sr = new StringReader(xmlInput);  
        XmlReader reader = XmlReader.Create(sr);  
        MemoryStream ms = new MemoryStream();  
        ushort i = 1;  
        while (reader.Read()) {  
            switch (reader.NodeType) {  
                case XmlNodeType.Element:  
                    _path.AddElement(reader.Name);  
                    _map[i++] = new XmlNodeItem() {  
                        Xpath = _path.ToString(),  
                        Text = reader.Name,  
                        ItemTypeItemType = ItemType.Element  
                    };  
                    if (reader.HasAttributes) {  
                        reader.MoveToFirstAttribute();  
                        _path.AddAttribute(reader.Name);  
                        _map[i++] = new XmlNodeItem() {  
                            Xpath = _path.ToString(),  
                            Text = reader.Name,  
                            ItemTypeItemType = ItemType.Attritube  
                        };  
                        _path.RemoveLastElement();  
                        while (reader.MoveToNextAttribute()) {  
                            _path.AddAttribute(reader.Name);  
                            _map[i++] = new XmlNodeItem() {  
                                Xpath = _path.ToString(),  
                                Text = reader.Name,  
                                ItemTypeItemType = ItemType.Attritube  
                            };  
                            _path.RemoveLastElement();  
                        }  
                        reader.MoveToElement();  
                    }  
                    if (reader.IsEmptyElement) _path.RemoveLastElement();  
                    break;  
                case XmlNodeType.EndElement:  
                    _path.RemoveLastElement();  
                    break;  
                default:  
                    break;  
            }  
        }  
        foreach (KeyValuePair<ushort, XmlNodeItem> pair in _map) {  
            _map2[pair.Value.Xpath] = pair.Key;  
        }  
    }  
 
    public byte[] XmlToBytes(string xmlInput) {  
        StringReader sr = new StringReader(xmlInput);  
        XmlReader reader = XmlReader.Create(sr);  
        MemoryStream ms = new MemoryStream();  
        BinaryWriter bw = new BinaryWriter(ms);  
        while (reader.Read()) {  
            ushort index;  
            byte[] bs;  
            switch (reader.NodeType) {  
                case XmlNodeType.Element:  
                    _path.AddElement(reader.Name);  
                    if (_map2.TryGetValue(_path.ToString(), out index)) {  
                        bw.Write(index);  
                    }  
                    if (reader.HasAttributes) {  
                        reader.MoveToFirstAttribute();  
                        _path.AddAttribute(reader.Name);  
                        if (_map2.TryGetValue(_path.ToString(), out index)) {  
                            _path.RemoveLastElement();  
                            bw.Write(index);  
                            bs = Encoding.UTF8.GetBytes(reader.Value);  
                            bw.Write((ushort)bs.Length);  
                            bw.Write(bs);  
                        }  
                        while (reader.MoveToNextAttribute()) {  
                            _path.AddAttribute(reader.Name);  
                            if (_map2.TryGetValue(_path.ToString(), out index)) {  
                                _path.RemoveLastElement();  
                                bw.Write(index);  
                                bs = Encoding.UTF8.GetBytes(reader.Value);  
                                bw.Write((ushort)bs.Length);  
                                bw.Write(bs);  
                            }  
                        }  
                        reader.MoveToElement();  
                    }  
                    if (reader.IsEmptyElement) {  
                        _path.RemoveLastElement();  
                        bw.Write(ushort.MaxValue);  
                    }  
                    break;  
                case XmlNodeType.EndElement:  
                    _path.RemoveLastElement();  
                    bw.Write(ushort.MaxValue);  
                    break;  
                case XmlNodeType.Text:  
                    bw.Write((ushort)0);  
                    bs = Encoding.UTF8.GetBytes(reader.Value);  
                    bw.Write((ushort)bs.Length);  
                    bw.Write(bs);  
                    break;  
                default:  
                    break;  
            }  
        }  
        bw.Close();  
        ms.Close();  
        reader.Close();  
        return ms.ToArray();  
    }  
 
    public string BytesToXml(byte[] bytes) {  
        MemoryStream ms = new MemoryStream(bytes);  
        BinaryReader br = new BinaryReader(ms);  
        StringBuilder sb = new StringBuilder();  
        StringWriter sw = new StringWriter(sb);  
        XmlWriterSettings settings = new XmlWriterSettings();  
        settings.Indent = true;  
        XmlWriter writer = XmlWriter.Create(sw, settings);  
 
        XmlNodeItem item;  
        while (br.PeekChar() != -1) {  
            ushort readFlag = br.ReadUInt16();  
            int len;  
            byte[] bs;  
            string str;  
            if (_map.TryGetValue(readFlag, out item)) {  
                if (item.ItemType == ItemType.Element)  
                    writer.WriteStartElement(item.Text);  
                else if (item.ItemType == ItemType.Attritube) {  
                    len = br.ReadUInt16();  
                    bs = br.ReadBytes(len);  
                    str = Encoding.UTF8.GetString(bs);  
                    writer.WriteAttributeString(item.Text, str);  
                }  
            }  
            else if (readFlag == 0) {  
                len = br.ReadUInt16();  
                bs = br.ReadBytes(len);  
                str = Encoding.UTF8.GetString(bs);  
                writer.WriteString(str);  
            }  
            else if (readFlag == ushort.MaxValue) {  
                writer.WriteEndElement();  
            }  
        }  
        writer.Flush();  
        writer.Close();  
        sw.Close();  
        br.Close();  
        return sb.ToString();  
    }  
}

【編輯推薦】

Servlet引擎的安裝
配置Servlet開發環境
標簽庫中JSP Servlet調用
學習Java Servlet時遇到的小問題
Servlet在session中共享鏈接

責任編輯：彭凡來源： JavaEye

XML壓縮