java - XML 序列化引用 - 重复

Question

[Java 或 C#] 我在序列化方面遇到了一些问题。如何不复制有关对象的所有信息并仅使用引用？

示例类:

class Author {
  public String id;
  public String name;
}

class Book {
  public String id;
  public Author author;
  public String title;
}

我必须像这里一样格式化输出文件:

<store>
  <authors>
   <author id="PK">
     <name>Philip Kindred</name>
    </author>
  </authors>

  <books>
    <book id="u1">
      <author>PK</author> <!--  use only ID -->
      <title>Ubik</title>
    </book>
  </books>
</store>

Answer 1

您面临的问题是在 XML 中表示聚合关系而不是组合关系。当您在父子关系中保持父子关系(即组合)时，XML 序列化非常简单。在这种情况下，一本书有一位(或多位)作者，但并不拥有这本书，因为一位作者可以是许多其他书籍的作者。

在那种情况下，你可以做一些类似于数据库中的事情，即有两个单独的条目并通过外键表达关系。请参见下面的示例:

[Serializable]
public class Author
{
    [XmlAttribute("id")]
    public String Id { get; set; }
    [XmlElement("name")]
    public String Name { get; set; }
}

[Serializable]
public class Book
{
    private Author _author;

    [XmlIgnore]
    public Author Author
    {
        get { return _author; }
        set
        {
            _author = value;
            AuthorId = _author != null ? _author.Id : null;
        }
    }

    [XmlAttribute("id")]
    public String Id { get; set; }

    [XmlElement("author")]
    public String AuthorId { get; set; }

     [XmlElement("title")]
    public String Title { get; set; }
}

[Serializable]
public class Store
{
    [XmlArray("authors")]
    [XmlArrayItem("author", Type = typeof(Author))]
    public List<Author> Authors { get; set; }

    [XmlArray("books")]
    [XmlArrayItem("book", Type = typeof(Book))]
    public List<Book> Books { get; set; }

    public Store()
    {
        Books = new List<Book>();
        Authors = new List<Author>();
    }
}


internal class Program
{
    private static void Main(string[] args)
    {
        // Create some authors
        var authors = new List<Author>
        {
            new Author{Id="PK", Name="Philip Kindred"},
            new Author{Id="WS", Name="William Shakespeare"},
        };

        // Create some books linked to the authors
        var books = new List<Book>
        {
            new Book{Id = "U1", Author = authors[0], Title = "Do Androids Dream of Electric Sheep?"},
            new Book{Id = "U2", Author = authors[1], Title = "Romeo and Juliet"}
        };

        var store = new Store {Authors = authors, Books = books};

        var success = Serialiser.SerialiseToXml(store, "store.xml");

        // Deserialize the data from XML
        var store2 = Serialiser.DeserialseFromXml<Store>("store.xml");

        // Resolve the actual Author instances from the saved IDs (foreign key equivalent in databases)
        foreach (var book in store2.Books)
            book.Author = store2.Authors.FirstOrDefault(author => author.Id == book.AuthorId);

        // Now variable 'store' and 'store2' have the same equivalent data
    }
}

// Helper class to serialize and deserialize the data to XML file
public static class Serialiser
{
    public static bool SerialiseToXml(object obj, string filename)
    {
        try
        {
            var ws = new XmlWriterSettings
            {
                NewLineHandling = NewLineHandling.Entitize,
                NewLineChars = Environment.NewLine,
                Indent = true,
                NewLineOnAttributes = false
            };
            var xs = new XmlSerializer(obj.GetType());
            using (var writer = XmlWriter.Create(filename, ws))
                xs.Serialize(writer, obj);

            return true;
        }
        catch(Exception ex)
        {
            return false;
        }
    }

    public static T DeserialseFromXml<T>(string filename) where T : new()
    {
        var typeofT = typeof(T);
        try
        {
            var xs = new XmlSerializer(typeofT);
            using (var reader = XmlReader.Create(filename))
                return (T)xs.Deserialize(reader);
        }
        catch(Exception ex)
        {
            return default(T);
        }
    }
}

“store.xml”将如下所示:

<?xml version="1.0" encoding="utf-8"?>
<Store xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<authors>
    <author id="PK">
    <name>Philip Kindred</name>
    </author>
    <author id="WS">
    <name>William Shakespeare</name>
    </author>
</authors>
<books>
    <book id="U1">
    <author>PK</author>
    <title>Do Androids Dream of Electric Sheep?</title>
    </book>
    <book id="U2">
    <author>WS</author>
    <title>Romeo and Juliet</title>
    </book>
</books>
</Store>