Simple-XML - 如何序列化集合中的派生类？

Question

我想序列化一个对象层次结构，包括从基类“Thing”派生的对象列表。这很好用，包括反序列化——但 XML-Simple 坚持编写一个属性来指定实际使用的 Java 类

当我用下面的java代码创建一个xml文件时，内容是这样的：

    <example1>
       <things>
          <fruit class="com.mumpitz.simplexmltest.Apple" id="17">
             <sugar>212</sugar>
          </fruit>
          <fruit class="com.mumpitz.simplexmltest.Orange" id="25" weight="11.2"/>
       </things>
    </example1>

但这不是我想要的。 我想拥有

    <example1>
       <things>
          <apple id="17">
             <sugar>212</sugar>
          </apple>
          <orange id="25" weight="11.2"/>
       </things>
    </example1>

'apple' 和 'orange' 元素没有 class 属性，而不是 'fruit' 有这样的属性。这可能吗？

（第二个 xml 符合现有架构；添加额外属性不是一种选择）

代码如下：

    package com.mumpitz.simplexmltest;

    import java.io.File;
    import java.util.ArrayList;

    import org.simpleframework.xml.Attribute;
    import org.simpleframework.xml.Element;
    import org.simpleframework.xml.ElementList;
    import org.simpleframework.xml.Root;
    import org.simpleframework.xml.Serializer;
    import org.simpleframework.xml.core.Persister;

    class Fruit {
        @Attribute(name = "id")
        protected final int id;
        Fruit(
                @Attribute(name = "id")
                int id) {
            this.id = id;
        }
        int getObjectId() {
            return id;
        }
    }

    @Root
    class Apple extends Fruit {
        private final int sugar;
        @Element(type = Fruit.class)
        public Apple(
                @Attribute(name = "id")
                int id,
                @Element(name = "sugar")
                int sugar) {
            super(id);
            this.sugar = sugar;
        }

        @Element(name = "sugar")
        public int getSugar() {
            return this.sugar;
        }

        @Override
        public String toString() {
            return "id: " + id + ", sugar: " + sugar;
        }
    }

    @Root
    class Orange extends Fruit {
        @Attribute
        public double weight;

        public Orange(
                @Attribute(name = "id")
                int id) {
            super(id);
        }

        @Override
        public String toString() {
            return "id: " + id + ", weight: " + weight;
        }
    }

    @Root
    public class Example1 {
        @ElementList
        public ArrayList<Fruit> things = new ArrayList<Fruit>();

        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append("things:\n");
            for (int i=0; i<things.size(); i++) {
                sb.append(" " + things.get(i).toString() + "\n");
            }
            return sb.toString();
        }


        //////////////////////////////////

        static Example1 createDummy() {
            Example1 d = new Example1();
            d.things.add(new Apple(17, 212));
            Orange or = new Orange(25);
            or.weight = 11.2;
            d.things.add(or);
            return d;
        }

        static String msg;
        static Example1 res;

        static public String getMessage() {
            String m = msg;
            msg = null;
            return m;
        }

        static public boolean write(String path) {
            Serializer serializer = new Persister();
            Example1 example = Example1.createDummy();
            File result = new File(path);

            try {
                serializer.write(example, result);
            } catch (Exception e) {
                e.printStackTrace();
                msg = e.getMessage();
                return false;
            }
            return true;
        }

        static public boolean read(String path) {
            Serializer serializer = new Persister();
            File source = new File(path);

            try {
                res = serializer.read(Example1.class, source);
            } catch (Exception e) {
                e.printStackTrace();
                msg = e.getMessage();
                return false;
            }
            return true;
        }

        public static Object getResult() {
            return res;
        }
    }

Answer 1

几个小时后，我找到了解决方案。你只需要

1. 阅读手册

2. 使用@ElementListUnion 注解

   package com.mumpitz.simplexmltest;
   
   import java.io.File;
   import java.util.ArrayList;
   import java.util.List;
   
   import org.simpleframework.xml.Attribute;
   import org.simpleframework.xml.Element;
   import org.simpleframework.xml.ElementList;
   import org.simpleframework.xml.ElementListUnion;
   import org.simpleframework.xml.Root;
   import org.simpleframework.xml.Serializer;
   import org.simpleframework.xml.core.Persister;
   
   // the base class
   @Element
   class Thing {
   
       static int count=0;
   
       Thing() {
           this.id = ++count;
       }
   
       @Attribute
       protected int id;
   
       public int getId() {
           return id;
       }
   }
   
   // first derived class
   @Element
   class Car extends Thing {
       @Attribute
       private String name;
   
       Car(@Attribute(name="name") String name) {
           this.name = name;
       }
   
       public String getName() {
           return name;
       }
       @Override
       public String toString() {
           return "ID: " + id + " Car: " + name;
       }
   }
   
   // second derived class
   @Element
   class House extends Thing {
       @Attribute
       private int price;
   
       House(@Attribute(name="price") int price) {
           this.price = price;
       }
   
       public int getPrice() {
           return this.price;
       }
       @Override
       public String toString() {
           return "ID: " + id + " House: " + price;
       }
   }
   
   
   // a class with a list of base class instances
   @Root(name="ListOfThings")
   public class Example4 {
   
       // specify the derived classes used in the list
       @ElementListUnion({
           @ElementList(entry="house", inline=true, type=House.class),
           @ElementList(entry="car", inline=true, type=Car.class)
       })
       private ArrayList<Thing> list = new ArrayList<Thing>();
   
       public void add(Thing t) {
           list.add(t);
       }
   
       public List<Thing> getProperties() {
           return list;
       }
   
       @Override
       public String toString() {
           StringBuilder sb = new StringBuilder();
           sb.append("Example4 contains " + list.size() + " elements:\n");
           for (Thing t : list) {
               sb.append(" " + t.toString() + "\n");
           }
           return sb.toString();
       }
   
   
       //////////////////////////////////
       // test code
       //////////////////////////////////
       static String msg;
       static Example4 res;
   
       static public String getMessage() {
           String m = msg;
           msg = null;
           return m;
       }
   
       static private Example4 createDummy() {
           Example4 d = new Example4();
           d.add(new Car("Mercedes"));
           d.add(new House(34000000));
           d.add(new Car("VW"));
           d.add(new House(230000));
           return d;
       }
   
       //////////////////////////////////
       // serialize / deserialize
       //////////////////////////////////
       static public boolean write(String path) {
   
           Serializer serializer = new Persister();
           File result = new File(path);
           Example4 example = Example4.createDummy();
   
           try {
               serializer.write(example, result);
           } catch (Exception e) {
               e.printStackTrace();
               msg = e.getMessage();
               return false;
           }
           return true;
       }
   
       static public boolean read(String path) {
           Serializer serializer = new Persister();
           File source = new File(path);
   
           try {
               res = serializer.read(Example4.class, source);
           } catch (Exception e) {
               e.printStackTrace();
               msg = e.getMessage();
               return false;
           }
           return true;
       }
   
       public static Object getResult() {
           return res;
       }
   }