Gson类型适配器与自定义反序列化器

Question

下面的例子显示了一个类(Club)，它包含一个抽象类(Member)的集合。我很困惑是否需要TypeAdapter或JsonDeserializer来使反序列化正确工作。序列化在没有任何帮助的情况下工作得很好，但反序列化会抛出异常。为了说明这一点，我构建了下面的"克隆"测试。如果有人能展示一个工作的例子，我将非常感激。

第一俱乐部班

package gson.test;
import java.util.ArrayList;
import com.google.gson.Gson;
public class Club {
    public static void main(String[] args) {
        // Setup a Club with 2 members
        Club myClub = new Club();
        myClub.addMember(new Silver());
        myClub.addMember(new Gold());
        // Serialize to JSON
        Gson gson = new Gson();
        String myJsonClub = gson.toJson(myClub); 
        System.out.println(myJsonClub);
        // De-Serialize to Club
        Club myNewClub = gson.fromJson(myJsonClub, Club.class);
        System.out.println(myClub.equals(myNewClub) ? "Cloned!" : "Failed");
    }
    private String title = "MyClub";
    private ArrayList<Member> members = new ArrayList<Member>();
    public boolean equals(Club that) {
        if (!this.title.equals(that.title)) return false;
        for (int i=0; i<this.members.size(); i++) {
            if (! this.getMember(i).equals(that.getMember(i))) return false;
        }
        return true;
    }
    public void addMember(Member newMember) { members.add(newMember); }
    public Member getMember(int i) { return members.get(i); }
}

抽象基类成员

package gson.test;
public abstract class Member {
    private int type;
    private String name = "";
    public int getType() { return type; }
    public void setType(int type) { this.type = type; }
    public boolean equals(Member that) {return this.name.equals(that.name);}
}

Member (Gold And Silver)的两个具体子类

package gson.test;
public class Gold extends Member {
    private String goldData = "SomeGoldData";
    public Gold() {
        super();
        this.setType(2);
    }
    public boolean equals(Gold that) {
        return (super.equals(that) && this.goldData.equals(that.goldData)); 
    }
}
package gson.test;
public class Silver extends Member {
    private String silverData = "SomeSilverData";
    public Silver() {
        super();
        this.setType(1);
    }
    public boolean equals(Silver that) { 
        return (super.equals(that) && this.silverData.equals(that.silverData)); 
    }
}

最后输出

    {"title":"MyClub","members":[{"silverData":"SomeSilverData","type":1,"name":""},{"goldData":"SomeGoldData","type":2,"name":""}]}
    Exception in thread "main" java.lang.RuntimeException: Failed to invoke public gson.test.Member() with no args
        at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:107)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:186)
...

Answer 1

两者都可以。选择哪一种取决于潜在的性能影响，以及你愿意写多少代码。

反序列化器更昂贵。这是因为反序列化器的输入是一个json树，GSon必须为匹配类的元素创建一个完整的JsonElement子树，然后才能将其传递给反序列化器。如果你的模型有很多嵌套，成本就会增加。对于普通对象，它可以忽略不计。

似乎您将根据将包含在目标对象中的type属性的值知道创建哪个类。反序列化器需要

• 查看传递的JsonElement对象，读取type属性，确定类型
• 使用传递给您的类和相同元素调用context.deserialize()
• 如果类型缺失或无效则抛出错误

类型适配器必须更复杂。类型适配器的输入是流，而不是元素/子树。你可以完全从流中加载下一个值，解析它，然后做反序列化器所做的，这没有意义，你可以直接使用反序列化器。或者，您可以读取流，查看有哪些属性，将它们保存到局部变量中，直到您获得type属性(您无法预测其位置)，然后完成读取其余属性，并根据类型创建最终的Gold/Silver对象，以及读取和保存的所有属性。

Answer 2

好，这是一个真实的工作示例(我很确定这次)。

俱乐部

package gson.test;
import java.util.ArrayList;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class Club {
    public static void main(String[] args) {
        // Setup a Club with 2 members
        Club myClub = new Club();
        myClub.addMember(new Silver("Jack"));
        myClub.addMember(new Gold("Jill"));
        myClub.addMember(new Silver("Mike"));
        // Get the GSON Object and register Type Adapter
        GsonBuilder builder = new GsonBuilder();
        builder.registerTypeAdapter(Member.class, new MemberDeserializer());
        builder.registerTypeAdapter(Member.class, new MemberSerializer());
        builder.setPrettyPrinting();
        Gson gson = builder.create();
        // Serialize Club to JSON
        String myJsonClub = gson.toJson(myClub); 
        // De-Serialize to Club
        Club myNewClub = gson.fromJson(myJsonClub, Club.class);
        System.out.println(myClub.equals(myNewClub) ? "Cloned!" : "Failed");
        System.out.println(gson.toJson(myNewClub));
    }
    private String title = "MyClub";
    private ArrayList<Member> members = new ArrayList<Member>();
    public boolean equals(Object club) {
        Club that = (Club) club;
        if (!this.title.equals(that.title)) return false;
        for (int i=0; i<this.members.size(); i++) {
            Member member1 = this.getMember(i);
            Member member2 = that.getMember(i);
            if (! member1.equals(member2)) return false;
        }
        return true;
    }
    public void addMember(Member newMember) { members.add(newMember); }
    public Member getMember(int i) { return members.get(i); }
}

成员抽象类

package gson.test;
public abstract class Member {
    private String clsname = this.getClass().getName() ;
    private int type;
    private String name = "unknown";
    public Member() { }
    public Member(String theName) {this.name = theName;}
    public int getType() { return type; }
    public void setType(int type) { this.type = type; }
    public boolean equals(Object member) {
        Member that = (Member) member;
        return this.name.equals(that.name);
    }
}

具体子类银和金

package gson.test;
public class Silver extends Member {
    private String silverData = "SomeSilverData";
    public Silver() { 
        super(); 
        this.setType(1); 
    }
    public Silver(String theName) {
        super(theName); 
        this.setType(1); 
    }
    public boolean equals(Object that) {
        Silver silver = (Silver)that;
        return (super.equals(that) && this.silverData.equals(silver.silverData)); 
    }
}
package gson.test;
public class Gold extends Member {
    private String goldData = "SomeGoldData";
    private String extraData = "Extra Gold Data";
    public Gold() {
        super(); 
        this.setType(2);
    }
    public Gold(String theName) { 
        super(theName); 
        this.setType(2); 
    }
    public boolean equals(Gold that) {
        Gold gold = (Gold) that;
        return (super.equals(that) && this.goldData.equals(gold.goldData)); 
    }
}

自定义成员序列化器

package gson.test;
import java.lang.reflect.Type;
import com.google.gson.JsonElement;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
public class MemberSerializer implements JsonSerializer<Member> {
    public JsonElement serialize(Member src, Type member, JsonSerializationContext context) {
        switch (src.getType()) {
            case 1: return context.serialize((Silver)src);
            case 2: return context.serialize((Gold)src);
            default: return null;
        }
    }
}

自定义反序列化器

package gson.test;
import java.lang.reflect.Type;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
public class MemberDeserializer implements JsonDeserializer<Member> {
    @Override
    public Member deserialize(JsonElement json, Type member, JsonDeserializationContext context) {
        int myType = json.getAsJsonObject().get("type").getAsInt();
        switch (myType) {
            case 1: return context.deserialize(json, Silver.class);
            case 2: return context.deserialize(json, Gold.class);
            default: return null;
        }
    }
}

…输出

Cloned!
{
  "title": "MyClub",
  "members": [
    {
      "silverData": "SomeSilverData",
      "clsname": "gson.test.Silver",
      "type": 1,
      "name": "Jack"
    },
    {
      "goldData": "SomeGoldData",
      "extraData": "Extra Gold Data",
      "clsname": "gson.test.Gold",
      "type": 2,
      "name": "Jill"
    },
    {
      "silverData": "SomeSilverData",
      "clsname": "gson.test.Silver",
      "type": 1,
      "name": "Mike"
    }
  ]
}

我应该注意到，我真正的用例是性能不应该是一个问题，我从jSon文本文件加载对象的缓存，所以这段代码的执行频率使得性能远不如可维护性重要。

Answer 3

看起来序列化/反序列化类层次结构是一个常见的问题。

甚至有一个"官方"的解决方案，在官方源代码仓库的extras目录中(不幸的是它不是Maven包的一部分)。

请检查:

• 说明:https://blog.novatec-gmbh.de/gson-object-hierarchies/
• 解决方案:https://github.com/google/gson/blob/master/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java建议直接复制/粘贴源代码。