Gson 类型适配器与自定义解串器

Question

下面的示例显示了一个 class（俱乐部），其中包含摘要 class（会员）的集合。我对是否需要 TypeAdapter 或 JsonDeserializer 来使反序列化正常工作感到困惑。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，我构建了以下 "clone" 测试。如果有人能展示一个工作示例，我将不胜感激。

第一俱乐部Class

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); }
}

现在是抽象基地 Class 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);}
}

以及会员（金牌和银牌）的两个具体子class

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.construct(ConstructorConstructor.java:107)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:186)
...

Answer 1

两者都可以。您选择哪一个实际上取决于潜在的性能影响，以及愿意编写多少代码。

反序列化器更贵。这是因为反序列化器的输入是一个 json 树，GSon 必须为与您的 class 匹配的元素创建一个完整的 JsonElement 子树，然后才能将其传递给您的反序列化器。如果您的模型有很多嵌套，则成本会增加。对于普通物体，它可以忽略不计。

看来你会根据将包含在目标对象中的type 属性的值知道创建哪个class。您的解串器需要

查看传入的JsonElement对象，读取type属性，判断类型
使用 class 和传递给您的相同元素调用 context.deserialize()
如果类型丢失或无效则抛出错误

您的类型适配器必须更复杂。类型适配器的输入是流，而不是 element/subtree。您可以完全从流中加载下一个值，解析它，然后完全按照反序列化器的方式执行，这没有意义，您可以改用反序列化器。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，直到到达 type 属性（您无法预测其位置），然后完成读取其余部分的属性，并根据类型创建最终的 Gold/Silver 对象，并读取和保存所有属性。

Answer 2

好的，真实的工作示例（这次我很确定）。

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); }
}

会员摘要Class

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Classes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)); 
    }
}

自定义成员序列化程序

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;
        }
    }
}

自定义解串器

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;
        }
    }
}

然后...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，我的真实用例是性能不应该成为问题的一个用例，我正在从 jSon 文本文件加载对象缓存，因此执行此代码的频率会提高性能远不如可维护性重要。

Answer 3

看起来 serializing/deserializing class 层次结构是一个常见问题。

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

请检查：

解释：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。建议仅 copy/paste 来源。

Gson 类型适配器与自定义解串器

Gson Type Adapter vs. Custom Deseralizer

java

gson