[英]Gson Type Adapter vs. Custom Deseralizer
下面的示例显示了一个包含抽象类(Member)集合的类(Club)。 我很困惑我是否需要TypeAdapter或JsonDeserializer来使反序列化正常工作。 序列化在没有任何帮助的情况下工作正常,但反序列化会抛出异常。 为了说明我已经构建了以下“克隆”测试。 如果有人能展示一个有效的例子,我将非常感激。
第一俱乐部课程
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);}
}
会员的两个具体子类(金银)
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)
...
你可以做到这两点。 您选择哪一个取决于潜在的性能影响,以及愿意编写多少代码。
反序列化器更昂贵。 这是因为反序列化器的输入是一个json树,并且GSon必须创建一个与您的类匹配的属性的完整JsonElement子树,然后才能将它传递给您的反序列化器。 如果你的课程有很多嵌套,那么这个成本会增加。 对于普通物体,它可以忽略不计。
您似乎将根据将包含在目标对象中的type
属性的值知道要创建的类。 你的解串器需要
JsonElement
对象,读取type
属性,确定类型 context.deserialize()
您的类型适配器必须更复杂。 类型适配器的输入是流,而不是元素/子树。 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程序执行的操作,这没有意义,您只需使用反序列化器接口即可。 或者,您可以阅读流,查看有哪些属性,将它们保存到局部变量中,直到找到type
属性(无法预测其位置),然后读完剩余的属性,然后创建最终属性基于类型的Gold
/ Silver
对象,以及读取和保存的所有属性。
好的,真正有效的例子(我很确定这一次)。
该俱乐部
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);
}
}
混凝土子类银和金
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));
}
}
定制会员Serailizer
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;
}
}
}
而......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,我的实际用例是性能不应该是一个问题,我正在从jSon文本文件加载对象的缓存,因此执行此代码的频率使得性能远不如可维护性重要。
看起来像序列化/反序列化类层次结构是一个常见问题。
甚至还有一个“官方”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)。
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