Gson解析JSON - Java

简介

Gson是一个Java库，用于将Java对象序列化为JSON格式，或将JSON格式反序列化为Java对象。Gson通过提供简单易用的API，使得处理JSON数据变得容易。

安装Gson库

在使用Gson之前，需要先将Gson库加入到项目中。

方式1: Maven

在项目的pom.xml文件中添加以下依赖：

<dependency>
   <groupId>com.google.code.gson</groupId>
   <artifactId>gson</artifactId>
   <version>2.8.6</version>
</dependency>

方式2: 手动下载

手动从https://github.com/google/gson/releases下载最新版的Gson，然后将jar文件添加到项目的classpath中。

将Java对象序列化为JSON格式

将Java对象序列化为JSON格式，可以使用Gson的toJson()方法。toJson()方法接收一个Java对象，然后返回一个JSON格式的字符串。

import com.google.gson.Gson;

public class GsonExample {
   public static void main(String[] args) {
      Gson gson = new Gson();
      User user = new User("Jack", "Smith", 25, "jack.smith@gmail.com", new String[]{"Java", "Python", "Ruby"});
      String json = gson.toJson(user);
      System.out.println(json);
   }
}

class User {
   private String firstName;
   private String lastName;
   private int age;
   private String email;
   private String[] programmingLanguages;
     
   public User(String firstName, String lastName, int age, String email, String[] programmingLanguages) {
      this.firstName = firstName;
      this.lastName = lastName;
      this.age = age;
      this.email = email;
      this.programmingLanguages = programmingLanguages;
   }
}

输出结果：

{
  "firstName": "Jack",
  "lastName": "Smith",
  "age": 25,
  "email": "jack.smith@gmail.com",
  "programmingLanguages": [
    "Java",
    "Python",
    "Ruby"
  ]
}

将JSON格式反序列化为Java对象

将JSON格式反序列化为Java对象，可以使用Gson的fromJson()方法。fromJson()方法接收一个JSON格式的字符串和一个Class对象作为参数，然后返回一个对应的Java对象。

import com.google.gson.Gson;

public class GsonExample {
   public static void main(String[] args) {
      Gson gson = new Gson();
      String json = "{\"firstName\":\"Jack\",\"lastName\":\"Smith\",\"age\":25,\"email\":\"jack.smith@gmail.com\",\"programmingLanguages\":[\"Java\",\"Python\",\"Ruby\"]}";
      User user = gson.fromJson(json, User.class);
      System.out.println(user.getFirstName());
   }
}

class User {
   private String firstName;
   private String lastName;
   private int age;
   private String email;
   private String[] programmingLanguages;
     
   public User(String firstName, String lastName, int age, String email, String[] programmingLanguages) {
      this.firstName = firstName;
      this.lastName = lastName;
      this.age = age;
      this.email = email;
      this.programmingLanguages = programmingLanguages;
   }
   
   public String getFirstName() {
      return firstName;
   }
}

输出结果：

Jack

处理未知JSON格式

有时候，我们需要处理一些不确定格式的JSON数据。Gson提供了JsonElement类来处理未知JSON格式的数据。JsonElement支持以下类型：

• JsonPrimitive
• JsonObject
• JsonArray
• JsonNull

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;

public class GsonExample {
   public static void main(String[] args) {
      Gson gson = new Gson();
      String json1 = "{\"firstName\":\"Jack\",\"lastName\":\"Smith\",\"age\":25,\"email\":\"jack.smith@gmail.com\",\"programmingLanguages\":[\"Java\",\"Python\",\"Ruby\"]}";
      String json2 = "{\"id\":1,\"name\":\"Apple\",\"price\":2.5}";

      JsonObject jsonObject1 = gson.fromJson(json1, JsonObject.class);
      JsonObject jsonObject2 = gson.fromJson(json2, JsonObject.class);

      System.out.println("jsonObject1");
      System.out.println(jsonObject1.getAsJsonPrimitive("firstName").getAsString());
      System.out.println(jsonObject1.getAsJsonPrimitive("lastName").getAsString());
      System.out.println(jsonObject1.getAsJsonPrimitive("age").getAsInt());
      System.out.println(jsonObject1.getAsJsonPrimitive("email").getAsString());
      System.out.println(jsonObject1.getAsJsonArray("programmingLanguages").get(0).getAsString());
      System.out.println(jsonObject1.getAsJsonArray("programmingLanguages").get(1).getAsString());
      System.out.println(jsonObject1.getAsJsonArray("programmingLanguages").get(2).getAsString());

      System.out.println("jsonObject2");
      System.out.println(jsonObject2.getAsJsonPrimitive("id").getAsInt());
      System.out.println(jsonObject2.getAsJsonPrimitive("name").getAsString());
      System.out.println(jsonObject2.getAsJsonPrimitive("price").getAsFloat());
   }
}

输出结果：

jsonObject1
Jack
Smith
25
jack.smith@gmail.com
Java
Python
Ruby
jsonObject2
1
Apple
2.5

自定义序列化和反序列化

有时候，我们需要自定义序列化和反序列化的行为，Gson提供了JsonSerializer和JsonDeserializer两个接口，可以实现自定义序列化和反序列化逻辑。

import com.google.gson.*;
import java.lang.reflect.Type;

public class GsonExample {
   public static void main(String[] args) {
      GsonBuilder builder = new GsonBuilder();
      builder.registerTypeAdapter(User.class, new UserAdapter());
      Gson gson = builder.create();

      String json = "{\"name\":\"Jack\",\"age\":\"25\"}";
      User user = gson.fromJson(json, User.class);
      System.out.println(user.getName());
   }
}

class User {
   private String name;
   private int age;

   public User(String name, int age) {
      this.name = name;
      this.age = age;
   }

   public String getName() {
      return name;
   }
}

class UserAdapter implements JsonSerializer<User>, JsonDeserializer<User> {
   @Override
   public User deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
      JsonObject jsonObject = json.getAsJsonObject();
      String name = jsonObject.get("name").getAsString();
      String age = jsonObject.get("age").getAsString();
      return new User(name, Integer.parseInt(age));
   }

   @Override
   public JsonElement serialize(User user, Type typeOfT, JsonSerializationContext context) {
      JsonObject jsonObject = new JsonObject();
      jsonObject.addProperty("name", user.getName());
      jsonObject.addProperty("age", user.getAge());
      return jsonObject;
   }
   
   private int getAge() {
      return age;
   }
}

输出结果：

Jack