使用spring boot开发时java对象和Json对象转换的问题

将java对象转换为json对象，市面上有很多第三方jar包，如下：

jackson(最常用)

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.11.2</version>
</dependency>

gson

<!-- https://mvnrepository.com/artifact/com.google.code.gson/gson -->
<dependency>
  <groupId>com.google.code.gson</groupId>
  <artifactId>gson</artifactId>
  <version>2.8.5</version>
</dependency>

fastjson

<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson -->
<dependency>
  <groupId>com.alibaba</groupId>
  <artifactId>fastjson</artifactId>
  <version>1.2.62</version>
</dependency>

一、构建测试项目

开发工具为：IDEA
后端技术：Spring boot ，Maven

引入依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.4.3</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>
  <groupId>com.example</groupId>
  <artifactId>json</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>json</name>
  <description>Demo project for Spring Boot</description>
  <properties>
    <java.version>1.8</java.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
      <groupId>org.projectlombok</groupId>
      <artifactId>lombok</artifactId>
      <optional>true</optional>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
        <configuration>
          <excludes>
            <exclude>
              <groupId>org.projectlombok</groupId>
              <artifactId>lombok</artifactId>
            </exclude>
          </excludes>
        </configuration>
      </plugin>
    </plugins>
  </build>

</project>

可以从上面看出，并未引入Jackson相关依赖，这是因为Spring boot的起步依赖spring-boot-starter-web 已经为我们传递依赖了Jackson JSON库。

当我们不用它，而采用其他第三方jar包时，我们可以排除掉它的依赖，可以为我们的项目瘦身。

<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
   <exclusions>
    <exclusion>
      <artifactId>jackson-core</artifactId>
       <groupId>com.fasterxml.jackson.core</groupId>
     </exclusion>
   </exclusions>
</dependency>

二、jackson转换

1.构建User实体类

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class UserEntity {


  private String userName;

  private int age;

  private String sex;
  
}

代码如下（示例）：

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns
import warnings
warnings.filterwarnings('ignore')
import ssl
ssl._create_default_https_context = ssl._create_unverified_context

2.controller类

Java对象转换为json对象

@Controller
public class JsonController {

  @GetMapping("/json1")
  //思考问题，正常返回它会走视图解析器，而json需要返回的是一个字符串
  //市面上有很多的第三方jar包可以实现这个功能，jackson，只需要一个简单的注解就可以实现了
  //@ResponseBody，将服务器端返回的对象转换为json对象响应回去
  @ResponseBody
  public String json1() throws JsonProcessingException {
    //需要一个jackson的对象映射器，就是一个类，使用它可以将对象直接转换成json字符串
    ObjectMapper mapper = new ObjectMapper();
    //创建对象
    UserEntity userEntity = new UserEntity("笨笨熊", 18, "男");
    System.out.println(userEntity);
    //将java对象转换为json字符串
    String str = mapper.writeValueAsString(userEntity);
    System.out.println(str);
    //由于使用了@ResponseBody注解，这里会将str以json格式的字符串返回。
    return str;
  }

  @GetMapping("/json2")
  @ResponseBody
  public String json2() throws JsonProcessingException {

    ArrayList<UserEntity> userEntities = new ArrayList<>();

    UserEntity user1 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user2 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user3 = new UserEntity("笨笨熊", 18, "男");

    userEntities.add(user1);
    userEntities.add(user2);
    userEntities.add(user3);

    return new ObjectMapper().writeValueAsString(userEntities);
  }
}

Date对象转换为json对象

@GetMapping("/json3")
  @ResponseBody
  public String json3() throws JsonProcessingException {
    ObjectMapper mapper = new ObjectMapper();
    //Date默认返回时间戳，所以需要关闭它的时间戳功能
    mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    //时间格式化问题 自定义时间格式对象
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    //让mapper指定时间日期格式为simpleDateFormat
    mapper.setDateFormat(simpleDateFormat);
    //写一个时间对象
    Date date = new Date();
    return mapper.writeValueAsString(date);

  }

提取工具类JsonUtils

public class JsonUtils {

  public static String getJson(Object object){
    return getJson(object,"yyyy-MM-dd HH:mm:ss");
  }
  public static String getJson(Object object,String dateFormat) {
    ObjectMapper mapper = new ObjectMapper();
    //Date默认返回时间戳，所以需要关闭它的时间戳功能
    mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    //时间格式化问题 自定义时间格式对象
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat(dateFormat);
    //让mapper指定时间日期格式为simpleDateFormat
    mapper.setDateFormat(simpleDateFormat);
    try{
      return mapper.writeValueAsString(object);
    }catch (JsonProcessingException e){
      e.printStackTrace();
    }
    return null;
  }
}

优化后：

@GetMapping("/json4")
  @ResponseBody
  public String json4() throws JsonProcessingException {
    Date date = new Date();
    return JsonUtils.getJson(date);

  }

三、gson转换

引入上述gson依赖

Controller类

@RestController
public class gsonController {
  @GetMapping("/gson1")
  public String json1() throws JsonProcessingException {

    ArrayList<UserEntity> userEntities = new ArrayList<>();

    UserEntity user1 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user2 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user3 = new UserEntity("笨笨熊", 18, "男");

    userEntities.add(user1);
    userEntities.add(user2);
    userEntities.add(user3);
    
    Gson gson = new Gson();
    String str = gson.toJson(userEntities);

    return str;
  }
}

四、fastjson转换

引入相关依赖

Controller类

@RestController
public class FastJsonController {
  @GetMapping("/fastjson1")
  public String json1() throws JsonProcessingException {

    ArrayList<UserEntity> userEntities = new ArrayList<>();

    UserEntity user1 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user2 = new UserEntity("笨笨熊", 18, "男");
    UserEntity user3 = new UserEntity("笨笨熊", 18, "男");

    userEntities.add(user1);
    userEntities.add(user2);
    userEntities.add(user3);

    String str = JSON.toJSONString(userEntities);

    return str;
  }
}

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