Spring Boot 返回 Json

在項目開發中，接口與接口之間，前后端之間數據的傳輸都使用Json格式，在 Spring Boot 中，接口返回Json格式的數據很簡單，在 Controller 中使用@RestController注解即可返回 Json 格式的數據。

Spring Boot 中默認使用的 JSON 解析框架是 Jackson。

在實際項目中，常用的數據結構無非有類對象、List 對象、Map 對象

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.json</groupId>
  <artifactId>RebackJson</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  
  <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.0.RELEASE</version>
        <relativePath /> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <!-- 聲明項目配置依賴編碼格式為 utf-8 -->
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <fastjson.version>1.2.24</fastjson.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>
        <dependency>
            <groupId>org.webjars</groupId>
            <artifactId>jquery</artifactId>
            <version>3.3.1</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>
  
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

package com.tszr.json.user;

public class User {
    private int id;
    private String name;
    private String password;

    public User(int id, String name, String password) {
        // TODO Auto-generated constructor stub
        this.id = id;
        this.name = name;
        this.password = password;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

}

package com.tszr.json.controller;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.tszr.json.user.User;

@RestController
@RequestMapping("/json")
public class UserController {
    @RequestMapping("/user")
    public User getUser() {
        return new User(10, "貝西", "11");
    }

    @RequestMapping("/list")
    public List<User> getUserList() {
        List<User> userlist = new ArrayList<>();
        User user1 = new User(1, "吳裕雄", "123456");
        User user2 = new User(1, "天生自然", "123456");
        userlist.add(user1);
        userlist.add(user2);
        return userlist;
    }

    @RequestMapping("/map")
    public Map<String, Object> getMap() {
        Map<String, Object> map = new HashMap<>(3);
        User user3 = new User(1, "天生自然", "123456");
        map.put("作者信息", user3);
        map.put("博客地址", "https://www.cnblogs.com/tszr");
        map.put("公眾號", "貝西奇談");
        map.put("B站", "貝西貝西");
        return map;
    }
}

package com.tszr.json;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.ServletComponentScan;

@ServletComponentScan
@SpringBootApplication
public class JsonApplication {
    public static void main(String[] args) {
        SpringApplication.run(JsonApplication.class, args);
    }
}

Jackson 中對 null 的處理
 在實際項目中，我們難免會遇到一些 null 值。當我們轉 JSON 時，不希望這些 null 出 現，比如我們希望所有的 null 在轉 JSON 時都變成空字符串。
  在 Spring Boot 中，我們做一下配置即可，新建一個 Jackson 的配置類：

package com.tszr.json.configuration;

import java.io.IOException;

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;

@Configuration
public class JacksonConfig {
    @Bean
    @Primary
    @ConditionalOnMissingBean(ObjectMapper.class)
    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        ObjectMapper objectMapper = builder.createXmlMapper(false).build();
        objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
            @Override
            public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
                    throws IOException {
                jsonGenerator.writeString("");
            }
        });
        return objectMapper;
    }

}