Wie gibt SpringBoot das Json-Datenformat zurück?

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Controller
@ResponseBody
public @interface RestController {
    String value() default "";
}

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-json</artifactId>
    <version>2.0.3.RELEASE</version>
    <scope>compile</scope>
</dependency>

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.6</version>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jdk8</artifactId>
    <version>2.9.6</version>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jsr310</artifactId>
    <version>2.9.6</version>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-parameter-names</artifactId>
    <version>2.9.6</version>
    <scope>compile</scope>
</dependency>

public class User {
    private Long id;
    private String username;
    private String password;
    /* 省略get、set和带参构造方法 */
}

import com.itcodai.course02.entity.User;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

@RestController
@RequestMapping("/json")
public class JsonController {

    @RequestMapping("/user")
    public User getUser() {
        return new User(1, "倪升武", "123456");
        //返回 {"id":1,"username":"倪升武","password":"123456"}
    }

    @RequestMapping("/list")
    public List<User> getUserList() {
        List<User> userList = new ArrayList<>();
        User user1 = new User(1, "倪升武", "123456");
        User user2 = new User(2, "达人课", "123456");
        userList.add(user1);
        userList.add(user2);
        return userList;
        //返回 [{"id":1,"username":"倪升武","password":"123456"},{"id":2,"username":"达人课","password":"123456"}]

    }

    @RequestMapping("/map")
    public Map<String, Object> getMap() {
        Map<String, Object> map = new HashMap<>(3);
        User user = new User(1, "倪升武", "123456");
        map.put("作者信息", user);
        map.put("博客地址", "http://blog.itcodai.com");
        map.put("CSDN地址", "http://blog.csdn.net/eson_15");
        map.put("粉丝数量", 4153);
        return map;
        //返回 {"作者信息":{"id":1,"username":"倪升武","password":"123456"},"CSDN地址":"http://blog.csdn.net/eson_15","粉丝数量":4153,"博客地址":"http://blog.itcodai.com"}

    }
}

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

@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;
    }
}

// 修改一下上面返回 Map 的接口，将几个值改成 null 测试一下：

@RequestMapping("/map")
public Map<String, Object> getMap() {
    Map<String, Object> map = new HashMap<>(3);
    User user = new User(1, "倪升武", null);
    map.put("作者信息", user);
    map.put("博客地址", "http://blog.itcodai.com");
    map.put("CSDN地址", null);
    map.put("粉丝数量", 4153);
    return map;
	// 返回 {"作者信息":{"id":1,"username":"倪升武","password":""},"CSDN地址":"","粉丝数量":4153,"博客地址":"http://blog.itcodai.com"}
	// 可以看到 Jackson 已经将所有 null 字段转成空字符串了。
}

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.35</version>
</dependency>