public interface HttpMessageConverter<T> {

  boolean canRead(Class<?> clazz, MediaType mediaType);

  boolean canWrite(Class<?> clazz, MediaType mediaType);

  List<MediaType> getSupportedMediaTypes();

  T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
      throws IOException, HttpMessageNotReadableException;

  void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
      throws IOException, HttpMessageNotWritableException;
}

@RequestMapping("/test")
@ResponseBody
public String test(@RequestBody String param) {
  return "param '" + param + "'";
}

@Controller
@RequestMapping("/user")
public class UserController {

  @RequestMapping("/testt")
  @ResponseBody
  public User testt() {
    User user = new User("name", 18);
    return user;
  }
}

public class User {

  private String username;

  private Integer age;
  
  private Integer phone;
  
  private String email;

  public User(String username, Integer age) {
  super();
  this.username = username;
  this.age = age;
  }
}

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.boot.autoconfigure.web.HttpMessageConverters;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;

import java.nio.charset.Charset;

@Configuration
public class HttpMessageConverterConfig {

  //引入Fastjson解析json，不使用默認(rèn)的jackson
  //必須在pom.xml引入fastjson的jar包，并且版必須大于1.2.10
  @Bean
  public HttpMessageConverters fastJsonHttpMessageConverters() {
    //1、定義一個(gè)convert轉(zhuǎn)換消息的對(duì)象
    FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();

    //2、添加fastjson的配置信息
    FastJsonConfig fastJsonConfig = new FastJsonConfig();

    SerializerFeature[] serializerFeatures = new SerializerFeature[]{
        //  輸出key是包含雙引號(hào)
//        SerializerFeature.QuoteFieldNames,
        //  是否輸出為null的字段,若為null 則顯示該字段
//        SerializerFeature.WriteMapNullValue,
        //  數(shù)值字段如果為null，則輸出為0
        SerializerFeature.WriteNullNumberAsZero,
        //   List字段如果為null,輸出為[],而非null
        SerializerFeature.WriteNullListAsEmpty,
        //  字符類(lèi)型字段如果為null,輸出為"",而非null
        SerializerFeature.WriteNullStringAsEmpty,
        //  Boolean字段如果為null,輸出為false,而非null
        SerializerFeature.WriteNullBooleanAsFalse,
        //  Date的日期轉(zhuǎn)換器
        SerializerFeature.WriteDateUseDateFormat,
        //  循環(huán)引用
        SerializerFeature.DisableCircularReferenceDetect,
    };

    fastJsonConfig.setSerializerFeatures(serializerFeatures);
    fastJsonConfig.setCharset(Charset.forName("UTF-8"));

    //3、在convert中添加配置信息
    fastConverter.setFastJsonConfig(fastJsonConfig);

    //4、將convert添加到converters中
    HttpMessageConverter<?> converter = fastConverter;

    return new HttpMessageConverters(converter);
  }
}