Spring MVC返回多重的Json数据

一、需求：

页面返回数据

{
  "code": 0,
  "msg": "",
  "count": "2",
  "data": [{
      "keywords": "广东木材提供有限公司1",
      "frequency": 4621,
      "userNums": 4235
    },
    {
      "keywords": "广东木材提供有限公司2",
      "frequency": 4621,
      "userNums": 4235
    }]
}

二、为此写了一个Firm类

public class Firm {

    private String keywords;
    private String frequency;
    private String userNums;
    
    public Testdata(String keywords, String frequency, String userNums) {
        this.keywords = keywords;
        this.frequency = frequency;
        this.userNums = userNums;
    }
}

三、在Controller方法中

@RequestMapping("/firm/listdata.action")
    public @ResponseBody Map<String, Object> listData(){
        
        List<Testdata> tdList = new ArrayList<>();
        Testdata td1 = new Testdata("广东木材提供有限公司1","4621","4235");
        Testdata td2 = new Testdata("广东木材提供有限公司1","4621","4235");
        tdList.add(td1);
        tdList.add(td2);
        tdList.add(td3);
        tdList.add(td4);
        tdList.add(td5);
        Map<String, Object> map = new HashMap<>();
        map.put("code", 0);
        map.put("msg", "");
        map.put("count", 2);
        map.put("data", tdList);
        return map;
    }

四、访问时浏览器报500异常

org.springframework.http.converter.HttpMessageNotWritableException: 
Could not write content: No serializer found for class cn.ssm.trading.domain.Firm and 
no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) 
(through reference chain: java.util.HashMap["data"]->java.util.ArrayList[0]); 
nested exception is com.fasterxml.jackson.databind.JsonMappingException: 
No serializer found for class cn.ssm.trading.domain.Firm and no properties discovered to create BeanSerializer 
(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) 
(through reference chain: java.util.HashMap["data"]->java.util.ArrayList[0])

五、解决

为Firm类实现序列化，为字段提供getter/setter以及无参构造方法

public class Firm implements Serializable {

    private String keywords;
    private String frequency;
    private String userNums;
    
    public String getKeywords() {
        return keywords;
    }
    public void setKeywords(String keywords) {
        this.keywords = keywords;
    }
    public String getFrequency() {
        return frequency;
    }
    public void setFrequency(String frequency) {
        this.frequency = frequency;
    }
    public String getUserNums() {
        return userNums;
    }
    public void setUserNums(String userNums) {
        this.userNums = userNums;
    }
    //无参构造方法
    public Testdata() {
        
    }
    
    public Testdata(String keywords, String frequency, String userNums) {
        this.keywords = keywords;
        this.frequency = frequency;
        this.userNums = userNums;
    }
}