Spring Boot GET请求返回200但响应体为空{}，数据库有有效数据

在Spring Boot开发中，调用/getclient（按ID查询）和/getclientbyname（按名称查询）两个GET接口时，请求均返回200 OK状态码，但Postman响应体始终为{}。MySQL数据库中已存在对应查询条件的有效数据，相关代码及请求响应详情如下：

实体类（Client.java） #

@Entity
@Table(name = "client")
public class Client {

    @Id
    // @GeneratedValue(strategy = GenerationType.IDENTITY)
    int clientId;
    @Column
    String clientName;
    @Column
    String clientEmail;
    @Column
    String clientPhone;

    public Client(){
        super();
    }

    public Client(int client_id,String client_name,String client_email,String client_phone){
        super();
        this.clientId = client_id;
        this.clientName = client_name;
        this.clientEmail = client_email;
        this.clientPhone = client_phone;
    }
    
    @Override
    public String toString() {
        return "Client{" + "client_id=" + clientId + ", client_name='" + clientName + '\'' + ", client_email='" + clientEmail + '\'' + ", client_phone='" + clientPhone + '\'' + '}';
    }
}

控制器（ClientController.java） #

@RestController
public class ClientController {

    @Autowired
    ClientService cserv;

    @GetMapping("/getclient")
    public ResponseEntity<Client> getClient(@RequestParam("client_id") int client_id) {
        return ResponseEntity.ofNullable(cserv.getClient(client_id));
    }

    @GetMapping("/getclientbyname/{client_name}")
    public ResponseEntity<Client> getClientByName(@PathVariable("client_name") String client_name) {
        return ResponseEntity.ofNullable(cserv.getClientByName(client_name));
    }
}

服务类（ClientService.java） #

@Service
public class ClientService {
    @Autowired
    ClientRepository crepo;

    public Client getClient(int client_id) {
        Client c;
        Optional<Client> optionalClient = crepo.findById(client_id);
        try{
            c = optionalClient.get();
        }
        catch(Exception e)
        {
            c=null;
        }
        return c;
    }
    
    public Client getClientByName(String client_name) {
       Client c;
       Optional<Client> cl = crepo.getClientByName(client_name);
       try{
        c = cl.get();
       }
       catch(Exception e)
       {
        c=null;
       }
       return c;
    }
}

仓库接口（ClientRepository.java） #

@Repository
public interface ClientRepository extends JpaRepository<Client,Integer> {

   public Optional<Client> findById(int client_id);

   @Query("select c from Client c where clientName=:client_name")
  public Optional<Client> getClientByName(String client_name); 
  
}

Postman请求响应详情 #

GET http://localhost:8080/getclientbyname/Client%201
200
122 ms

请求头
User-Agent: PostmanRuntime/7.35.0
Accept: /
Postman-Token: 6186ba22-b040-4bcb-9da9-4932c662d41f
Host: localhost:8080
Accept-Encoding: gzip, deflate, br
Connection: keep-alive

响应头
Content-Type: application/json
Transfer-Encoding: chunked
Date: Mon, 04 Dec 2023 18:31:17 GMT
Keep-Alive: timeout=60
Connection: keep-alive

响应体
{}

出现问题的接口：

• http://localhost:8080/getclient?client_id=1
• http://localhost:8080/getclientbyname/mahendra

问题根源与解决方案 #

1. 实体类字段无序列化权限 #

当前Client类的所有字段都是默认包访问权限，Spring Boot依赖的Jackson序列化框架无法访问非public字段，导致序列化后返回空JSON对象。

解决： 为字段添加private修饰符并提供对应的getter方法（符合JavaBean规范）：

@Entity
@Table(name = "client")
public class Client {

    @Id
    // @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int clientId;
    @Column
    private String clientName;
    @Column
    private String clientEmail;
    @Column
    private String clientPhone;

    public Client(){
        super();
    }

    public Client(int clientId, String clientName, String clientEmail, String clientPhone){
        super();
        this.clientId = clientId;
        this.clientName = clientName;
        this.clientEmail = clientEmail;
        this.clientPhone = clientPhone;
    }
    
    // 添加getter方法
    public int getClientId() {
        return clientId;
    }

    public String getClientName() {
        return clientName;
    }

    public String getClientEmail() {
        return clientEmail;
    }

    public String getClientPhone() {
        return clientPhone;
    }

    @Override
    public String toString() {
        return "Client{" + "clientId=" + clientId + ", clientName='" + clientName + '\'' + ", clientEmail='" + clientEmail + '\'' + ", clientPhone='" + clientPhone + '\'' + '}';
    }
}

2. 仓库接口冗余方法与参数绑定问题 #

JpaRepository已自带findById(Integer id)方法，无需重复定义；同时JPQL查询需添加@Param注解确保参数绑定正确：

@Repository
public interface ClientRepository extends JpaRepository<Client,Integer> {

   @Query("select c from Client c where clientName=:client_name")
   Optional<Client> getClientByName(@Param("client_name") String client_name); 
}

3. 服务类代码简化 #

用Optional的orElse(null)替代try-catch逻辑，代码更简洁：

@Service
public class ClientService {
    @Autowired
    ClientRepository crepo;

    public Client getClient(int client_id) {
        return crepo.findById(client_id).orElse(null);
    }
    
    public Client getClientByName(String client_name) {
       return crepo.getClientByName(client_name).orElse(null);
    }
}

内容的提问来源于stack exchange，提问作者mahendra kumar