java - 如果我搜索数据库中不存在的用户 ID，则获取 org.springframework.http.converter.HttpMessageNotWritableException

Question

我正在 Spring boot+JPA(MYSQL) 中构建 RESTAPI。如果我搜索数据库中不存在的用户 ID，我会收到以下错误响应:

{
    "timestamp": 1545657449608,
    "status": 500,
    "error": "Internal Server Error",
    "exception": "**org.springframework.http.converter.HttpMessageNotWritableException**",
    "message": "Could not write content: Unable to find com.bookmyshow.user.User with id asd (through reference chain: com.bookmyshow.user.User_$$_jvst809_0[\"username\"]); nested exception is **com.fasterxml.jackson.databind.JsonMappingException**: Unable to find com.bookmyshow.user.User with id asd (through reference chain: com.bookmyshow.user.User_$$_jvst809_0[\"username\"])",
    "path": "/users/asd"
}

用户注册 Controller

package com.bookmyshow.user;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserRegistrationController {

@Autowired
private UserDAO userDAO;

@RequestMapping("/users/{id}") 
public ResponseEntity<?> getUser(@PathVariable String id) {

    User user= userDAO.getUser(id);

    if(user==null) {
        return ResponseEntity.notFound().build();
    }

    return ResponseEntity.ok().body(user);
    }

UserDAO

package com.bookmyshow.user;
import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;


@Service
public class UserDAO {

@Autowired
UserRepository userRepository;  

public User getUser(String id) {

     System.out.println("USERDAO:"+id);
          return userRepository.getOne(id);
}

Answer 1

getOne 方法仅返回来自数据库的引用(延迟加载)。所以基本上你在事务之外(不考虑你在服务类中声明的事务)，并且发生错误。请参阅documentation

改用findOne。

另请参阅when-use-getone-and-findone-methods-spring-data-jpa