Springboot Service 类返回空引用

Question

我创建了一个 Employee Service 实现类以及一个 Employee Service 接口，作为 Springboot 学习的一部分。我的数据库表中有数据，但出现以下错误

java.lang.NullPointerException：无法调用“com.example.demo.implementation.EmployeeServiceImpl.listEmployees(int)”，因为“this.employeeService”为空

这是我的服务实施课程

import com.example.demo.model.Employee;
import com.example.demo.repository.EmployeeRepository;
import com.example.demo.service.EmployeeService;
import jakarta.transaction.Transactional;
import lombok.RequiredArgsConstructor;
import lombok.extern.slf4j.Slf4j;
import org.springframework.data.domain.PageRequest;
import org.springframework.stereotype.Service;
import org.springframework.web.servlet.support.ServletUriComponentsBuilder;

import java.util.Collection;
import java.util.Random;

@RequiredArgsConstructor
@Service
@Transactional
@Slf4j
public class EmployeeServiceImpl implements EmployeeService {

    private final EmployeeRepository employeeRepository;
    @Override
    public Employee createEmployee(Employee employee) {
        log.info("Saving new employee: {}", employee.getEmployeeId());
        employee.setImageUrl(setEmployeeImageUrl());
        return employeeRepository.save(employee);
    }

    @Override
    public Collection<Employee> listEmployees(int limit) {
        log.info("Fetching all employees");
        return employeeRepository.findAll(PageRequest.of(0, limit)).toList();
    }

    @Override
    public Employee getEmployee(String employeeId) {
        log.info("Fetching employee by ID: {}", employeeId);
        return employeeRepository.findByEmployeeId(employeeId);
    }

    @Override
    public Employee updateEmployee(Employee employee) {
        log.info("Updating employee {}", employee.getEmployeeId());
        return employeeRepository.save(employee);
    }

    @Override
    public Boolean deleteEmployee(String employeeId) {
        log.info("Deleting employee {}", employeeId);
        employeeRepository.deleteById(employeeId);
        return Boolean.TRUE;
    }

    private String setEmployeeImageUrl() {
        String[] imageNames = {"face1.png"};
        return ServletUriComponentsBuilder.fromCurrentContextPath().path("/server/image/" + imageNames[new Random().nextInt(1)]).toUriString();
    }
}

这是我的员工服务界面

import com.example.demo.model.Employee;
import org.springframework.stereotype.Service;

import java.util.Collection;

@Service
public interface EmployeeService {
    Employee createEmployee(Employee employee);
    Collection<Employee> listEmployees(int limit);
    Employee getEmployee(String employeeId);
    Employee updateEmployee(Employee employee);
    Boolean deleteEmployee(String employeeId);
}

这是我的控制器

import com.example.demo.implementation.EmployeeServiceImpl;
import com.example.demo.model.Employee;
import com.example.demo.model.Response;
import jakarta.validation.Valid;
import lombok.RequiredArgsConstructor;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.Map;

import static java.time.LocalDateTime.now;
import static java.util.Map.*;
import static java.util.Map.of;
import static org.springframework.http.HttpStatus.CREATED;
import static org.springframework.http.HttpStatus.OK;

@RestController
@RequestMapping("/employee")
@RequiredArgsConstructor
public class EmployeeController {
    private EmployeeServiceImpl employeeService;

    // Return the list of employees
    // Return a response entity of OK with a body of
    // builder method of model Response
    // with current timestamp LocalDateTime.now()
    // and then map the data with the key employee and the value returned by listEmployees of employeeService
    // with embeded success message, status, and statusCode. Then build the entire package
    @GetMapping("/list")
    public ResponseEntity<Response> getEmployees() {
        return ResponseEntity.ok(
                Response.builder()
                        .timestamp(now())
                        .data(of("employees", employeeService.listEmployees(30)))
                        .message("Employee retrieved")
                        .status(OK)
                        .statusCode(OK.value())
                        .build()
        );
    }

    // save an employee
    @PostMapping("/save")
    public ResponseEntity<Response> saveEmployee(@RequestBody @Valid Employee employee) {
        return ResponseEntity.ok(
                Response.builder()
                        .timestamp(now())
                        .data(of("employee", employeeService.createEmployee(employee)))
                        .message("Employee created")
                        .status(CREATED)
                        .statusCode(CREATED.value())
                        .build()
        );
    }

    // Return a specific employee with employeeID
    @GetMapping("/getemployee/{employeeId}")
    public ResponseEntity<Response> getEmployee(@PathVariable("employeeId") String employeeId) {
        return ResponseEntity.ok(
                Response.builder()
                        .timestamp(now())
                        .data(of("employee", employeeService.getEmployee(employeeId)))
                        .message("Employee retrieved")
                        .status(OK)
                        .statusCode(OK.value())
                        .build()
        );
    }

    @DeleteMapping("/deleteemployee/{employeeId}")
    public ResponseEntity<Response> deleteEmployee(@PathVariable @Valid String employeeId) {
        return ResponseEntity.ok(
                Response.builder()
                        .timestamp(now())
                        .data(of("deleted", employeeService.deleteEmployee(employeeId)))
                        .message("Employee deleted")
                        .status(OK)
                        .statusCode(OK.value())
                        .build()
        );
    }


}

感谢您的帮助，谢谢！

Answer 1

问题源于您使用EmployeeServiceImpl.listEmployees(int)。它可能位于您调用服务方法的controller类之一中。

作为错误的原因，我猜测服务实现没有正确注入到控制器中的 EmployeeService 实例。由于它无法正确实例化，因此它为 null 并且引用 null 对象会导致您提到的 NPE。

如果你写 public static final EmployeeServiceemployeeService;在控制器中，确保添加lombok的注释@RequiredArgsConstructor。