我一直在尝试从运行在我电脑上的spa获取一个数组请求localhost:8080，它必须是正确的端口，因为它在控制台中被告知，如果我尝试请求其他端口，它（ Postman ）会说请求失败。我很难找到404的原因，因为我没有可以处理的错误。
休息控制器：

@RestController
public class Controller {

    private AtomicInteger rCounter;
    private AtomicInteger pCounter;
    private List<Registration> registrationList = new ArrayList<Registration>();
    private List<Project> projectList = new ArrayList<Project>();

    @PostConstruct
    public void init() {

        this.rCounter = new AtomicInteger();
        this.pCounter = new AtomicInteger();
    }

    // THIS SECTION IS DEDICATED TO THE REGISTRATION REQUESTS

    // get all Registrations
    @GetMapping("/registrations")
    public List<Registration> getAllRegistrations() {
        return this.registrationList;
    }

    // get a single Registration
    @GetMapping("/registrations/{id}")
    public Registration getRegistration(@PathVariable("id") int id) {

        for (Registration reg : registrationList) {
            if (reg.getId() == id) {
                return reg;
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND,
                String.format("Registration with ID %s not found!", id));
    }

    // create a Registration
    @PostMapping("/createRegistration")
    @ResponseStatus(HttpStatus.CREATED)
    public void createRegistration(@Valid @RequestBody Registration requestBody) {
        Registration reg = new Registration(rCounter.getAndIncrement(), requestBody.getStudents(),
                requestBody.getPreferredProjects());
        this.registrationList.add(reg);
    }

    // update a Registration
    @PutMapping("/updateRegistration/{id}")
    public void updateRegistration(@PathVariable("id") int id, @Valid @RequestBody Registration requestBody) {
        requestBody.setId(id);
        for (Registration reg : registrationList) {
            if (reg.getId() == id) {
                this.registrationList.set(id, requestBody);
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND,
                String.format("Registration with ID %s not found!", id));
    }

    // delete a Registration
    @DeleteMapping("/deleteRegistration/{id}")
    public void deleteRegistration(@PathVariable("id") int id) {

        for (Registration reg : registrationList) {
            if (reg.getId() == id) {
                this.registrationList.remove(id);
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND,
                String.format("Registration with ID %s not found!", id));

    }

    // THIS SECTION IS DEDICATED TO THE PROJECT REQUESTS

    // get all Projects
    @GetMapping("/projects")
    public List<Project> getAllProjects() {
        return this.projectList;
    }

    // get a single Project
    @GetMapping("/projects/{id}")
    public Project getProject(@PathVariable("id") int id) {

        for (Project project : projectList) {
            if (project.getId() == id) {
                return project;
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND, String.format("Project with ID %s not found!", id));
    }

    // create a Project
    @PostMapping("/createProject/{id}")
    @ResponseStatus(HttpStatus.CREATED)
    public void createProject(@Valid @RequestBody Project requestBody) {
        Project project = new Project(pCounter.getAndIncrement(), requestBody.getName());
        this.projectList.add(project);
    }

    // update a Project
    @PutMapping("/updateProject/{id}")
    public void updateProject(@PathVariable("id") int id, @Valid @RequestBody Project requestBody) {
        requestBody.setId(id);
        for (Project project : projectList) {
            if (project.getId() == id) {
                this.projectList.set(id, requestBody);
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND, String.format("Project with ID %s not found!", id));
    }

    // delete a Project
    @DeleteMapping("/deleteProject/{id}")
    public void deleteProject(@PathVariable("id") int id) {

        for (Project project : projectList) {
            if (project.getId() == id) {
                this.projectList.remove(id);
            }
        }

        throw new ResponseStatusException(HttpStatus.NOT_FOUND, String.format("Project with ID %s not found!", id));

    }

}

api bean：

// enable cross-origin resource sharing (CORS)
@Bean
public WebMvcConfigurer corsConfigurer() {
    return new WebMvcConfigurer() {
        @Override
        public void addCorsMappings(CorsRegistry registry) {
            // allow CORS requests for all resources and HTTP methods from the frontend
            // origin
            registry.addMapping("/**").allowedMethods("OPTIONS", "HEAD", "GET", "PUT", "POST", "DELETE")
                    .allowedOrigins("http://localhost:8080");
        }
    };
}

如果你需要看更多的只是写一个评论，我会通过编辑提供它。谢谢你的帮助！