What is `ResponseEntity` does?

ResponseEntity 在 Spring MVC 中是一个用于表示 HTTP响应的类。
可以设置HTTP响应的状态码（如200、401、500等），
还可以设置返回给客户端的具体内容（内容可以是任意类型的数据，包括字符串、对象、甚至是文件流）

使用样例

返回JSON数据：

@RestController
public class MyController {

    @GetMapping("/users/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        User user = userService.getUserById(id); // 假设userService是获取用户的方法

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

        return ResponseEntity.ok(user);
    }
}

自定义错误信息：

@GetMapping("/invalid")
public ResponseEntity<String> handleInvalidRequest() {
    String errorMessage = "This is an invalid request.";
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(errorMessage);
}

下载文件：

@GetMapping("/download")
public ResponseEntity<Resource> downloadFile(@PathVariable String fileName) throws FileNotFoundException {
    Resource file = resourceLoader.getResource("classpath:files/" + fileName);

    HttpHeaders headers = new HttpHeaders();
    headers.add(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + fileName + "\"");

    return ResponseEntity.ok()
                         .headers(headers)
                         .contentLength(file.contentLength())
                         .contentType(MediaType.APPLICATION_OCTET_STREAM)
                         .body(file);
}

处理分页结果：

@GetMapping("/users/page")
public ResponseEntity<Page<User>> getUsersByPage(Pageable pageable) {
    Page<User> usersPage = userService.getUsersByPage(pageable);
    
    if (usersPage.hasContent()) {
        return ResponseEntity.ok(usersPage);
    } else {
        return ResponseEntity.notFound().build();
    }
}

处理成功或失败时返回不同状态码的样例：

@RestController
public class MyResourceController {

    @DeleteMapping("/resources/{id}")
    public ResponseEntity<Void> deleteResource(@PathVariable Long id) {
        try {
            resourceService.deleteResource(id); // 假设resourceService是资源删除服务
            return ResponseEntity.noContent().build(); // 成功删除时返回204 No Content
        } catch (ResourceNotFoundException e) {
            return ResponseEntity.notFound().build(); // 资源未找到时返回404 Not Found
        } catch (ResourceDeletionException e) {
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
                                 .body(new ErrorResponse("Error deleting resource", e.getMessage()));
        }
    }

    // 自定义错误响应类
    static class ErrorResponse {
        private String error;
        private String message;

        public ErrorResponse(String error, String message) {
            this.error = error;
            this.message = message;
        }

        // getters and setters...
    }
}