英文:
Spring Boot Controller return generic types (A or B)
问题
您的控制器函数中使用的ResponseMessage<?>中的问号?并不是最好的方式，您可以使用Java的通配符<? extends T>来更好地表示返回值，其中T是ResponseMessage的类型参数。这将更准确地表明您的函数可能返回不同类型的ResponseMessage。这是一个更新后的示例：
public ResponseMessage<? extends Object> findRecent(@AuthenticationPrincipal LoginUser user) {Optional<TodoResponse> todo = todoService.findRecent(user.getId());return todo.isPresent() ? ResponseMessage.ok(todo.get()) : ResponseMessage.noContent();}
在这种情况下，您的findRecent函数可以返回ResponseMessage<TodoResponse>或ResponseMessage<String>，具体取决于todoService.findRecent的返回值。这种方式更具灵活性，同时仍然提供了类型安全。
英文:
I develop simple todo-list application with spring boot. And I wanna send common response dto from all request. The data filed in ReponseMessage is Generic because it can be various type.
public class ResponseMessage<T> {private int statusCode = HttpStatus.OK.value();private String message = "";protected T data; // this is generic. it can be many response dto.public static ResponseMessage<String> ok() {return new ResponseMessage<>("");}public static <T> ResponseMessage<T> ok(T data) {return new ResponseMessage<>(data);}public static ResponseMessage<Object> noContent() {return new ResponseMessage<>(HttpStatus.NO_CONTENT.value(), HttpStatus.NO_CONTENT.getReasonPhrase(), "");}protected ResponseMessage(T data) {this.data = data;}private ResponseMessage(int statusCode, String message, T data) {this.statusCode = statusCode;this.message = message;this.data = data;}protected ResponseMessage() {}public int getStatusCode() {return statusCode;}public String getMessage() {return message;}public T getData() {return data;}}
Now i need to make a feature that find a recend todo.
If todo exists, send ResponseMessage<TodoResponse>.
{"statusCode": 200,"message": "","data": [{"id": 1,"title": "test","status": "TODO"}]}
If todo doesn't exist, send ResponseMessage<Srring>.
{"statusCode": 200,"message": "","data": []}
Now my controller function return like this.
public ResponseMessage<?> findRecent(@AuthenticationPrincipal LoginUser user) {Optional<TodoResponse> todo = todoService.findRecent(user.getId());return todo.isPresent() ? ResponseMessage.ok(todo.get()) : ResponseMessage.noContent();}
I know that the question mark '?' is not good. so what is better way to return A or B in this case? plz help me.
答案1
得分: 1
如果将noContent().data设置为null，您可以安全地分配任何类型：
public static <T> ResponseMessage<T> noContent() {return new ResponseMessage<>(HttpStatus.NO_CONTENT.value(), HttpStatus.NO_CONTENT.getReasonPhrase(), null);}
另外，请注意调用Optional.isPresent()然后再调用get()有点类似反模式。在您的情况下，您可以简化为：
return todoService.findRecent(user.getId()).map(ResponseMessage::ok).orElseGet(ResponseMessage::noContent);
英文:
If you set noContent().data to null, you can safely assign it any type:
public static <T> ResponseMessage<T> noContent() {return new ResponseMessage<>(HttpStatus.NO_CONTENT.value(), HttpStatus.NO_CONTENT.getReasonPhrase(), null);}
Also note that calling Optional.isPresent() and then get() is something of an antipattern. In your case, you can simplify it to this:
return todoService.findRecent(user.getId()).map(ResponseMessage::ok).orElseGet(ResponseMessage::noContent);
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