无法构造“类名”的实例（尽管至少在 Creator 上存在）

Question

我有以下类，我将其用作请求有效负载：public class SampleRequest {    private String fromDate;    private String toDate;    // Getters and setters removed for brevity.}我正在尝试将其与以下资源一起使用（只是尝试将其打印到屏幕上以查看发生的情况）：@PostMapping("/getBySignatureOne")public ResponseEntity<?> getRequestInfo(@Valid @RequestBody SampleRequest signatureOneRequest) {    System.out.println(signatureOneRequest.getToDate);    System.out.println(signatureOneRequest.getFromDate);}这是我发送的 JSON 请求：{    "fromDate":"2019-03-09",    "toDate":"2019-03-10"}这是我得到的错误：Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of `com.test.app.payload.SampleRequest` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('fromDate'); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.test.app.payload.SampleRequest` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('fromDate') at [Source: (PushbackInputStream); line: 1, column: 2]]我很想知道这里出了什么问题，我怀疑这是构造函数的问题，或者我在某处遗漏了一些注释，但老实说我不确定我哪里出错了。

Answer 1

您需要一个带有所有参数的构造函数：

public SampleRequest(String fromDate, String toDate) {

    this.fromDate = fromDate;

    this.toDate = toDate;

}

或使用@AllArgsConstructor或@Data来自龙目岛。

Answer 2

您需要编写自定义反序列化器，因为它无法将字符串（fromDate 和 toDate）解析为 Date

{ "fromDate":"2019-03-09", "toDate":"2019-03-10" }

这个链接有一个教程开始使用自定义反序列化器https://www.baeldung.com/jackson-deserialization

反序列化器可以这样写。

public class CustomDateDeserializer extends StdDeserializer<Date> {

private static SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss");

public CustomDateDeserializer() {

    this(null);

}

public CustomDateDeserializer(Class<?> vc) {

    super(vc);

}

@Override

public Date deserialize(JsonParser jsonparser, DeserializationContext context) throws IOException {

    String date = jsonparser.getText();

    try {

        return formatter.parse(date);

    } catch (ParseException e) {

        throw new RuntimeException(e);

    }

}}

您可以像这样在 Class 本身注册反序列化器。

@JsonDeserialize(using = ItemDeserializer.class)

public class Item {  ...}

或者您可以像这样手动注册自定义反序列化器

ObjectMapper mapper = new ObjectMapper();

SimpleModule module = new SimpleModule();

module.addDeserializer(Item.class, new ItemDeserializer());

mapper.registerModule(module);