问题
I have following code in my @RestController class:
@RequestMapping("api/")
@RestController
public class RecommendationsController {
@PostMapping(path = "cart")
public List<RecommendationDTO> getCartRecommendations(@NonNull @RequestBody List<CartItemDTO> cart){
System.out.println(cart);
return null;
}
}
This is the code in my CartItemDTO class:
public class CartItemDTO {
private String productId;
private Double quantity;
public CartItemDTO(String productId, Double quantity) {
this.productId = productId;
this.quantity = quantity;
}
public String getProductId() {
return productId;
}
public Double getQuantity(){
return quantity;
}
And this is the request that I send with postman:
[
{
"productId": "20000010",
"quantity": 5.0;
},
{
"productId": "20000011",
"quantity": 7.0;
}
]
This is working but when I change my code like following, I get Bad Request: bad syntax
public class CartItemDTO {
private String productId;
public CartItemDTO(String productId) {
this.productId = productId;
}
public String getProductId() {
return productId;
}
}
with request:
[
{
"productId": "20000010"
},
{
"productId": "20000011"
}
]
Someone any idea what could be wrong?
回答1:
The main issue is Jackson unable to construct instance of a DTO.
Two ways to solve this issue:
1. Specify the default constructor:
- when you specify parameterized constructor, then the Java compiler will not add default constructor.
Now your 1st request:
curl --location --request POST 'localhost:8080/cart' \
--header 'Content-Type: application/json' \
--data-raw '[
{
"productId": "20000010",
"quantity": 5.0
},
{
"productId": "20000011",
"quantity": 7.0
}
]'
Throws error:
"timestamp": "2020-04-27T12:08:28.497+0000",
"status": 500,
"error": "Internal Server Error",
"message": "Type definition error: [simple type, class hello.dto.CartItemDTO]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `hello.dto.CartItemDTO` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)\n at [Source: (PushbackInputStream); line: 3, column: 9] (through reference chain: java.util.ArrayList[0])",
"path": "/cart"
on adding default constructor to DTO, everything works fine as expected.
public class CartItemDTO {
private String productId;
private Double quantity;
public CartItemDTO() {
}
public CartItemDTO(String productId, Double quantity) {
this.productId = productId;
this.quantity = quantity;
}
public String getProductId() {
return productId;
}
public Double getQuantity() {
return quantity;
}
}
Since OP does not have RecommendationDTO
object, adding just System.out.println as output:
[hello.dto.CartItemDTO@145e35d6, hello.dto.CartItemDTO@25df553f]
- Only ProductId in DTO
public class CartItemDTO {
private String productId;
public CartItemDTO() {
}
public CartItemDTO(String productId, Double quantity) {
this.productId = productId;
}
public String getProductId() {
return productId;
}
}
Request:
curl --location --request POST 'localhost:8080/cart' \
--header 'Content-Type: application/json' \
--data-raw '[
{
"productId": "20000010"
},
{
"productId": "20000011"
}
]'
output:
[hello.dto.CartItemDTO@42ad1f23, hello.dto.CartItemDTO@777d8eb3]
- Solution Two: Need to instruct Jackson to create an dto object using a constructor with instance fields as below:
change DTO to
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class CartItemDTO {
private String productId;
private Double quantity;
@JsonCreator
public CartItemDTO(@JsonProperty(value = "productId", required = true) String productId,
@JsonProperty(value = "quantity", required = true) Double quantity) {
this.productId = productId;
this.quantity = quantity;
}
public String getProductId() {
return productId;
}
public Double getQuantity() {
return quantity;
}
}
OR with only productId
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class CartItemDTO {
private String productId;
@JsonCreator
public CartItemDTO(@JsonProperty(value = "productId", required = true) String productId) {
this.productId = productId;
}
public String getProductId() {
return productId;
}
}
回答2:
There seems to be an issue with fasterxml jackson, when it tries to auto-guess the JsonCreator when the constructor only has 1 argument.
The best solution that I found, is to add the @JsonCreator annotation to the constructor, so jackson won't have to guess which creator should it use.
Like so:
import com.fasterxml.jackson.annotation.JsonCreator;
public class CartItemDTO {
private String productId;
@JsonCreator
public CartItemDTO( String productId) {
this.productId = productId;
}
public String getProductId() {
return productId;
}
}
Update:
Apparently this is a known limitation in fasterxml jackson: https://github.com/FasterXML/jackson-modules-java8/issues/8
来源:https://stackoverflow.com/questions/61455098/spring-post-400-bad-request-postman-when-changing-class-from-2-to-1-parameter