How do you return an error response from a binding result validation failure?

Question

I am trying to get the validation message from a form object validation failure, put the message into an error response and return it but I havn't been able to figure out how looking at alot of articles and github repos...

ProductController.java

@PostMapping("/products")

    public ProductResponse createProduct(@Validated @RequestBody ProductForm productForm, BindingResult bindingResult) {

//trying to make the code below work but I get an "incompatible type" error.

        if (bindingResult.hasErrors()) {
            List<String> errors = bindingResult.getAllErrors().stream().map(e -> e.getDefaultMessage()).collect(Collectors.toList());
            return new ErrorResponse("404", "Validation failure", errors);
        }

        Product product = productForm.convertToProduct();
        Product createdProduct = productRepository.save(product);
        return new ProductResponse(createdProduct, "Product created");
    }

ErrorResponse.java

package com.assignment.restapi.web.response;


import java.util.ArrayList;
import java.util.List;

public class ErrorResponse {

    private String statusCode;
    private String errorContent;
    private List<String> messages;

    public ErrorResponse(String statusCode, String errorContent, String messages) {
        this.statusCode = statusCode;
        this.errorContent = errorContent;
        this.messages = new ArrayList<>();
        this.messages.add(messages);
    }

    public ErrorResponse(String statusCode, String errorContent, List<String> messages) {
        this.statusCode = statusCode;
        this.errorContent = errorContent;
        this.messages = messages;
    }

    public String getStatusCode() {
        return statusCode;
    }

    public void setStatusCode(String statusCode) {
        this.statusCode = statusCode;
    }

    public String getErrorContent() {
        return errorContent;
    }

    public void setErrorContent(String errorContent) {
        this.errorContent = errorContent;
    }

    public List<String> getMessages() {
        return messages;
    }

    public void setMessages(List<String> messages) {
        this.messages = messages;
    }
}

ProductForm.java

package com.assignment.restapi.web.view;

import com.assignment.restapi.domain.Product;

import javax.validation.constraints.Min;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.Size;

public class ProductForm {

    String productImage;

    @NotBlank(message = "cannot be blank")
    @Size(max = 100, message = "has to be over 100 characters")
    String productTitle;

    @Size(max = 500, message = "has to be over 500 characters")
    String productDescription;

    @Min(value = 1, message = "has to be larger than 1")
    Integer productPrice;

    public ProductForm() {

    }

    public ProductForm(String productImage, String productTitle, String productDescription, Integer productPrice) {
        this.productImage = productImage;
        this.productTitle = productTitle;
        this.productDescription = productDescription;
        this.productPrice = productPrice;
    }

    public String getProductImage() {
        return productImage;
    }

    public void setProductImage(String productImage) {
        this.productImage = productImage;
    }

    public String getProductTitle() {
        return productTitle;
    }

    public void setProductTitle(String productTitle) {
        this.productTitle = productTitle;
    }

    public String getProductDescription() {
        return productDescription;
    }

    public void setProductDescription(String productDescription) {
        this.productDescription = productDescription;
    }

    public Integer getProductPrice() {
        return productPrice;
    }

    public void setProductPrice(Integer productPrice) {
        this.productPrice = productPrice;
    }

    //turns productForm into Product object.
    public Product convertToProduct() {
        //step by step debug mode, new object constructor function in Product.java gets called.
        //setter methods get called and values of the ProductForm object gets passed and becomes the new value of the Product object.
        Product product = new Product();
        product.setProductTitle(this.productTitle);
        product.setProductImage(this.productImage);
        product.setProductDescription(this.productDescription);
        product.setProductPrice(this.productPrice);
        return product;
    }
}

Any idea how I can return a ErrorResponse when there is a validation error on the form object?

Was thinking to create an class that extends both ErrorResponse and a ProductResponse class so that I won't get the error "incompatible types."

Answer 1

The reason why you are getting "incompatible type" is the method is expected to return "ProductResponse" but you are trying to return "ErrorResponse". This can be fixed by using ResponseEntity.

@PostMapping("/products")
public ResponseEntity<Object> createProduct(@Validated @RequestBody ProductForm productForm, BindingResult bindingResult) {
    //trying to make the code below work but I get an "incompatible type" error.

    if (bindingResult.hasErrors()) {
        List<String> errors = bindingResult.getAllErrors().stream().map(e -> e.getDefaultMessage()).collect(Collectors.toList());
        // Here you can change ok to badRequest depending on your use case.
        return ResponseEntity.ok(new ErrorResponse("404", "Validation failure", errors));
        // In case if you want to fail the request, you need to use the below: 
        // return ResponseEntity.badRequest().body(new ErrorResponse("404", "Validation failure", errors));
    }

    Product product = productForm.convertToProduct();
    Product createdProduct = productRepository.save(product);
    return ResponseEntity.ok(new ProductResponse(createdProduct, "Product created"));
}