CORS, Content Negotiation and File Uploads in Spring Boot

//WebFeaturesApp.java
package com.javabrahman.rest.web;

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.math.BigDecimal;
import java.nio.charset.StandardCharsets;
import java.util.List;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.ByteArrayResource;
import org.springframework.core.io.Resource;
import org.springframework.http.ContentDisposition;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.http.converter.AbstractHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.servlet.config.annotation.CorsRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

record Book(long id, String title, BigDecimal price) {
}

// A wrapper type, so that the CSV converter can recognise "a list of books"
record BookList(List<Book> books) {
}

// ---- 1. CORS for every /api endpoint ----
@Configuration
class CorsConfig implements WebMvcConfigurer {
  @Override
  public void addCorsMappings(CorsRegistry registry) {
    registry.addMapping("/api/**")
        .allowedOrigins("https://shop.example.com")
        .allowedMethods("GET", "POST", "PUT", "DELETE")
        .allowedHeaders("Content-Type", "Authorization")
        .exposedHeaders("X-Total-Count")
        .maxAge(3600);
  }
}

// ---- 2. A message converter that writes BookList as CSV ----
class BookListCsvConverter extends AbstractHttpMessageConverter<BookList> {
  static final MediaType TEXT_CSV = MediaType.parseMediaType("text/csv");

  BookListCsvConverter() {
    super(StandardCharsets.UTF_8, TEXT_CSV);
  }

  @Override
  protected boolean supports(Class<?> clazz) {
    return BookList.class.equals(clazz);
  }

  @Override
  protected BookList readInternal(Class<? extends BookList> clazz, HttpInputMessage input) {
    throw new HttpMessageNotReadableException("Reading CSV is not supported", input);
  }

  @Override
  protected void writeInternal(BookList list, HttpOutputMessage output) throws IOException {
    Writer writer = new OutputStreamWriter(output.getBody(), StandardCharsets.UTF_8);
    writer.write("id,title,price\n");
    for (Book b : list.books()) {
      writer.write(b.id() + ",\"" + b.title().replace("\"", "\"\"") + "\"," + b.price() + "\n");
    }
    writer.flush();
  }
}

@RestController
@RequestMapping("/api")
class ApiController {

  @GetMapping("/books")
  ResponseEntity<BookList> books() {
    BookList list = new BookList(List.of(
        new Book(1, "Effective Java", new BigDecimal("45.99")),
        new Book(2, "Java Concurrency in Practice", new BigDecimal("49.99"))));
    return ResponseEntity.ok().header("X-Total-Count", "2").body(list);
  }

  // ---- 3. File upload and download ----
  @PostMapping(path = "/files", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
  String upload(@RequestParam("file") MultipartFile file, @RequestParam String description) throws IOException {
    String firstLine = new String(file.getBytes(), StandardCharsets.UTF_8).lines().findFirst().orElse("");
    return "received '" + file.getOriginalFilename() + "' (" + file.getContentType() + ", " + file.getSize()
        + " bytes), description '" + description + "', first line: " + firstLine;
  }

  @GetMapping("/files/{name}")
  ResponseEntity<Resource> download(@PathVariable String name) {
    byte[] content = ("Order report for " + name + "\nOrders: 42\nRevenue: 1890.50\n").getBytes(StandardCharsets.UTF_8);
    return ResponseEntity.ok()
        .contentType(MediaType.TEXT_PLAIN)
        .header(HttpHeaders.CONTENT_DISPOSITION,
            ContentDisposition.attachment().filename(name + ".txt").build().toString())
        .body(new ByteArrayResource(content));
  }
}

@SpringBootApplication
public class WebFeaturesApp {
  @Bean
  BookListCsvConverter bookListCsvConverter() {   // Spring Boot adds HttpMessageConverter beans to Spring MVC
    return new BookListCsvConverter();
  }

  public static void main(String[] args) {
    SpringApplication.run(WebFeaturesApp.class, args);
  }
}

spring.mvc.contentnegotiation.default-content-types=application/json
spring.mvc.contentnegotiation.favor-parameter=true
spring.mvc.contentnegotiation.media-types.csv=text/csv