spring_boot_starter_web 0.5.0

Spring MVC for Dart: @RestController with @GetMapping and @PostMapping on dart:io, JSON bodies, @Valid, ProblemDetail errors, server-sent events, filters, CORS.

spring_boot_starter_web

No backward compatibility before 1.0. Any release may rename or change APIs, properties and behaviour; the breaking changes are listed in each package's CHANGELOG.

A web server for spring_boot_dart applications. You write controller classes whose methods carry annotations such as @GetMapping('/books/{isbn}'); the starter routes each HTTP request to the right method, fills its parameters from the path, query, headers and body, and turns what it returns into the response.

It takes care of the work around a REST API that you would otherwise write by hand:

• JSON in and out, through the ObjectMapper of spring_core_dart;
• validation of request bodies with @Valid();
• errors as standard ProblemDetail JSON bodies;
• server-sent events from a Stream;
• file uploads (multipart/form-data) and downloads with Range support;
• filters, interceptors, CORS and sessions.

Use it for a backend in Dart, or for a local server inside a Flutter app on mobile or desktop (a local API, a companion server). The server is dart:io's HttpServer, behind a factory you can replace. The generator writes all the routing code at build time: there is no reflection.

@RestController()
@RequestMapping('/api/books')
class BookController {
  BookController(this.books);             // injected like any bean
  final BookService books;

  // GET /api/books/978-0 → the book as JSON
  @GetMapping('/{isbn}')
  Future<Book> find(@PathVariable() String isbn) => books.find(isbn);

  // GET /api/books?q=dune&page=2
  @GetMapping()
  Future<List<Book>> search(
    @RequestParam('q') String query, {
    @RequestParam('page') int page = 0,   // missing: the Dart default
  }) => books.search(query, page: page);

  // POST /api/books → 201 Created with a Location header; an invalid body is a 400
  @PostMapping()
  Future<ResponseEntity<Book>> create(@RequestBody() @Valid() NewBook book) async {
    final created = await books.create(book);
    return ResponseEntity.created(Uri.parse('/api/books/${created.isbn}'))
        .body(created);
  }

  // DELETE /api/books/978-0 → 204 No Content
  @DeleteMapping('/{isbn}')
  @ResponseStatus(HttpStatus.noContent)
  void delete(@PathVariable() String isbn) => books.delete(isbn);

  // A browser's EventSource gets every change as it happens.
  @GetMapping('/changes')
  EventStream<Book> changes() => EventStream(
    () => books.changes().map((b) => ServerSentEvent(b, id: b.isbn)),
  );
}

class NewBook {                           // introspected for JSON: the controller uses it
  const NewBook({required this.isbn, required this.title});

  @NotBlank()
  @Size(min: 10, max: 13)
  final String isbn;

  @NotBlank()
  final String title;
}

# lib/resources/application.properties
server.port=8080

An error is answered with a ProblemDetail, for example when books.find throws a ResponseStatusException(HttpStatus.notFound, 'No book 123'):

GET /api/books/123      → 404 application/problem+json
{"type":"about:blank","title":"Not Found","status":404,"detail":"No book 123","instance":"/api/books/123"}

Spring (Java):

@RestController
@RequestMapping("/api/books")
class BookController {
  @GetMapping("/{isbn}")
  Book find(@PathVariable String isbn) { ... }

  @PostMapping
  ResponseEntity<Book> create(@RequestBody @Valid NewBook book) { ... }
}

In the guides: server, 3. Web.

Setup

dependencies:
  spring_boot_starter_web: ^0.5.0

dev_dependencies:
  build_runner: ^2.17.0
  spring_core_dart_generator: ^0.5.0   # required: describes the controllers
  spring_boot_starter_test: ^0.5.0     # MockMvc, springBootTest
  test: ^1.25.0

// bin/main.dart
import 'package:spring_boot_starter_web/spring_boot_starter_web.dart';

import 'main.spring.dart';

@SpringBootApplication()
void main(List<String> args) => runApplication(args);

dart run build_runner build
dart run bin/main.dart
# INFO  Web server started on port 8080 (http)

Nothing else to switch on (there is no @EnableWebMvc): every @RestController in the scanned package is mapped, and the server starts once the context is ready. One import is enough: package:spring_boot_starter_web/spring_boot_starter_web.dart also exports spring_boot_dart, spring_web_dart (the annotations, ResponseEntity, ProblemDetail, the server request and response) and spring_boot_starter_logging. As with every Spring Boot starter, it is the only dependency the application needs.

The classes the controllers take and return are introspected for JSON automatically when they are in the same package.

Stopping with Ctrl+C. For Ctrl+C or SIGTERM to stop the server gracefully, the application must close its context on those signals: set spring.main.register-shutdown-hook=true. It is false by default here, unlike in Spring Boot; see Configuration.

Controllers

A controller is a bean whose methods answer HTTP requests. @RestController() declares one; @RequestMapping('/api/books') on the class is the path prefix of every method, and its produces, consumes and headers apply to all of them. A plain @Controller() is mapped the same way: there are no views, so the return value is always the response body.

Annotation	Request
@GetMapping('/{isbn}')	GET
@PostMapping(), @PutMapping, @PatchMapping, @DeleteMapping	POST, PUT, PATCH, DELETE; without a path, the class's path
@RequestMapping.of(path: '/x', method: HttpMethod.head)	any method
@GetMapping.of(path: '/x', produces: 'text/csv', consumes: ..., headers: ['X-Api=2'])	the same, with conditions: produces must suit the Accept header and consumes the Content-Type (comma-separated media types); headers are Name, !Name, Name=value, Name!=value

Paths are path patterns (PathPattern of spring_web_dart), and ${...} placeholders in them are filled from the configuration:

Pattern	Matches
/books	the path itself; a trailing slash matters (/books/ is another path, as in Spring 6)
/books/{isbn}	one segment, captured as isbn (decoded)
/books/{isbn:\d+}	one segment matching the regular expression
/files/{*path}	the rest of the path, with its leading /
/static/**, /b?ok, /*.json	wildcards: any number of segments, one character, part of a segment

Matrix parameters are left out of the matching, as in Spring: /books/1111;v=1 is /books/1111. The server answers a path with an encoded /, \ or NUL (%2F, %5C, %00) with a 400 before any filter, as Tomcat does, so ..%2F..%2Fsecret never reaches a path variable; so is a . or .. segment that matrix parameters hide (/files/..;/secret), and a % escape in the path or the query that is not UTF-8 (/%FF, /%C0%AF, ?id=%FF). A raw \ (not encoded) never reaches the server: dart:io reads it as / first (see Differences from Spring).

When several mappings match, the most specific pattern wins (/books/new before /books/{isbn}), then the mapping with more conditions. As in Spring, every GET mapping also answers HEAD (the headers and Content-Length, no body), and OPTIONS is answered with an Allow header.

Parameters

Each parameter of a controller method says where its value comes from.

Parameter	Value
@PathVariable() String isbn	{isbn} of the path; @PathVariable('id') for another name
@RequestParam('q') String query	a query parameter, or else a field of a form body (application/x-www-form-urlencoded) or a text field of a multipart form. List<int> ids takes ?id=1&id=2 or ?id=1,2; a Map<String, String> takes all of them; a MultipartFile takes the uploaded file of that name
@RequestHeader('X-Trace') String? trace	a header; an HttpHeaders or Map parameter takes all of them
@CookieValue('session') String session	a cookie
@RequestBody() Book book	the body: JSON through the ObjectMapper (application/json, *+json). Without a Content-Type the body is application/octet-stream, as in Spring: a 415 for an object in a POST, PUT or PATCH, a missing body when there is none. Also a String, Uint8List or List<int> (any Content-Type), a Stream<Uint8List> (unread, for big uploads), or a Map from a form (application/x-www-form-urlencoded)
@RequestBody() @Valid() NewBook book	the body, validated (@Validated([Group]) for groups): a violation is a 400
@RequestPart('file') MultipartFile file	a part of a multipart/form-data upload: a file, a text field or a JSON part; see Files
ServerHttpRequest, ServerHttpResponse	the request and the response
HttpHeaders, HttpMethod, Uri	the request's headers, method and URI
Principal	the user an authentication filter set (request.principal); a non-nullable Principal without one is a 401
a simple value or a MultipartFile without annotation (String q, int page)	the query parameter (or uploaded file) of that name, as if it had @RequestParam

Simple values are String, int, double, num, bool (true, on, yes, 1, ...), BigInt, DateTime (ISO 8601), Duration (10s), Uri, enums (by name in any case, or by @JsonValue), and lists or sets of them. A text that does not convert is a 400 that names the parameter.

Required or optional comes from the Dart parameter, where Spring has required and defaultValue:

Value missing	Result
nullable parameter (String? q)	null
optional named or positional parameter ({int page = 0})	left out of the call: its Dart default applies
otherwise	400 Required parameter 'q' is not present. (also for headers and cookies)

So @RequestParam(name = "sort", required = false) String sort is @RequestParam('sort') String? sort, and @RequestHeader(name = "X-Lang", defaultValue = "en") String lang is @RequestHeader('X-Lang') String lang = 'en'.

@Valid() uses the application's Validator bean. With several Validator beans and none of them @Primary, no validator is chosen and @Valid() bodies are not checked, without a warning.

Results

What a method returns decides the response:

Return type	Response
void, null, Future<void>	the status (200, or @ResponseStatus), no body
an object, List, Map	JSON through the ObjectMapper (application/json, or the mapping's produces); 406 when Accept excludes it
String	text/plain;charset=UTF-8 (or produces)
Uint8List	application/octet-stream (or produces)
Resource (FileSystemResource, ByteArrayResource)	a download: the media type from the file name, Content-Length, Range and Last-Modified; see Files
Stream<Uint8List>, Stream<List<int>>	raw bytes as they come (application/octet-stream, or produces)
StreamingResponseBody	bytes the handler writes itself
ResponseEntity<T>	its status, headers and body (ResponseEntity.ok(x), .created(uri).body(x), .notFound().build()); with an ETag or Last-Modified, conditional GETs and HEADs get a 304 (see below)
ProblemDetail, an ErrorResponse	application/problem+json with its status
EventStream<T>, Stream<T> (other than bytes)	server-sent events (text/event-stream); NDJSON with produces: 'application/x-ndjson'
SseEmitter	server-sent events sent from anywhere

A Future of any of these is awaited. A Future of a stream is not allowed: return the stream itself (the startup check reports it).

@ResponseStatus(HttpStatus.created) sets the status of a normal answer. With a reason, @ResponseStatus(HttpStatus.gone, reason: 'Retired'), the result is ignored and the answer is an error with that detail.

A ResponseEntity with a status of 200 and an ETag or Last-Modified (.eTag(book.version), .lastModified(book.updatedAt)) answers a browser or cache that revalidates, as Spring's HttpEntityMethodProcessor does, for GET and HEAD: If-None-Match with its tag (compared weakly: W/ is ignored; a list, or several headers) is a 304 without a body, and so is, without If-None-Match, an If-Modified-Since at or after Last-Modified (whole seconds); an If-Unmodified-Since before it is a 412. The 304 keeps the other headers (Cache-Control, Vary, the CORS ones). As in Spring, If-None-Match: * is not a match for a GET, other methods' If-Match and If-Unmodified-Since are not checked (the handler checks them), and neither are the bodies written as they come (event streams, StreamingResponseBody).

Checked at startup

Every controller is checked when the context starts, so a mistake shows up at once instead of on the first request. The application stops with the whole list when it finds:

• a {variable} without a @PathVariable parameter, or the other way round;
• a parameter nothing can fill, or a @RequestParam that is not a simple value;
• two @RequestBody parameters;
• two methods mapping the same request (method, pattern and conditions);
• an invalid produces;
• an @ExceptionHandler without an exception;
• a method returning a Future of a stream.

Invalid controllers:
  - BookController.find: the path variable {isbn} of "/api/books/{isbn}" has no @PathVariable parameter
  - Ambiguous mapping: OrderController.list and OrderController.all both map GET /api/orders

Set logging.level.spring.web.mvc=debug to see every mapping (Mapped GET /api/books/{isbn} -> BookController.find).

Errors: ProblemDetail

Errors are answered with a problem (RFC 9457, application/problem+json): a JSON body with the status, a title and a detail, which clients can read the same way for every error. It is Spring 6's ProblemDetail. Two kinds of answers are plain text instead, as in Spring: a refused CORS request (a 403 with Invalid CORS request, as Spring's DefaultCorsProcessor answers), and a WebSocket handshake the WebSocket starter refuses (a 403 for an origin or a HandshakeInterceptor, a 400 for a request that asks for no upgrade or a path variable that does not convert, as Spring's handshake handler). The server's own checks of a handshake (a repeated or malformed Sec-WebSocket-Key: 400; another Sec-WebSocket-Version: 426 with Sec-WebSocket-Version: 13) are problems.

Cause	Status
no mapping for the path	404
the path is mapped, but not for this method	405 with Allow
Content-Type not in consumes, not JSON (or missing) for a @RequestBody object, or not a media type at all	415 with Accept
nothing in produces (or JSON) suits Accept	406
a required value missing, a value that does not convert, an unreadable body, a form with a % escape that is malformed or not UTF-8, unmet headers conditions	400
a request body that sends nothing for server.connection-timeout (60 s)	408
@Valid violations	400 with errors: [{"field": "isbn", "message": "...", "rejectedValue": ""}] (messages from the MessageSource)
a body above server.max-http-request-body-size	413
an upload above spring.servlet.multipart.max-file-size or max-request-size, or with more than max-part-count parts	413 (MaxUploadSizeExceededException)
a malformed multipart request, or @RequestPart on a request that is not multipart	400 (MultipartRequestException)
throw ResponseStatusException(HttpStatus.notFound, 'No book 9')	its status and detail
an ErrorResponse or ErrorResponseException	its status, headers and problem
any other exception	500 Internal Server Error, with no detail; the exception is logged with its stack trace under spring.web.mvc
answered by the server before any filter: a method HttpMethod does not know, a missing client certificate (server.ssl.client-auth=need), a malformed or missing Host (or one that is not the host of an absolute target), an encoded /, \ or NUL in the path, a . or .. segment behind matrix parameters, a % escape that is not UTF-8	501, 403, 400, 400, 400, 400
a handler fails after the response was committed	no error answer can follow: the connection is dropped without finishing the response, as Tomcat does

A 5xx answer that an exception asks for (a ResponseStatusException, the 503 of a refused session) is logged as a warning without a stack trace, at most once every 10 seconds for the same kind; only unexpected exceptions are logged as errors.

To answer an exception your own way, write an @ExceptionHandler method. On a controller it handles the errors of that controller's methods; on a @ControllerAdvice or @RestControllerAdvice bean it handles them for every controller. The method takes the exception, and optionally the ServerHttpRequest, ServerHttpResponse or HttpHeaders, and returns whatever a handler may return:

@RestControllerAdvice()
class ApiErrors {
  // The parameter's type picks the exceptions; the isbn helps the client.
  @ExceptionHandler()
  @ResponseStatus(HttpStatus.notFound)
  ProblemDetail notFound(BookNotFoundException e) =>
      ProblemDetail.forStatusAndDetail(HttpStatus.notFound, 'No book ${e.isbn}')
        ..setProperty('isbn', e.isbn);

  // A ResponseEntity when the answer needs its own headers or status.
  @ExceptionHandler()
  ResponseEntity<ProblemDetail> conflict(StateError e) =>
      ResponseEntity.status(HttpStatus.conflict)
          .body(ProblemDetail.forStatusAndDetail(HttpStatus.conflict, e.message));

  // Listed types match by exact runtime type; nothing to say but the status.
  @ExceptionHandler([TimeoutException])
  @ResponseStatus(HttpStatus.gatewayTimeout)
  void timeout() {}
}

A controller's own handlers come first. Then the advice beans are tried in @Order, as in Spring: the first bean that has a handler for the error answers it, even when a later bean has a more specific one. Within one controller or bean, the handler for the most specific type wins: an exact type listed in @ExceptionHandler([...]) first, then the parameter type that is a subtype of the others. Advice handlers also see the 404, 405, 406 and 415 errors of the lookup and the errors of other handler mappings (WebSocket handshakes). An exception thrown by a filter skips them and gets the default answer.

An advice applies to every controller unless @ControllerAdvice.of (or @RestControllerAdvice.of) narrows it, as Spring's attributes do: to the controllers of some libraries (basePackages, prefixes of library URIs instead of Java packages), of some types (assignableTypes, their subtypes too) or carrying some annotation (annotations). A narrowed advice does not see errors raised before a controller method was chosen (a 404, a 405).

// Only the admin controllers answer validation errors this way.
@RestControllerAdvice.of(basePackages: ['package:shop/admin/'])
@Order(1)
class AdminErrors { ... }

Server-sent events

Server-sent events let the server push a series of messages over one long response, which a browser reads with EventSource: notifications, progress, live data. Return an EventStream<T>, or any Stream<T> that is not a byte stream (see Files). Each item is one event, its data written as JSON (a String as it is). ServerSentEvent<T> items keep their id, event, retry and comment. The stream is listened to when the response starts and cancelled when the client goes away. It is paused while an event is being written, so a client that reads slowly slows the stream down instead of filling the server's memory. A ResponseEntity of an EventStream, a Stream or an SseEmitter writes the same events with its own status and headers.

// Every change, with the isbn as the event id.
@GetMapping('/changes')
EventStream<Book> changes() => EventStream(
  () => books.changes().map((book) => ServerSentEvent(book, id: book.isbn)),
);

// A reconnecting EventSource sends the last id it saw: resume from there.
@GetMapping('/resume')
Stream<Book> resume(@RequestHeader('Last-Event-ID') String? lastId) =>
    books.changesAfter(lastId);

// One JSON object per line instead of events.
@GetMapping.of(path: '/export', produces: 'application/x-ndjson')
Stream<Book> export() => books.all();

Any method may answer with a stream, POST included, as MCP's Streamable HTTP does.

SseEmitter is for events that come from elsewhere, later: a service or an event listener. The controller method creates and returns the emitter, the application keeps it, and other code calls send. Events sent before the response is ready are buffered.

@GetMapping('/notifications')
SseEmitter subscribe() {
  final emitter = SseEmitter(timeout: const Duration(minutes: 30));
  notifications.add(emitter);
  // Forget the emitter once the client is gone or it timed out.
  emitter.onCompletion(() => notifications.remove(emitter));
  return emitter;
}

// Elsewhere, when an order is placed:
for (final emitter in notifications) {
  emitter.send(ServerSentEvent(order, event: 'order', id: order.id));
}

An emitter has send, complete(), completeWithError(e), onCompletion, onTimeout, onError and isComplete. Sending after the client went away throws a StateError. Await send to go at the client's pace; when sends are not awaited and 1000 events wait for a client that reads nothing, send fails with a StateError and the emitter ends. Without its own timeout, an emitter ends after spring.mvc.async.request-timeout, or never when that is not set (in Spring on Tomcat, the container's 30 seconds apply).

Proxies often close connections that stay quiet. spring.mvc.sse.heartbeat-interval=15s writes a comment line at that interval to keep idle streams open (a property of this starter, not of Spring Boot).

dart:io sends the response headers only with the first bytes of the body. So that the client sees the response before the first event, every text/event-stream response starts with an empty comment line (:), which clients ignore. When the server stops, open streams are closed.

Files: uploads and downloads

Uploads: multipart/form-data

A browser form with enctype="multipart/form-data", curl -F or a Feign client sends files as a multipart request. The starter reads it once per request and binds its parts to the handler's parameters:

@PostMapping.of(path: '/api/books', consumes: MediaType.multipartFormData)
Future<Book> create(
  @RequestPart('book') @Valid() NewBook book,          // a JSON part, validated
  @RequestPart('cover') MultipartFile cover,           // a file
  @RequestParam('note') String note, {                 // a text field
  @RequestPart('pages') List<MultipartFile>? pages,     // optional
}) async {
  final created = await books.create(book, note: note);
  // Keep the cover: temporary files are deleted when the request ends.
  await cover.transferTo('/var/shop/covers/${created.isbn}.png');
  return created;
}

Parameter	Value
@RequestPart('file') MultipartFile file	the file of that part (the first one if there are several)
@RequestPart('files') List<MultipartFile> files	every file of that name. When there is none: a 400 if required, null if nullable, an empty list for a non-nullable optional parameter
@RequestPart('note') String note, int, ...	a text field, converted like a request parameter (or the text of a file part)
@RequestPart('data') Uint8List data	the raw bytes of the part (List<int> too)
@RequestPart('book') @Valid() NewBook book	a part read as JSON into the type through the ObjectMapper, validated with @Valid
@RequestPart('cover') MultipartFile? cover	optional: null when there is none
@RequestParam('file') MultipartFile file	the same as @RequestPart, for files
@RequestParam('note') String note	a text field of the form (the query string is looked at first)
MultipartFile file without annotation	the file named like the parameter

A missing required part is a 400 Required part 'cover' is not present., and @RequestPart on a request that is not multipart is a 400.

MultipartFile (from spring_web_dart) has name, originalFilename, contentType, size, isEmpty, headers, bytes (a Future), openRead() (a stream, without loading the file) and transferTo(path), which moves the temporary file to path (or copies it when it cannot be moved). After transferTo, the file can no longer be read or moved again.

Limits and temporary files use Spring Boot's spring.servlet.multipart.* keys:

spring.servlet.multipart.max-file-size=5MB        # one file (or part); 1MB by default, -1: no limit
spring.servlet.multipart.max-request-size=20MB    # the whole request; 10MB by default, -1: no limit
spring.servlet.multipart.max-part-count=100       # files and fields together; 50 by default, -1: no limit
spring.servlet.multipart.file-size-threshold=64KB # smaller files stay in memory; 0B by default: all go to disk
spring.servlet.multipart.location=/var/tmp/shop   # a private directory is made in it; the system's temporary directory by default
spring.servlet.multipart.enabled=false            # don't parse multipart requests

• Above a limit, the upload is a 413 (MaxUploadSizeExceededException) and reading stops there. The resolver reads uploads with max-request-size instead of server.max-http-request-body-size; anything else that reads a multipart body (a @RequestBody() String) keeps the server's limit. max-part-count is this starter's own key: Spring Boot sets the same limit on Tomcat with server.tomcat.max-part-count (50).
• Files are streamed to temporary files as they arrive, so a whole upload is never held in memory. They are deleted when the request ends, whatever the handler did: keep a file with transferTo or by copying openRead(). An upload that fails on the way (a limit, a broken body, a client that goes away) leaves nothing behind either. The files are in a directory of the process that only its user can open (0700), with random names. Without dart:io (on the web), files stay in memory.

To send files from a client, see the Feign starter's Files and multipart.

Downloads: Resource

To serve a file, return a Resource from spring_core_dart (FileSystemResource, ByteArrayResource). The starter streams it and answers the HTTP caching and range headers for you:

// A cover image from disk. The pattern lets only digits into the file
// name: a path variable is decoded, so never put it into a path as it is.
@GetMapping(r'/api/books/{isbn:\d+}/cover')
Resource cover(@PathVariable() String isbn) =>
    FileSystemResource('/var/shop/covers/$isbn.png');

// A generated report the browser saves as report.csv.
@GetMapping('/api/report')
ResponseEntity<Resource> report() => ResponseEntity.status(HttpStatus.ok)
    .contentType('text/csv')
    .header('Content-Disposition', ['${ContentDisposition.attachment(filename: 'report.csv')}'])
    .body<Resource>(ByteArrayResource(utf8.encode(reports.csv())));

• Headers: Content-Type from the file name (MediaTypeFactory; application/octet-stream when unknown) unless the ResponseEntity sets one, Content-Length, and Accept-Ranges: bytes. A ResponseEntity<Resource> keeps its status and headers, such as a Content-Disposition that names the download.
• A file is streamed from disk (FileSystemResource.openRead(start, end)), never read whole. A resource that does not exist is a 404.
• Last-Modified (files): a GET or HEAD with an If-Modified-Since at or after it is a 304 without a body.
• Range: one range (bytes=0-99, bytes=500-, bytes=-100) is a 206 with Content-Range; a range past the end is a 416 (Content-Range: bytes */length). Several ranges or a malformed header get the whole content (200), as RFC 9110 allows. If-Range with a date applies the range only if the file has that Last-Modified.
• HEAD answers the same headers without the body.
• A path variable in a file name: the server refuses an encoded /, \ or NUL in the path and . / .. segments (MockMvc does not), but a variable can still hold characters a file name should not; restrict it with a pattern ({isbn:\d+}) or check it before building the path.

Byte streams and StreamingResponseBody

A handler declared to return Stream<Uint8List> or Stream<List<int>> writes the chunks as raw bytes, as they come (application/octet-stream, or the mapping's produces). Any other Stream<T> is still server-sent events. For a body produced on the fly, return a StreamingResponseBody: the handler writes to a ResponseBodySink (add(bytes), write(text), flush(), addStream(stream)):

// A CSV export written row by row, never held whole in memory.
@GetMapping.of(path: '/api/books.csv', produces: 'text/csv')
StreamingResponseBody export() => StreamingResponseBody((out) async {
  await for (final book in books.all()) {
    out.write('${book.isbn},${book.title}\n');
    await out.flush();
  }
});

@GetMapping('/api/backup')
Stream<Uint8List> backup() => storage.openBackup();   // bytes, not events

Both can be the body of a ResponseEntity, for example to add a Content-Disposition.

Testing uploads and downloads

await app.mockMvc.multipart('/api/books', files: [
  MockMultipartFile('cover', 'dune.png', 'image/png', pngBytes),
  MockMultipartFile('book', null, 'application/json', utf8.encode('{"isbn": "9780441013593", "title": "Dune"}')),
], params: {'note': 'first edition'}).andExpect(status: 200);

final result = await app.mockMvc
    .get('/api/books/9780441013593/cover', headers: {'Range': 'bytes=0-99'})
    .andExpect(status: 206, headers: {'Content-Range': 'bytes 0-99/48213'})
    .andReturn();
expect(result.response.bodyBytes, hasLength(100));

A MockMultipartFile without a file name is a plain part; with application/json it is a JSON part for @RequestPart. More in docs/testing.md.

Sessions: HttpSession

HTTP forgets everything between two requests. A session remembers data for one client across requests, such as a shopping cart or the logged-in user: the data stays on the server, and the client only holds a random id in a cookie (JSESSIONID by default) that it sends back. Browsers need this; APIs called by programs usually send a token instead and never create a session. The security starter keeps the logged-in user in one.

Code that has the request gets the session with request.getSession(). The first call creates the session and sends its cookie; getSession(create: false) only looks and returns null when there is none.

// Each visitor has a cart that lives as long as the session.
@PostMapping('/cart')
void add(ServerHttpRequest request, @RequestParam() String item) {
  final session = request.getSession()!;
  final cart = session.getAttribute('cart') as List<String>? ?? [];
  session.setAttribute('cart', [...cart, item]);
}

A session has id, creationTime, lastAccessedTime, maxInactiveInterval, isNew, getAttribute, setAttribute (null removes), removeAttribute, attributeNames, invalidate() (as a logout does) and isInvalidated. After a login, call await request.changeSessionId(): the session keeps its data under a new id, so an id someone saw before the login stops working. The client never chooses an id: an unknown id from a cookie is ignored.

server.servlet.session.timeout=15m              # without a request; 30m by default, seconds without a unit
server.servlet.session.cookie.name=SHOPSESSION  # JSESSIONID by default
server.servlet.session.cookie.same-site=strict  # lax by default; strict, none, or unset
server.servlet.session.max-sessions=50000       # 10000 by default; -1: no limit

Property	Default
server.servlet.session.timeout	30m
server.servlet.session.cookie.name	JSESSIONID
server.servlet.session.cookie.path	the context path, or /
server.servlet.session.cookie.domain	none
server.servlet.session.cookie.http-only	true
server.servlet.session.cookie.secure	secure when the request came over HTTPS
server.servlet.session.cookie.same-site	lax
server.servlet.session.max-sessions	10000 (-1: no limit)

The number of sessions is limited (max-sessions). At the limit, a new session first removes every expired one; when the store is still full, the new session is refused with a TooManyActiveSessionsException, a 503, and a warning is logged (at most once a minute). A live session is never removed to make room. As in WebFlux's InMemoryWebSessionStore (maxSessions, expired ones removed, then refused). The security starter creates a session only at a login, so anonymous requests cannot fill the store; raise the limit when logged-in users alone fill it.

Sessions live in memory by default (InMemoryHttpSessionRepository): they are lost when the process stops and are not shared between several instances of the application. To keep them in a database or Redis, declare your own HttpSessionRepository bean; the in-memory one then backs off. When a repository answers with Futures, use request.getSessionAsync().

Spring (Java), where the session can also be a handler parameter:

@PostMapping("/cart")
void add(HttpSession session, @RequestParam String item) {
  List<String> cart = (List<String>) session.getAttribute("cart");
  ...
  session.setAttribute("cart", cart);
}

Spring Boot leaves SameSite unset and has no session limit of its own (Tomcat's maxActiveSessions, unlimited by default, refuses every new session above it). Here the cookie is SameSite=Lax, and the number of sessions is limited so clients cannot fill the memory by opening sessions; at the limit a session without a login makes room (with a warning, at most once a minute), as described above.

Filters

A filter runs around every request, before the controller is even looked up: request ids, logging, authentication. Declare a WebFilter bean; filters run in @Order. The security starter's filter chain is one of them. OncePerRequestFilter adds shouldNotFilter to skip some requests:

// Every answer carries an id the logs can be searched by.
@Component()
@Order(Ordered.highestPrecedence)
class RequestIdFilter extends OncePerRequestFilter {
  @override
  bool shouldNotFilter(ServerHttpRequest request) => request.path == '/health';

  @override
  Future<void> doFilterInternal(request, response, chain) {
    response.headers.set('X-Request-Id', newId());
    return chain.doFilter(request, response);
  }
}

A filter can answer itself by not calling the chain, or pass a wrapped request or response on: ServerHttpRequestDecorator and ServerHttpResponseDecorator forward everything and let you override one member (the path, the headers, the principal).

Every request runs in its own Zone, so RequestContextHolder.currentRequest is the request anywhere in its handling, across awaits.

Interceptors and argument resolvers: WebMvcConfigurer

A bean that mixes in WebMvcConfigurer customizes how controllers are called:

• interceptors run before and after the controller method, and know which method it is (unlike a filter);
• CORS mappings set CORS rules for path patterns (see CORS);
• argument resolvers fill parameters of your own types.

@Configuration()
class WebConfig with WebMvcConfigurer {
  @override
  void addInterceptors(InterceptorRegistry registry) => registry
      .addInterceptor(TimingInterceptor())
      .addPathPatterns(['/api/**'])
      .excludePathPatterns(['/api/health']);

  @override
  void addCorsMappings(CorsRegistry registry) => registry
      .addMapping('/api/**')
      .allowedOrigins(['https://shop.example'])
      .allowedMethods(['GET', 'POST']);

  @override
  void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) =>
      resolvers.add(TenantResolver());
}

// Logs how long each controller method took.
final class TimingInterceptor with HandlerInterceptor {
  @override
  bool preHandle(request, response, handler) {   // false: the interceptor answered
    request.attributes['start'] = DateTime.now();
    return true;
  }

  @override
  void afterCompletion(request, response, handler, error) =>
      log.info('${handler.description} took '
          '${DateTime.now().difference(request.attributes['start'] as DateTime)}');
}

// A `Tenant tenant` parameter gets the tenant of the X-Tenant header.
final class TenantResolver implements HandlerMethodArgumentResolver {
  @override
  bool supportsParameter(MethodParameter parameter) => parameter.type.type == Tenant;

  @override
  Object? resolveArgument(parameter, request, response) =>
      Tenant(request.headers['X-Tenant'] ?? 'default');
}

• HandlerInterceptor: preHandle, postHandle (with the result, before it is written), afterCompletion (also after an error; it gets the exception even when an @ExceptionHandler answered it). As in Spring, an interceptor runs only when a WebMvcConfigurer registers it: a HandlerInterceptor bean alone is not picked up.
• HandlerMethodArgumentResolver: supportsParameter is asked once per parameter at startup (parameter.type, parameter.name, parameter.annotation<A>()), resolveArgument once per request (it may return a Future).

CORS

A browser keeps a page of https://shop.example from reading the answers of https://api.example unless the API allows it with Access-Control-Allow-... headers (CORS). For most requests the browser first asks with an OPTIONS "pre-flight" request. The starter answers both from the rules you declare, in one of three ways:

// On a controller (or a method): the shop's pages may call it.
@RestController()
@CrossOrigin(origins: ['https://shop.example'], maxAge: 3600)
class BookController { ... }

// For many paths at once, a bean (its rules alone, where it has some):
@Bean()
CorsConfigurationSource corsConfigurationSource() => UrlBasedCorsConfigurationSource()
  ..registerCorsConfiguration(
    '/api/**',
    CorsConfiguration(
      allowedOriginPatterns: ['https://*.example'],
      allowCredentials: true,
    )..applyPermitDefaultValues(), // GET, HEAD, POST and every header
  );

// Or WebMvcConfigurer.addCorsMappings, shown above.

• @CrossOrigin on a class or method takes origins, originPatterns, methods, allowedHeaders, exposedHeaders, allowCredentials, allowPrivateNetwork and maxAge (in seconds). What it leaves out gets these defaults, as in Spring: every origin, the method the handler is mapped to (DELETE for a @DeleteMapping; GET alone for a @GetMapping, so a cross-origin HEAD needs methods: ['GET', 'HEAD']), every request header, and a pre-flight cache of 30 minutes.
• WebMvcConfigurer.addCorsMappings gets the defaults of applyPermitDefaultValues() for what a mapping leaves out: every origin, GET, HEAD and POST, every header, 30 minutes. Its list methods add to what is there; Spring's replace it.
• A CorsConfigurationSource bean named corsConfigurationSource is the rules the security starter's CORS filter applies, as Spring Security applies the bean of that name: where it has a configuration for a request, that configuration alone decides, neither @CrossOrigin nor the WebMvcConfigurer mappings widen it (Spring MVC leaves a request the filter answered as it is). The router applies it the same way, so CORS works the same with and without the security starter. Where it has none (null), @CrossOrigin and the mappings apply. The name counts, not the type: a @Bean method corsConfigurationSource() gives it; a bean of the type with another name (apiCors, or a library's) is ignored, as by Spring Security, and several such beans never fail the start. (Spring MVC alone does not read such a bean.)
• A CorsConfiguration you build is used as it is: empty lists allow no origin and no request header, and empty allowedMethods allow only GET and HEAD. Call applyPermitDefaultValues() for the defaults above.

Every cross-origin request is checked, as Spring's DefaultCorsProcessor checks it: its origin, and its method, the one a pre-flight announces or an actual request's own. So an allowed origin's DELETE, PUT, HEAD or OPTIONS that the rules do not list is refused with a 403 before the handler runs (with the security starter, before the security checks), even if the browser sent no pre-flight. allowPrivateNetwork (Chrome's Private Network Access, for a server on localhost or an office network called from a public site) answers a pre-flight with Access-Control-Request-Private-Network: true with Access-Control-Allow-Private-Network: true; like allowCredentials, it cannot go with all origins (*): @CrossOrigin stops the startup, a mapping fails every request to its paths (500) and a bean every cross-origin one, as in Spring.

Behind a proxy that terminates TLS (or that changes the host or the port), the server sees http and its own address, and the starter does not read Forwarded or X-Forwarded-* headers yet. Browsers send an Origin with every POST, PUT, PATCH and DELETE, to their own site too, so there the application's own pages count as another origin, and their requests are checked as cross-origin ones: the origin, and the method. Under addCorsMappings rules for /** with the defaults (GET, HEAD, POST), a page's own PUT or DELETE is a 403; with origins that leave out the site's own, its POST too (with or without the security starter). Spring does the same without server.forward-headers-strategy. Give CORS rules only to the paths other sites call (/api/public/** rather than /**), or list the site's own origin and every method its pages send (allowedOrigins(['https://shop.example', ...]), allowedMethods(['GET', 'HEAD', 'POST', 'PUT', 'DELETE'])), or terminate TLS in the application (server.ssl.*). The security starter's DPoP proofs have the same limit (see its README).

Origins are checked as Spring's CorsConfiguration checks them, by AllowedOrigins (from spring_web_dart; the WebSocket starter checks handshakes with it too):

• origins / allowedOrigins are whole origins, compared without case (https://shop.example); * allows any, but not with allowCredentials.
• originPatterns / allowedOriginPatterns are compared with case: * matches any text, as Spring's .* (https://*.shop.example takes https://eu.shop.example, not https://shop.example; *.shop.example takes any scheme and *localhost* any localhost origin), and a port list at the end takes the ports listed (http://localhost:[3000,3001]) or, with [*], any port or none (http://localhost:[*] takes http://localhost too). The origin is echoed back, so patterns work with allowCredentials.
• A value may list several, separated by commas ('https://a.example, https://b.example', handy when it comes from one property), and a trailing / is ignored, in the values and in the Origin header. The allowed origin is echoed as the browser sent it.

As in Spring, https://*.example.com also takes https://evil.com/.example.com: no browser sends such an Origin, and the CORS answer binds browsers only (any other client sends what it likes). Two differences from Spring:

• An Origin longer than 1024 characters is refused (AllowedOrigins.maxOriginLength; a real origin is under 300: a host has at most 253). A pattern is matched in time that grows with the origin's length, however many * it has, where Spring's regular expressions take the square or the cube of it with two or three.
• An Origin with a character other than visible US-ASCII (a space, a control character, a letter beyond ASCII) is refused with a 403, as Spring refuses one it cannot parse, and so is a pre-flight that asks for such a method or header, even where * allows any: no browser sends one, and dart:io could not echo it (Spring echoes it).

The rules are combined as in Spring: the global ones (the WebMvcConfigurer mappings) first, then the class, then the method. Values one side sets are merged with the other side's (a * makes the result *), but the defaults of @CrossOrigin or applyPermitDefaultValues() never widen what the other side sets, as Spring's default markers: @CrossOrigin(methods: ['PUT']) on a handler under /api/** keeps the global origins, and adds PUT to the global methods. @CrossOrigin(allowCredentials: true) without origins or originPatterns stops the startup, as in Spring (all origins cannot be allowed with credentials), and so does allowPrivateNetwork: true.

Pre-flight requests (OPTIONS with Origin and Access-Control-Request-Method) are answered for mapped paths, never passed to a handler; one for a method no handler of the path maps is decided by the global rules alone (a 403 when they refuse it, never a 405), as in Spring. A pre-flight to a mapped path without CORS rules gets a 200 with no CORS header and no Vary, as in Spring: the browser then blocks the request it announced. On a path with CORS rules, a cross-origin request from an origin they do not allow is a 403. On a path without rules, the request is handled without CORS headers, and the browser keeps the answer from the page.

A pre-flight cannot say which Content-Type or Accept the request it announces will send, so it is answered from the rules of the first handler of its path and method (the most specific pattern, then the most conditions). This differs from Spring whenever more than one handler matches the pre-flight's path and announced method and all of them have @CrossOrigin rules, whatever their specificity: the handlers mapped to exactly that path when there are some (GET /items producing JSON for one origin, CSV for another), else every pattern that matches (/items/{id} and /items/** for /items/7). Spring then allows the pre-flight from any origin, with credentials, and leaves the check to the actual request. Here an origin the first handler's rules do not allow gets a 403 for the pre-flight (safer: no origin gets more than one handler's rules); give such handlers the same CORS rules. The actual request is checked against the rules of the handler that answers it, chosen with all its conditions, with and without the security starter.

Every answer of a handler with CORS rules gets Vary: Origin, Access-Control-Request-Method, Access-Control-Request-Headers, a CORS request or not (no Origin, the server's own origin, an allowed or a refused one), as Spring's DefaultCorsProcessor adds it: a cache in between then keeps the answers to different origins apart. Each name is added once, next to the names already in Vary (a filter's, in any case). A handler's own Vary (of a ResponseEntity or an ErrorResponse) adds its names to these, as Spring's HttpEntityMethodProcessor merges them, rather than replacing them; on a handler without CORS rules it is written as it is. With the security starter (its CORS filter is on by default), the filter adds them to every request with CORS rules, before the security checks, so its own 401 and 403 answers have them too. Answers of handlers without rules get none.

The server

The webServerFactory bean is an IoWebServerFactory (dart:io's HttpServer, from package:spring_boot_starter_web/io.dart), configured from the server.* properties:

server.port=8080                         # 0: a free port
server.address=127.0.0.1                 # all interfaces by default (127.0.0.1 in Flutter apps)
server.shutdown=graceful                 # or immediate
spring.lifecycle.timeout-per-shutdown-phase=20s
server.servlet.context-path=/shop        # a prefix of every mapping
server.max-http-request-body-size=10MB   # 413 above (10MB by default), whatever the type; -1: no limit
server.connection-timeout=60s            # a body that sends nothing this long: 408; idle connections close; 0 or -1: no limit
server.compression.enabled=true          # gzip, for clients that accept it
server.compression.mime-types=text/html,application/json   # Spring Boot's list by default
server.compression.min-response-size=2KB # the default
server.websocket.max-text-message-size=64KB    # 8KB by default; -1: no limit
server.websocket.send-buffer-size-limit=1MB    # 512KB by default: more waiting drops the connection
server.websocket.send-time-limit=20s           # 10s by default
server.ssl.certificate=/etc/shop/cert.pem
server.ssl.certificate-private-key=/etc/shop/key.pem

Every property, with YAML next to it, is in the configuration reference.

Behind a reverse proxy

A proxy that terminates TLS (nginx, Traefik, a cloud load balancer) passes plain HTTP: without help the server takes every request for http://, so cookies miss Secure, redirect URIs are http:// and the client's address is the proxy's. As in Spring Boot, set:

server.forward-headers-strategy=native   # or framework; none by default
# Whose headers are believed: loopback and private networks by default.
server.forward-headers.trusted-proxies=10.0.0.0/8,192.168.1.10

ForwardedHeaderFilter then applies Forwarded (RFC 7239), or X-Forwarded-Proto, -Host, -Port and -For, before every other filter, and only when the request comes from a trusted proxy: a client that reaches the server directly cannot claim to be on HTTPS or to come from another address. The client's address is the first one from the right of X-Forwarded-For that is not a trusted proxy. The proxy must set these headers itself (replacing what a client sent). Spring's ForwardedHeaderFilter believes every peer; here, as Tomcat's RemoteIpValve, only trusted ones. X-Forwarded-Prefix is not read.

With several instances behind the proxy, route each client to the same instance (a sticky cookie): sessions live in the memory of one process. The cookies kept before a login are signed: give every instance the same spring.security.cookie.secret.

• Start and stop. The webServerStartStopLifecycle bean starts the server when the context is ready and stops it when the context closes. It is a SmartLifecycle in the same phase as Spring Boot's WebServerStartStopLifecycle (SmartLifecycle.defaultPhase - 2048). dart:io binds the socket asynchronously; its started future completes once the server listens. Then the port is published as the local.server.port property and in a WebServerInitializedEvent (@EventListener() void on(WebServerInitializedEvent e)).

• Graceful shutdown (the default): the server takes no new connections, running requests get up to spring.lifecycle.timeout-per-shutdown-phase (30 s by default), open event streams and WebSocket sessions are closed at once (1001), then whatever is left. The same property is the timeout of the context's lifecycle processor (the lifecycleProcessor bean of spring_boot_dart, as in Spring Boot), so the beans are destroyed only after the requests finished. server.shutdown=immediate closes everything right away.

• A port in use fails the start, as in Spring Boot: the application waits for the server to listen before the ApplicationStartedEvent, the runners and the ApplicationReadyEvent, so none of them happen; the ApplicationFailedEvent is published, the context closes, runAsync throws the PortInUseException (from run, it is thrown uncaught) and the program ends with a non-zero exit code. The report:

  Web server failed to start. Port 8080 was already in use.
  
  Action: Identify and stop the process that is listening on port 8080, or
  configure this application to listen on another port (server.port; 0 picks a free one).
  

• No server: spring.main.web-application-type=none keeps the beans (and MockMvc) but starts no server, for example for a command-line task of the same application.

• TLS with PEM files (server.ssl.*); server.ssl.client-auth=need (or want) asks for client certificates. HTTP/1.1 only: dart:io has no HTTP/2.

• Compression (server.compression.enabled=true) gzips the responses of server.compression.mime-types (Spring Boot's list: HTML, XML, plain text, CSS, JavaScript, JSON) of at least min-response-size (2 KB), or of a length not known yet, for clients that send Accept-Encoding: gzip. Event streams are compressed only when text/event-stream is listed (each flush sends what is compressed so far). As with Tomcat, an answer with a strong ETag is never compressed (the tag promises those very bytes; a weak W/"..." one is), and every answer that would be compressed for a client that takes gzip gets Vary: Accept-Encoding, also when this client does not take it, so a cache keeps the two apart (once, and a * among the Vary names leaves Vary: * alone).

• Bodies the handler does not read (or stops reading, at a limit), such as an upload answered with a 413 or a 415: the answer goes out first, then up to 2 MB (Tomcat's max-swallow-size) are read and dropped, for at most 5 seconds, so the client sees the answer and the connection can serve the next request. A longer or slower body closes the connection.

• Stalled bodies and idle connections: a body that sends nothing for server.connection-timeout (60 s, Tomcat's connectionTimeout) while it is read ends with a 408 problem, and the connection closes; the same timeout closes idle keep-alive connections. 0 or a negative value (-1) means no limit, as in Tomcat. There is no limit on the number of connections (Tomcat's max-connections, 8192): put a proxy in front, or rely on the system's limit of open files.

• An error after the response was committed (the status and headers, maybe part of the body, are out): no error answer can follow, so the connection is dropped without finishing the response (a chunked body gets no final chunk), as Tomcat does; the client sees a broken response, not a short one that looks complete.

• Answers without a body or a type: an answer whose media type the application did not set has no Content-Type (dart:io's default text/plain; charset=utf-8 is not sent), as with Tomcat. A 204, a 304 and the 101 of a WebSocket handshake have no Content-Length (RFC 9110 forbids it on a 1xx or 204) and no Transfer-Encoding, a 304 no Content-Type either, so a cache that updates its stored answer with a 304's headers keeps the right type; what a handler writes to them is dropped, as Tomcat drops it. A HEAD keeps the Content-Type and Content-Length of the GET.

• Host: an HTTP/1.1 request without one, a malformed one, or an absolute target (GET http://a.example/ HTTP/1.1) whose host is not the Host header's is a 400, as Tomcat answers. dart:io keeps the last of two Host headers, so such a request is served for the last one (Tomcat answers 400).

• % escapes: one that does not decode to UTF-8 (/%FF, ?id=%FF) is a 400. A % without two hex digits after it is read as a plain % by dart:io (/a%G1 is the path /a%25G1), where Tomcat answers 400; it can never decode to a separator.

• Content-Length and Transfer-Encoding together: dart:io accepts such a request and reads the body as chunked, where RFC 9112 asks a server to refuse it (Tomcat answers 400). Behind a proxy that reads it the other way, requests can be smuggled: use a proxy that refuses these requests (most do), or none.

To change the settings in code, declare a WebServerFactoryCustomizer bean. To replace the server entirely, declare your own WebServerFactory bean; the default one then backs off (@ConditionalOnMissingBean):

@Component()
class PortCustomizer implements WebServerFactoryCustomizer {
  @override
  void customize(ConfigurableWebServerFactory factory) =>
      factory.settings = factory.settings.copyWith(port: 9090);
}

@Configuration()
class ServerConfig {
  @Bean()
  WebServerFactory webServerFactory() => MyServerFactory();   // getWebServer(HttpHandler)
}

Everything above the server (routing, binding, filters, errors) works on ServerHttpRequest and ServerHttpResponse from spring_web_dart and never imports dart:io. Another server therefore needs only a WebServer and its factory, and MockMvc is one more caller of the same HttpHandler.

Handler mappings

The dispatcherHandler bean plays the role of Spring's DispatcherServlet: it runs the filters, then asks every HandlerMapping bean in order for a handler. The controllers come at order 0, the WebSocket endpoints of spring_boot_starter_websocket at order 1. A starter or the application adds its own by declaring a HandlerMapping bean:

// A health check answered without a controller.
@Component()
@Order(10)
class HealthMapping implements HandlerMapping {
  @override
  HttpHandler? getHandler(ServerHttpRequest request) => request.path == '/health'
      ? FunctionHttpHandler((request, response) => response.send(200, 'UP'))
      : null;
}

Flutter

The starter works in a Flutter app on Android, iOS, macOS, Windows and Linux. It runs in the main isolate with the same beans as the widgets. HttpServer's I/O is asynchronous and does not block the UI, but heavy synchronous work in a handler does: move it into Isolate.run.

• 127.0.0.1 by default: an app opens no port to the network unless server.address says so.
• In the background the context pauses: the server stops and frees the port, and starts again when the app comes back (spring.flutter.lifecycle.pause-in-background).
• Platform permissions: macOS needs the com.apple.security.network.server entitlement; Android needs INTERNET in the release manifest; on iOS, binding beyond localhost asks for local network access (NSLocalNetworkUsageDescription).
• The web has no server sockets: the application fails at startup with an error that says so. Use the clients (Feign, WebSocket) there, or set spring.main.web-application-type=none.

WebSocket endpoints

With spring_boot_starter_websocket in the dependencies too, @WebSocketEndpoint beans and WebSocketConfigurer handlers are served on the same port, and their handshakes pass through the same filters:

@WebSocketEndpoint('/ws/rooms/{room}')
class ChatEndpoint {
  // Each message is answered with an acknowledgement, sent back as JSON.
  @OnMessage()
  Ack message(@PathVariable() String room, ChatMessage message) => Ack(room);
}

The WebSocket starter's Server endpoints section describes them. The server reads at most 8 KB per message (Tomcat's default in Spring): a larger text or binary message is not read, and the session is closed with 1009 (message too big). Raise the limits with server.websocket.max-text-message-size and server.websocket.max-binary-message-size (this starter's keys; Spring sets them on the container). The text limit counts UTF-8 bytes, where Tomcat counts characters. No extension is negotiated, so messages are never compressed (permessage-deflate).

What a session sends waits in memory until the client takes it. As Spring's ConcurrentWebSocketSessionDecorator does, a session gives up when more than server.websocket.send-buffer-size-limit (512 KB) waits, or when what was sent has not left within server.websocket.send-time-limit (10 s): the connection is dropped (a close frame could not get through either), and the handler's afterConnectionClosed gets 4500 (CloseStatus.sessionNotReliable) with the reason. The pongs that answer pings wait with the messages, so a client that sends pings and reads nothing reaches the limit too. The session goes on reading the client while messages wait, so a client that sends a lot while the server does too is not held up. A burst larger than the limit that the network cannot take at once drops the session too, as in Spring: raise the limit for such bursts, or await flush() on the session (IoWebSocketSession, the delegate of a DefaultServerWebSocketSession) after each message to send at the client's pace. A client that goes away while messages wait (a reset) ends the session with 1006 (CloseStatus.noCloseFrame) and the error as the reason, also over TLS, where the input often ends before the write fails (the session waits for the write, a second at most); after close(status) from the server, the handler gets that status.

Testing

spring_boot_starter_test calls the controllers in memory with MockMvc: no socket, but the same dispatcher, filters and error handling. It can also start a real server on a free port.

import 'package:spring_boot_starter_test/spring_boot_starter_test.dart';
import 'package:test/test.dart';

import '../bin/main.spring.dart';

void main() {
  final app = springBootTest(springApplication);   // WebEnvironment.mock: no server

  test('finds a book', () async {
    await app.mockMvc.get('/api/books/1111').andExpect(
      status: 200,
      contentType: MediaType.applicationJson,
      json: {'isbn': '1111', 'title': 'Dune'},
    );
  });

  test('rejects an invalid book', () async {
    await app.mockMvc
        .post('/api/books', json: {'isbn': '', 'title': ''})
        .andExpect(status: 400, contentType: MediaType.applicationProblemJson);
  });

  test('streams changes', () async {
    final result = await app.mockMvc.get('/api/books/changes').andReturn();
    // Skip the leading ':' comment line: wait for the first real event.
    final next = result.response.eventStream.firstWhere((e) => e.data != null);
    await app.mockMvc.post('/api/books', json: {'isbn': '5555', 'title': 'Vurt'});
    expect((await next).id, '5555');
    await result.cancel();
  });
}

springBootTest(springApplication, webEnvironment: WebEnvironment.randomPort) starts the real server on a free port (app.localServerPort), for a Feign client or dart:io's HttpClient to call. In a test class: @SpringBootTest(webEnvironment: ...), @Autowired() late MockMvc mvc;, @LocalServerPort() late int port;. Web tests are described in full in docs/testing.md.

ApplicationContextRunner starts no server unless the test sets spring.test.web-environment=random-port.

How it works

@GetMapping, @PostMapping, ..., @ExceptionHandler are marked @BeanMethodAnnotation() (from spring_core_dart). For a bean with such methods, the generator writes a description of each one into the bean definition (BeanDefinition.annotatedMethods: the parameters with their names, types and annotations, the result type, and an invoker). It also introspects the parameter and result types of the bean's package for JSON. At startup, RequestMappingHandlerMapping reads these descriptions, checks them and builds the routes. Handlers are called on the bean as the context holds it, so AOP advice on a controller (@Cacheable, method validation, the security starter's @PreAuthorize) applies.

Differences from Spring

• Errors are ProblemDetail (application/problem+json), except the plain-text answers Spring gives too (a refused CORS request or WebSocket handshake). Spring Boot's default is the /error JSON (timestamp, status, error, path) unless spring.mvc.problemdetails.enabled=true; here there is no /error endpoint and no ErrorController.
• @ControllerAdvice.of(basePackages: ...) takes library URI prefixes (package:shop/admin/) instead of Java packages, and annotations match a controller's annotation by its exact type; there is no basePackageClasses.
• @ExceptionHandler([types]) matches exact runtime types, because Dart cannot test whether a Type is a subtype of another. Subtypes match through the type of the exception parameter (@ExceptionHandler() void on(StateError e)).
• @ResponseStatus goes on handler methods or the controller class, not on exception classes, whose annotations cannot be read at runtime: throw a ResponseStatusException or implement ErrorResponse instead.
• A pre-flight that several handlers match (with @CrossOrigin each; no handler of exactly that path, or several) gets the first one's CORS rules; Spring allows it from any origin with credentials, whatever the handlers' specificity (see CORS).
• CORS values outside visible US-ASCII are refused with a 403: an Origin, or a pre-flight's requested method or headers, with a space, a control character or a letter beyond ASCII, even where * allows any. Spring echoes such a method or header (no browser sends one), and refuses such an Origin too.
• An Origin longer than 1024 characters is never allowed (CORS and WebSocket handshakes: a 403), whatever the rules, * included; a real origin is under 300 characters. Spring checks any length, and its patterns' regular expressions slow down with it.
• Custom argument resolvers are asked before the built-in ones (Spring asks them after), so a starter can take over any parameter, as the security starter's @AuthenticationPrincipal does.
• No @ModelAttribute and no binding of forms to objects. Form fields reach a handler through @RequestParam (one field, a list, or all of them as a Map) or a @RequestBody() Map<String, String>.
• No HttpSession parameter: a handler gets the session with request.getSession(). The session cookie is SameSite=Lax and the number of sessions is limited (server.servlet.session.max-sessions; at the limit, expired ones are removed, then a new session is refused with 503).
• Multipart without a servlet container: the spring.servlet.multipart.* keys keep Boot's names, but the temporary files are this starter's own (file-size-threshold is 0B: every file goes to disk). Only multipart/form-data parts are bound, not multipart/mixed, for @RequestPart.
• A declared Stream<Uint8List> or Stream<List<int>> result is raw bytes. Spring MVC has no byte stream result; Spring WebFlux writes a Flux<DataBuffer> the same way. Other streams are server-sent events.
• Resources: one byte range per request. Several ranges get the whole content, not multipart/byteranges, and resources have no ETag.
• Event streams start with a : comment line, because dart:io sends the headers with the first body bytes.
• An SseEmitter without a timeout never times out when spring.mvc.async.request-timeout is not set: there is no servlet container whose default applies (Tomcat's 30 seconds in Spring). It ends when it is completed, when the client goes away, or when the server stops. Give it a timeout, or set the property.
• WebSocket message limits and compression are the server's: server.websocket.max-text-message-size and max-binary-message-size (8 KB each, Tomcat's defaults) are this starter's keys; Spring sets them on the container (ServletServerContainerFactoryBean). The text limit counts UTF-8 bytes; Tomcat counts characters. permessage-deflate is not offered, while Tomcat negotiates it.
• Every WebSocket session has send limits (server.websocket.send-buffer-size-limit, 512 KB, and send-time-limit, 10 s; past them the connection is dropped with 4500), Spring's ConcurrentWebSocketSessionDecorator defaults. In Spring they apply to STOMP sessions and to sessions you decorate; a plain handler on Tomcat blocks the sending thread instead (up to Tomcat's 20 s blocking send timeout). Await IoWebSocketSession.flush() after sendMessage to wait the same way.
• Stalled request bodies get a 408 after server.connection-timeout (60 s); with Tomcat the read fails after its connectionTimeout and Spring answers 400. There is no connection limit (Tomcat's max-connections). Two Host headers and a % without two hex digits are accepted (dart:io), where Tomcat answers 400.
• A raw \ in the request target is a /: dart:io parses the target before the server sees it and keeps no raw form, so /files/..\..\secret arrives as /secret (its .. resolved), and raw bytes past ASCII are read as Latin-1 (/café sent as UTF-8 arrives as /caf%C3%83%C2%A9). Tomcat answers 400 to both. Filters, security rules and controllers all see the same path, but a proxy in front whose rules look at paths must refuse a \ itself.
• HTTP/1.1 only; server.http2.enabled is not read.
• TLS from PEM file paths, not classpath: locations; no keystores or SSL bundles.
• More startup checks than Spring: a path variable without a parameter, a parameter nothing can fill, an invalid @ExceptionHandler.
• server.max-http-request-body-size, spring.mvc.sse.heartbeat-interval, spring.servlet.multipart.max-part-count (Boot's server.tomcat.max-part-count), server.connection-timeout (Boot 2.0's key; Boot 4's is server.tomcat.connection-timeout) and server.websocket.* are this starter's own properties.
• MockMvc takes named expectations (andExpect(status: 200, json: ...), matchers allowed) instead of status().isOk() and jsonPath(...).

Not yet

• @ConditionalOnWebApplication, test slices (@WebMvcTest).
• ResponseBodyEmitter (a Stream<T> result covers SSE and NDJSON).
• Several byte ranges as multipart/byteranges, ETag for resources, @RequestPart with multipart/mixed.
• Static files (lib/resources/static/), forwarded headers (server.forward-headers-strategy), spring.web.cors.*, server.max-http-request-header-size.
• Flutter: a warning when a handler blocks the main isolate, decoding big JSON bodies in another isolate, the whole server in a background isolate (server.isolate: background).
• Other servers (shelf, Rust over FFI), STOMP, the actuator over HTTP. See the roadmap.