spring_boot_dart 0.2.0

Spring Boot for Dart and Flutter: @SpringBootApplication, SpringApplication.run, @ConfigurationProperties, auto-configuration and starters, runners, failure reports.

spring_boot_dart

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.

Spring Boot for Dart, on top of spring_core_dart (re-exported: one import is enough). A separate package so it can live in its own repository later; it depends only on spring_core_dart.

dependencies:
  spring_boot_dart: ^0.1.0
dev_dependencies:
  build_runner: ^2.17.0
  spring_core_dart_generator: ^0.1.0

The whole application entry point

// bin/main.dart (or lib/main.dart in Flutter)
import 'package:spring_boot_dart/spring_boot_dart.dart';

import 'main.spring.dart'; // generated next to this file

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

dart run build_runner build scans the whole package (lib/ and the directory of main) — no @ComponentScan, no module variables — and writes main.spring.dart with the beans, the lib/resources/ files, the auto-configuration of the dependencies and runApplication / runApplicationAsync.

runApplication(args) (SpringApplication.run):

1. command-line arguments (--key=value) and OS environment variables;
2. config data: classpath:application.properties / application.yml (= lib/resources/), application-{profile} files, then the external group ./, ./config/, ./config/*/ overriding the packaged files (Boot's location groups), imports, profile groups, ${ENV_VAR:default} and ${random.uuid} / ${random.int[1,10]} placeholders; relaxed names (app.read-timeout also finds app.readTimeout, app.read_timeout, APP_READTIMEOUT) and indexed keys (spring.profiles.include[0], SPRING_PROFILES_INCLUDE_0_; a list comes whole from the highest source). Placeholders in a file's imports and on-profile guards see the command line first; an optional: import with an unresolvable placeholder is skipped;
3. logging.level.root=debug → SpringLog;
4. the context, with ApplicationArguments as a bean and @ConfigurationProperties binding;
5. startup info through SpringLog at INFO, like Spring Boot's (Starting neon-books using Dart 3.9.0 with PID 4242, The following 1 profile is active: "dev", Started neon-books in 0.07 seconds; spring.main.log-startup-info=false turns it off) — or an APPLICATION FAILED TO START report explaining a missing bean, an ambiguity, a cycle, an unresolved placeholder or invalid properties (your own FailureAnalyzers, as beans or on SpringApplication(failureAnalyzers:), are asked first);
6. ApplicationRunner / CommandLineRunner beans, one after another in @Order order, as in Spring Boot: a runner returning a Future is finished before the next starts; a failing runner is reported (Application run failed) and the context is closed.

Application events are published along the way (see Application events).

@Component()
@Order(1)
class Migrations implements ApplicationRunner {
  Migrations(this.db);
  final Database db;

  @override
  Future<void> run(ApplicationArguments args) => db.migrate(); // awaited
}

@Component()
@Order(2)
class Warmup implements ApplicationRunner { ... }       // starts after Migrations

runApplication returns once the synchronous part is done (the runners then go on one after another); await runApplicationAsync(args) completes after the last runner, like Spring Boot's run.

Shutdown and exit codes

With spring.main.register-shutdown-hook=true (the property wins) or SpringApplication(registerShutdownHook: true) the context is closed when the process gets SIGINT or SIGTERM (Boot's shutdown hook, through the context's registerShutdownHook()): @PreDestroy methods and ContextClosedEvent listeners run, then the process ends with 130 / 143. Unlike Boot it is off by default: a Dart program that listens for signals does not end by itself, so a program that is done would never exit. Turn it on in programs that run until stopped (scheduled tasks, servers); the listener goes away when the context closes. It listens through dart:io and does nothing on the web.

SpringApplication.exit(context) closes the context and returns the exit code, like Boot 3+: the generators passed as the second argument and the ExitCodeGenerator beans, sorted by order (Ordered, @Order), give the first non-zero code; a generator that throws gives 1 unless a later one has a code. A code other than 0 is also published as an ExitCodeEvent before the context closes.

@Component()
class FailedJobs implements ExitCodeGenerator {
  int failed = 0;
  @override
  int getExitCode() => failed > 0 ? 2 : 0;
}

Future<void> main(List<String> args) async {
  final context = await runApplicationAsync(args);
  exit(SpringApplication.exit(context)); // dart:io
}

When a runner fails, ExitCodeExceptionMapper beans map the exception to a code (the exception may also implement ExitCodeGenerator itself); the code is published as an ExitCodeEvent and returned by SpringApplication.exitCodeOf(error), for main to pass to exit.

Resources (lib/resources/ = the classpath)

lib/resources/
  application.properties     (or application.yml)
  application-prod.properties
  messages/messages.properties
  templates/welcome.txt

The generator embeds these files into the generated code, so classpath: works the same in dart run, AOT executables, Flutter and the web — no asset declarations in pubspec.yaml. Files above 1 MB are skipped with a warning (ship them as assets).

@Component()
class Mailer {
  Mailer(@Value('classpath:templates/welcome.txt') this.template);
  final Resource template;
}

lib/resources/ rather than a top-level resources/: build_runner only sees lib/, bin/, test/, ... without extra configuration.

@ConfigurationProperties

@ConfigurationProperties('shop')
@Validated()
class ShopProperties {
  @NotBlank()
  String name = 'NEON Books';
  Duration deliveryTime = const Duration(days: 3);
  @Min(1)
  int maxBasketSize = 10;
}

Bound from shop.max-basket-size (or shop.maxBasketSize, --shop.max-basket-size=…, SHOP_MAX_BASKET_SIZE) and converted to the field types; with @Validated() the constraints are checked at startup, as in Spring Boot. The class is a bean and introspected automatically.

Collections and nested objects bind like Boot's Binder:

@ConfigurationProperties('app')
class AppProperties {
  List<String> hosts = [];               // app.hosts=a,b  or  app.hosts[0]=a
  List<Server> servers = [];             // app.servers[0].name=eu-1
  Map<String, String> headers = {};      // app.headers.X-Shop=1, app.headers[x.y]=2
  Map<String, Backend> backends = {};    // app.backends.eu.url=https://...
}

app:
  hosts: [a, b]          # YAML lists are indexed keys: app.hosts[0], ...
  servers:
    - name: eu-1

• A list comes whole from the highest-precedence source that has it (a comma value or indexed keys, APP_HOSTS_0_ as an environment variable): --app.hosts[0]=x replaces a three-element list from application.yml, it does not change its first element. Indexes start at 0 without gaps.
• Map entries are merged across sources, the higher source winning per key; keys keep the form they are written in (X-Shop), [x.y] keeps the dots, and for scalar values the rest of the name is the key (app.headers.a.b=1 → a.b).
• Elements, keys and values are converted through the conversion service (List<Duration>, Map<String, int>, enums by lenient name); a value that cannot be converted fails the start naming the property (app.ports[0]: failed to convert to int (was "x")).

A class whose only constructor has parameters is bound through it (constructor binding, Boot 3's default for immutable classes), also for nested classes:

@ConfigurationProperties('pool')
class PoolProperties {
  const PoolProperties(
    this.url, {                          // required: pool.url must be set
    @DefaultValue('10') required this.maxSize,
    this.timeout = const Duration(seconds: 5),
    this.owner,                          // nullable: null when not set
    this.replicas = const [],            // pool.replicas[0].host=...
  });
  final String url;
  final int maxSize;
  final Duration timeout;
  final String? owner;
  final List<Endpoint> replicas;
}

A parameter without a property takes its @DefaultValue (Boot's), its Dart default value, or null when nullable; a required non-nullable one without any of them fails the start (pool.url: no value bound for the required constructor parameter 'url' of PoolProperties ...). A nullable nested value object none of whose own keys is set stays null. Binder.get(environment).bind('app.hosts', ListType<String>(...)) binds outside of a properties class (BindResult.map / ifBound / orElseGet / orElseThrow, and a BindHandler told about each property).

More of Boot's binding:

• @ConfigurationProperties('pool') on a @Bean method binds the object it returns, also a class of another package (the generator writes its introspection); @Validated() on the method validates it.
• @Name('import') binds a field or constructor parameter from another property name; @DurationUnit(ChronoUnit.seconds) / @DataSizeUnit(DataUnit.megabytes) give plain numbers a unit (DataSize properties read 10MB, 512KB).
• Environment variables bind nested objects under dashed names (APP_MAIN_SERVER_HOST → app.main-server.host).
• @ConfigurationProperties('db', ignoreUnknownFields: false) fails the start for properties under the prefix that bind to nothing (The elements [db.urll] were left unbound.; OS environment variables are not checked); ignoreInvalidFields: true keeps the default of a value that cannot be converted.
• Validation: a Validator bean named configurationPropertiesValidator checks every bound object it supports, @Validated() classes are checked against their constraints, and a properties class that is itself a Validator validates itself.

SpringApplication(environmentPostProcessors: [...]) customizes the environment before the context exists (Boot's EnvironmentPostProcessor): one ordered before EnvironmentPostProcessor.configDataOrder runs before the config files are read (it can add imports, profiles, sources), the others after them. SPRING_APPLICATION_JSON (or --spring.application.json) adds its JSON as properties ({"app": {"hosts": ["a"]}} is app.hosts[0]), below the command line and above the OS environment, like Boot's.

A banner is printed at startup (SpringBootBanner, a classpath:banner.txt or spring.banner.location, or SpringApplication(banner:)); spring.main.banner-mode / bannerMode: is console (default), log or off. With runAsync a banner.txt that can only be read asynchronously (a Flutter asset) is printed too.

A Validator (defaultValidator, a BeanValidator) and a methodValidationPostProcessor (method validation of @Validated beans, with the unique BeanValidator) are auto-configured, like Boot's ValidationAutoConfiguration; your own BeanValidator bean replaces the default one (plain form Validator beans do not).

spring.main.lazy-initialization=true makes all singletons lazy (a LazyInitializationExcludeFilter bean keeps the beans it excludes eager, LazyInitializationExcludeFilter.forBeanTypes([CacheWarmer])); spring.main.allow-bean-definition-overriding=true lets a definition replace another of the same name (off by default).

JSON: the ObjectMapper bean

A jacksonJsonMapper bean (Boot 4's name; the core's ObjectMapper) is configured from spring.jackson.*, like Spring Boot's JacksonAutoConfiguration. It knows every @Introspected class of the application and adds every JsonSerializer / JsonDeserializer bean (Boot's @JsonComponent) and every JsonModule bean (custom serializers). Declare your own ObjectMapper bean to replace it.

@Service()
class Catalog {
  Catalog(this.mapper);
  final ObjectMapper mapper;

  String export(List<Book> books) => mapper.writeValueAsString(books);
  List<Book> import(String json) => mapper.readList<Book>(json);
}

Property	Default	Meaning
spring.jackson.property-naming-strategy	LOWER_CAMEL_CASE	SNAKE_CASE, UPPER_SNAKE_CASE, UPPER_CAMEL_CASE, KEBAB_CASE, LOWER_CASE, LOWER_DOT_CASE
spring.jackson.default-property-inclusion	always	non_null, non_absent, non_empty, non_default
spring.jackson.deserialization.fail-on-unknown-properties	false	reject JSON properties a class does not have
spring.jackson.deserialization.accept-float-as-int	true	1.5 into an int truncates; false rejects it
spring.jackson.deserialization.fail-on-null-for-primitives	true	a JSON null (or missing constructor value) for a non-nullable int, double, num or bool fails, like Jackson 3; false reads 0 / false
spring.jackson.datatype.datetime.write-dates-as-timestamps (Boot 4) or spring.jackson.serialization.write-dates-as-timestamps (Boot 3; the former wins)	false	DateTime as epoch seconds with the fraction (Jackson 3)
spring.jackson.datatype.datetime.write-durations-as-timestamps (or serialization.)	false	Duration as seconds
spring.jackson.datatype.datetime.write-date-timestamps-as-nanoseconds (or serialization.)	true	false: timestamps in milliseconds
spring.jackson.datatype.datetime.read-date-timestamps-as-nanoseconds (or deserialization.)	true	integer timestamps are seconds; false: milliseconds
spring.jackson.mapper.default-view-inclusion	false	properties without @JsonView are written with a view
spring.jackson.serialization.indent-output	false	pretty-print
spring.jackson.mapper.sort-properties-alphabetically	true	properties sorted by JSON name, constructor parameters first (Jackson 3's / Boot 4's order); false: declaration order
spring.jackson.time-zone	each date in its own zone	UTC or a fixed offset such as +02:00 (no named zones: Dart has no tz database)

Any other spring.jackson.* key (Boot binds Jackson's feature flags, date-format, locale, ...) is logged as a warning naming the key and ignored. Time zones are UTC / Z or a fixed offset: Dart has no time zone database, so a named zone such as Europe/Berlin fails the startup with a message saying so.

JsonMapperBuilderCustomizer beans (Boot 4's) change the builder after the properties are applied:

@Bean()
JsonMapperBuilderCustomizer indented() =>
    JsonMapperBuilderCustomizer.of(
      (builder) => builder.enable(SerializationFeature.indentOutput),
    );

objectMapperFromProperties(environment) builds the same mapper by hand.

Auto-configuration and starters

A library ("starter") ships auto-configuration in lib/spring_auto_configuration.dart (Spring Boot's META-INF/spring/...AutoConfiguration.imports):

// lib/spring_auto_configuration.dart of package `neon_cache`
import 'package:spring_boot_dart/spring_boot_dart.dart';
import 'src/cache.spring.dart';

@AutoConfigurationModule()
final BeanModule neonCacheAutoConfiguration = $neonCacheBeans;

// lib/src/cache.dart
@AutoConfiguration()
class CacheAutoConfiguration {
  @Bean()
  @ConditionalOnMissingBean(Cache)
  Cache cache() => InMemoryCache();
}

The generator of every application that depends on the library — directly or transitively — finds these modules and passes them to the generated springApplication; nothing is listed by hand. They come after the application's classes, so @ConditionalOnMissingBean backs off when the application declares its own bean. Like Boot's OnBeanCondition, @ConditionalOnBean, @ConditionalOnMissingBean and @ConditionalOnSingleCandidate are decided when the configuration class is read (Spring's registerBean phase), class by class — a scanned @Component and an @Imported class count as classes too, at their place in that order: by then every earlier class has registered its @Bean methods and run its @Imported BeanRegistrar / ImportBeanDefinitionRegistrar, so a bean registered any of those ways counts (a component declared before the class whose bean it looks for is decided first, as in Spring). On a class, the condition decides the class, its @Bean methods and its imports once: a skipped class runs no registrar and what it selects is not registered; a class whose condition looks for the bean its imported class defines stays registered (Boot's PooledDataSourceConfiguration pattern); its @PropertySources load anyway (they are read while parsing, as in Spring). @ConditionalOnBean / @ConditionalOnMissingBean take a type, .named('...'), or .where(types:, names:, annotations:, search:) (ignored: too for the missing one); without arguments they check the @Bean method's return type, and search: SearchStrategy.current looks at this context only (Boot's SearchStrategy). Also: @ConditionalOnSingleCandidate, @ConditionalOnProperty (with prefix:, several names with .names([...])), @ConditionalOnBooleanProperty, @ConditionalOnExpression (SpEL, placeholders resolved first), @ConditionalOnResource and @ConditionalOnCloudPlatform (CloudPlatform.kubernetes, heroku, cloudFoundry, azureAppService, amazonEcs, ..., detected from environment variables like Boot's, or set with spring.main.cloud-platform). Like Boot 3.4+, beans declared with autowireCandidate: false or defaultCandidate: false count for neither @ConditionalOnBean nor @ConditionalOnMissingBean. With --debug (or debug=true) the condition evaluation report (positive and negative matches, ConditionEvaluationReport) is logged at INFO after the refresh or a failed start.

Auto-configurations are registered in Boot's order (AutoConfigurationSorter): by name, then order (@AutoConfigureOrder), then before / after (a cycle fails the start with AutoConfigure cycle detected between ...). A module is named like its BeanModule (by convention the variable); an @AutoConfiguration class in it can be named too:

@AutoConfigurationModule(
  order: 10,
  after: ['springBootAutoConfiguration'],   // module or @AutoConfiguration class
  before: ['neonMetricsAutoConfiguration'],
)
final BeanModule neonCacheAutoConfiguration = $neonCacheBeans;

Exclude auto-configurations before their conditions run: with @SpringBootApplication(exclude: [CacheAutoConfiguration]) (classes) or excludeName: ['springBootStarterCacheAutoConfiguration'] (the name of a module or of a class), or with the property spring.autoconfigure.exclude (comma-separated or a YAML list), which adds to them. Excluding a class removes only its beans; excluding a module removes all of it. Excluding a type that is no auto-configuration fails the start; an unknown name is ignored. The built-in ones are MessageSourceAutoConfiguration, JacksonAutoConfiguration and ValidationAutoConfiguration:

spring.autoconfigure.exclude=JacksonAutoConfiguration

A configuration class's conditions are evaluated once for the class and all of its @Bean methods (all or nothing), like in Spring.

spring_boot_dart itself is a starter: MessageSourceAutoConfiguration registers a messageSource for classpath:<spring.messages.basename> (default messages) when the context (not a parent) has no bean named messageSource, with spring.messages.encoding, use-code-as-default-message, always-use-message-format, common-messages, fallback-to-system-locale and cache-duration.

Starters of this repository:

Starter	What adding the dependency gives
spring_boot_starter_logging	Console output with patterns, logging.* properties, an in-memory LogBuffer for an in-app log screen.
spring_boot_starter_test (dev)	springBootTest (@SpringBootTest), BeanOverride (@MockitoBean), ApplicationContextRunner, bean matchers, captureLogs.

Test a starter's auto-configuration with ApplicationContextRunner from spring_boot_starter_test.

Writing your own starter

Any package can be a starter. An application that adds it to pubspec.yaml and re-runs build_runner gets its beans with no code at all — exactly how spring_boot_starter_logging is picked up by the example.

1. Declare the module in lib/spring_auto_configuration.dart (the file name is fixed): a public top-level BeanModule variable annotated with @AutoConfigurationModule.

// lib/spring_auto_configuration.dart of package `neon_cache`
import 'package:spring_boot_dart/spring_boot_dart.dart';
import 'src/cache.spring.dart';

@AutoConfigurationModule(requires: '^0.1.0') // compatible spring_boot_dart versions
final BeanModule neonCacheAutoConfiguration = $neonCacheBeans;

2. Describe the beans, with the generator (@AutoConfiguration + @Bean + conditions, scanned by a @ComponentScan in the starter) or as hand-written BeanDefinitions (both starters of this repository do that):

// lib/src/cache.dart
@AutoConfiguration()
class CacheAutoConfiguration {
  @Bean()
  @ConditionalOnMissingBean(Cache)              // the application's own Cache wins
  @ConditionalOnProperty('neon.cache.enabled', matchIfMissing: true)
  Cache cache(@Value(r'${neon.cache.size:100}') int size) => InMemoryCache(size);
}

3. Publish the generated code. The generator writes files only into the package being built (build_to: source), never into dependencies, so run dart run build_runner build in the starter and publish its *.spring.dart with the package.

4. Test it with ApplicationContextRunner (spring_boot_starter_test): the starter alone, with an application bean that makes it back off, with properties that switch it off.

How it is found: when an application with @SpringBootApplication is built, the generator walks every package of its dependency graph (direct and transitive), reads lib/spring_auto_configuration.dart where present and lists the annotated modules in the generated springApplication (by package name; SpringApplication puts them in order, see above):

// generated bin/main.spring.dart of the application
final springApplication = SpringApplication(
  [$application],
  autoConfigurations: [
    neonCacheAutoConfiguration,   // found in neon_cache
    springBootAutoConfiguration,  // spring_boot_dart
  ],
  // only where a starter sets order, before or after
  autoConfigurationMetadata: {neonCacheAutoConfiguration: AutoConfigurationModule(order: 10)},
);

Good to know:

• Only with @SpringBootApplication. Applications that create the context by hand (AnnotationConfigApplicationContext) list the module themselves.
• Rebuild after adding the dependency. Discovery happens at build time, not by scanning a classpath at startup as in Java: until build_runner runs again, the generated file does not know the new starter.
• Incompatible versions stop the build: if requires does not match the application's spring_boot_dart, the generator says which version to use (see below).
• Registration order: application classes first, then auto-configuration in Boot's order (see above); bean conditions are decided class by class in that order, so @ConditionalOnMissingBean sees what the application declared, including the beans of its @Imported registrars. A hand-written auto-configuration bean needs a source (its class, library#Class, as the generator sets it) or a factoryMethodName (a @Bean method) for its bean conditions to be decided in that order too; a definition with neither is decided at once.
• Keep third-party constraints of the starter wide (dio: '>=5.0.0 <7.0.0') so applications can resolve them together with other starters.

Logging levels

SpringApplication applies logging.level.* to the SpringLog facade (Spring Boot's LoggingApplicationListener), with or without the logging starter:

logging.level.root=info
logging.level.neon.checkout=debug          # and every neon.checkout.* logger
logging.group.payments=neon.card,neon.cash # a log group (or a YAML list)
logging.level.payments=trace

LOGGING_LEVEL_NEON_CHECKOUT=debug (an environment variable) works too. A value that is no level (trace, debug, info, warn, error, fatal, off) fails the start, like Boot's binding.

Context initializers and test properties

SpringApplication(..., initializers: [...]) runs ApplicationContextInitializers before the refresh, sorted by Ordered (a class implementing initialize, or ApplicationContextInitializer.of((context) => ...)), and run(args, properties: {...}) adds properties above every other source (what @SpringBootTest(properties = ...) does). spring_boot_starter_test is built on both. SpringApplication(defaultProperties: {...}) / setDefaultProperties adds the defaultProperties source below every other one, config files included (Boot's setDefaultProperties), and additionalProfiles: [...] activates profiles on top of the configured ones (Boot's setAdditionalProfiles).

Application events

Like Spring Boot, run / runAsync publish, in this order: ApplicationStartingEvent, ApplicationEnvironmentPreparedEvent, ApplicationContextInitializedEvent, ApplicationPreparedEvent, (the context's ContextRefreshedEvent), ApplicationStartedEvent, the runners, ApplicationReadyEvent — or ApplicationFailedEvent (with the exception) before the failure report and the rethrow.

The early events happen before any bean exists, so their listeners are registered on the application:

void main(List<String> args) => (springApplication
      ..addListener<ApplicationEnvironmentPreparedEvent>(
        (event) => print(event.environment.activeProfiles),
      ))
    .run(args);

// or SpringApplication([...], listeners: [ApplicationListenerAdapter.of<ApplicationStartingEvent>(...)])

From ApplicationPreparedEvent on these listeners are also listeners of the context. ApplicationStartedEvent, ApplicationReadyEvent and the ApplicationFailedEvent of a failing runner go through the context, so @EventListener methods and ApplicationListener beans receive them:

@Component()
class Warmup {
  @EventListener()
  void onReady(ApplicationReadyEvent event) => cache.fill();
}

Versions and starter compatibility

Java solves library conflicts with a BOM. pub resolves exactly one version of every package per application, so conflicts cannot hide at runtime — pub get fails and names the culprits. What remains is keeping the family consistent and making incompatibilities readable:

• Release train. spring_core_dart, spring_core_dart_generator, spring_boot_dart and the spring_boot_starter_* packages always share one version and depend on each other with ^<version> (dart run tool/version.dart 0.2.0, checked in CI).
• Starters declare the Boot versions they support:

  @AutoConfigurationModule(requires: '^0.4.0')
  final BeanModule neonCacheAutoConfiguration = $neonCacheBeans;
  

  The generator compares the range with springBootVersion and stops the build: Incompatible starter: neon_cache requires spring_boot_dart ^0.4.0, but 0.1.0 is used. Upgrade spring_boot_dart to a version in ^0.4.0, or use a version of neon_cache made for spring_boot_dart 0.1.0.
• Starters keep third-party constraints wide (dio: '>=5.0.0 <7.0.0') so the solver finds a common version; CI resolves the minimum versions (dart pub downgrade) and analyzes them.
• Escape hatch: dependency_overrides in the application's pubspec.yaml, like overriding a managed version in Maven.

Hot reload (Flutter)

A hot reload does not run main again, so the running context would keep the old beans. The generator writes springApplication and the bean module as getters plus a fingerprint of the generated code, and reloadApplication(context) decides what to do:

• only method bodies changed (same fingerprint): the context is kept, beans keep their state and already run the new code;
• beans, annotations or lib/resources/ changed (with build_runner watch running): the context is closed and started again with the same arguments.

Call it from reassemble() of the root widget's State, which Flutter calls on every hot reload:

late ConfigurableApplicationContext context;

@SpringBootApplication()
Future<void> main() async {
  context = await runApplicationAsync(const []);
  runApp(const App());
}

class _AppState extends State<App> {
  @override
  void reassemble() {
    super.reassemble();
    context = reloadApplication(context); // generated next to main
  }

  @override
  Widget build(BuildContext context) => ... // look beans up here, not in initState
}

Objects you keep in fields (a bean fetched once in initState) still point to the old context after a restart; look beans up in build or after reassemble. A hot restart always starts from scratch.

Runners after a restart (runners:)

Runners are beans whose code runs once, right after the application has started — the place for "on startup" work:

@Component()
class StartupTasks implements CommandLineRunner {
  StartupTasks(this.db, this.analytics);
  final Database db;
  final Analytics analytics;

  @override
  void run(List<String> args) {
    db.migrate();                    // update the database schema
    analytics.track('app_started');  // "the app was opened"
    print('Shop is open!');
  }
}

When a hot reload restarts the context (beans, annotations or properties changed), the context starts like a fresh launch, so by default the runners run again:

hot reload after adding a bean
  → old context closed, new one started
  → StartupTasks.run(): migrate() again, a second 'app_started' event,
    "Shop is open!" printed again

The app was not really opened twice, so this is often unwanted. Pass runners: false to restart the beans without repeating the startup work:

@override
void reassemble() {
  super.reassemble();
  context = reloadApplication(context, runners: false);
}

	runners: true (default)	runners: false
Beans after a restart	new	new
CommandLineRunner / ApplicationRunner	run again	skipped
Good for	runners that only prepare in-memory state the new beans need	migrations, analytics events, notifications, network calls

Notes:

• When only method bodies changed, the context is not restarted, so runners never run on such a reload, whatever the flag says.
• runApplication / runApplicationAsync (a real start) always run the runners.
• The default matches Spring Boot DevTools, which runs the runners after every automatic restart.

Flutter

runApplicationAsync(args) (also generated) reads the config files asynchronously; resources from lib/resources/ are embedded, so no asset declarations are needed for them. Other assets plug in as a ProtocolResolver on the context's resource loader (for asset: or package: locations); config imports and @PropertySource files from it are read asynchronously too.

Differences from Spring

Spring Boot	Here	Why
Constructor binding passes null for a missing property without @DefaultValue	A missing non-nullable scalar fails the start; a missing collection is empty and a missing nested object is created with its own defaults (what Boot does with an empty @DefaultValue)	Dart's non-nullable types
@ConstructorBinding picks one of several constructors	Constructor binding only for a class whose only public generative constructor has parameters	Not implemented yet
.properties config files are read as ISO-8859-1	UTF-8 (pass PropertiesPropertySourceLoader(encoding: latin1) to ConfigDataEnvironment for ISO-8859-1)	Dart files are UTF-8
YAML 1.1 (SnakeYAML)	YAML 1.2 (package:yaml): on, off, yes, no stay strings (converting them to bool still works), 0777 is 777	The Dart team's YAML parser
No spring.messages.default-locale	spring.messages.default-locale: the locale tried before the system locale	Apps (Flutter) often have no meaningful system locale
Started app in 0.07 seconds (process running for 0.3)	Without the process uptime; logged only at INFO (visible with spring_boot_starter_logging or logging.level.root=info)	Dart has no process start time; no logging backend by default
The shutdown hook is registered by default and runs when the JVM exits	Off by default (spring.main.register-shutdown-hook=true turns it on); only on SIGINT / SIGTERM (then exit(130) / exit(143)); the listener keeps the process alive until the context closes	Dart has no exit hooks, and a signal listener is an open handle that would keep a finished program running
EnvironmentPostProcessors listed in META-INF/spring.factories	SpringApplication(environmentPostProcessors: [...]); ordered by Ordered around config data (EnvironmentPostProcessor.configDataOrder)	No class path to scan
@DurationUnit(ChronoUnit.X), @DataSizeUnit(DataUnit.X)	The same with Dart enums (ChronoUnit.seconds, DataUnit.megabytes; no months / years)	Dart naming; Duration has no calendar units
BindHandler.onFailure returns a replacement value or rethrows	onFailure(name, error) returns whether the failure counts (false leaves the property unbound)	Errors are collected and reported together
JsonMapperBuilderCustomizer gets Jackson's JsonMapper.Builder	It gets the core JsonMapperBuilder (naming, inclusion, time zone, features, serializers, modules); Jackson's other builder methods are missing	Only what the Dart mapper supports
The condition evaluation report explains each outcome (found bean 'x', @ConditionalOnProperty (a=b) matched)	It names each condition and whether it matched; a configuration class's conditions are listed under its key	Conditions are plain Dart objects
The banner is printed to System.out; banner.txt may use ${application.version}, ${AnsiColor.*}	print (spring.main.banner-mode=log logs it); banner.txt gets ${spring-boot.version}, ${spring-boot.formatted-version} and environment placeholders	Dart has no manifest version or ANSI property source
@AutoConfiguration(before:, after:) and @AutoConfigureOrder on the class	before, after and order of @AutoConfigurationModule (a module is the unit; classes can be named in them)	The generator discovers modules, not classes
spring.main.allow-circular-references=true allows circular references	Not supported: a cycle is always an error (mark an injection point @Lazy() or inject an ObjectProvider)	No early singleton references
@ConditionalOnBean(annotation = Service.class) also finds meta-annotated beans	annotations: [Service] matches that exact annotation class	Annotations are plain objects at runtime

Not yet

AvailabilityChangeEvent, test slices (@WebMvcTest-like), Actuator, Web.