Kora di runtime

Kora DI Runtime — Runtime Injection Behavior

Kora Version: 1.x
Focus: Runtime DI — lifecycle, tags, collections, lazy dependencies, interception.

Read this first when:

• Marking components as @Root for automatic startup
• Implementing Lifecycle for init/release logic
• Disambiguating with @Tag, collecting with All<T>
• Using ValueOf<T> for lazy dependencies
• Wrapping components with GraphInterceptor

References: See references/ for detailed guides on each topic.

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Quick Start

@Root — Self-Starting Components

Only components that are dependencies of other components, or marked @Root (ru.tinkoff.kora.common.annotation.Root), are instantiated at runtime. @Root goes on a @Component class or on a @KoraApp/@Module factory method.

When to use @Root: HTTP/gRPC servers, Kafka consumers, cache warmers, schedulers, startup tasks — anything that must run even though nothing depends on it.

A self-starting worker:

import ru.tinkoff.kora.application.graph.Lifecycle;
import ru.tinkoff.kora.common.Component;
import ru.tinkoff.kora.common.annotation.Root;

@Root
@Component
public final class ServerRunner implements Lifecycle {
    private final ServerConfig config; // a @ConfigSource interface, injected as a component

    public ServerRunner(ServerConfig config) {
        this.config = config;
    }

    @Override
    public void init() throws Exception {
        System.out.println("Starting server on port " + config.port());
        // Bind socket, start accepting traffic
    }

    @Override
    public void release() throws Exception {
        System.out.println("Stopping server");
        // Stop server, release connections
    }
}

Config is supplied as a separate @ConfigSource component (see kora-config-hocon), never as a constructor-parameter annotation:

@ConfigSource("server")
public interface ServerConfig {
    int port();
}

Warning: Forget @Root and a non-dependency worker won't start — the application exits silently after the Graph is built.

Learn more: references/root-component-reference.md — Complete @Root guide with patterns and troubleshooting.

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Component Lifecycle

Components with initialization/cleanup logic implement Lifecycle (ru.tinkoff.kora.application.graph.Lifecycle):

@Root
@Component
public final class DatabasePool implements Lifecycle {
    private final DataSource dataSource;
    private final ScheduledExecutorService scheduler;
    
    public DatabasePool(DataSource dataSource) {
        this.dataSource = dataSource;
        this.scheduler = Executors.newSingleThreadScheduledExecutor();
    }
    
    @Override
    public void init() throws Exception {
        try (Connection conn = dataSource.getConnection()) {
            System.out.println("Database connection OK");
        }
        scheduler.scheduleAtFixedRate(this::cleanup, 5, 5, TimeUnit.MINUTES);
    }
    
    @Override
    public void release() throws Exception {
        scheduler.shutdown();
        if (!scheduler.awaitTermination(30, TimeUnit.SECONDS)) {
            scheduler.shutdownNow();
        }
        if (dataSource instanceof AutoCloseable) {
            ((AutoCloseable) dataSource).close();
        }
    }
}

Key points:

• init() is called after component creation, before use; release() during graceful shutdown (SIGTERM)
• Release order is the reverse of init order
• Components are initialized as parallel as the dependency Graph allows
• For factory methods, wrap with LifecycleWrapper — see Advanced DI Patterns for exact signature

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Generic Factories

Generic methods in @Module interfaces become "generic factories" usable for ANY matching type. Use deliberately, or move helpers to top-level private static methods. See Advanced DI Patterns.

Learn more: references/lifecycle-reference.md — LifecycleWrapper, graceful shutdown, post-commit actions.

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Tags and Collections

@Tag for Disambiguation

When multiple implementations of one interface exist, use @Tag (ru.tinkoff.kora.common.Tag) to select which one to inject. The tag is a marker class, not a string literal, so navigation and refactoring stay safe:

public final class RedisTag {}
public final class CaffeineTag {}

@Tag(RedisTag.class)
@Component
public final class RedisCache implements Cache {}

@Tag(CaffeineTag.class)
@Component
public final class CaffeineCache implements Cache {}

@Component
public final class UserService {
    private final Cache redisCache;
    private final Cache localCache;
    
    public UserService(
        @Tag(RedisTag.class) Cache redisCache,
        @Tag(CaffeineTag.class) Cache localCache
    ) {
        this.redisCache = redisCache;
        this.localCache = localCache;
    }
}

All<T> for Collections

Inject every untagged implementation of a type. All<T> (ru.tinkoff.kora.application.graph.All) extends List<T>:

import ru.tinkoff.kora.application.graph.All;

@Component
public final class NotificationService {
    private final List<Notifier> notifiers;

    public NotificationService(All<Notifier> notifiers) {
        this.notifiers = List.copyOf(notifiers);
    }

    public void notify(String message) {
        notifiers.forEach(n -> n.send(message));
    }
}

To collect every implementation including tagged ones, request @Tag(Tag.Any.class):

public NotificationService(@Tag(Tag.Any.class) All<Notifier> notifiers) { ... }

@Tag(SomeTag.class) All<T> collects only the components registered under SomeTag.

Learn more:

• references/tag-injection-reference.md — @Tag patterns
• references/collection-injection-reference.md — All<T>, Tag.Any patterns

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ValueOf<T> — Lazy Dependencies

ValueOf<T> (ru.tinkoff.kora.application.graph.ValueOf) provides lazy access via get() and decouples lifecycles: depending on ValueOf<B> tells the Graph that this component is NOT refreshed when B changes. ValueOf also exposes refresh().

import ru.tinkoff.kora.application.graph.ValueOf;

@Component
public final class ApiClient {
    private final ValueOf<AuthConfig> config;

    public ApiClient(ValueOf<AuthConfig> config) {
        this.config = config;
    }

    public Response get(String url) {
        var currentConfig = config.get(); // always the latest instance
        // ApiClient is NOT recreated when AuthConfig refreshes
        return doGet(url, currentConfig);
    }
}

Use cases: breaking refresh cascades, breaking circular dependencies, lazy access to a component that may not be ready at construction.

Optional dependencies use a @Nullable constructor parameter (any @Nullable: jakarta.annotation.Nullable, javax.annotation.Nullable, or org.jetbrains.annotations.Nullable); the Graph then tolerates the absence of that component instead of failing the build.

Learn more: references/optional-dependency-reference.md — @Nullable and ValueOf patterns.

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GraphInterceptor — Component Wrapping

GraphInterceptor<T> (ru.tinkoff.kora.application.graph.GraphInterceptor) wraps or modifies a component during graph construction. Its contract mirrors Lifecycle except init/release return the (possibly replaced) instance that other components will then receive:

@Component
public final class MetricsInterceptor implements GraphInterceptor<HttpClient> {
    private final MeterRegistry meterRegistry;
    
    public MetricsInterceptor(MeterRegistry meterRegistry) {
        this.meterRegistry = meterRegistry;
    }
    
    @Override
    public HttpClient init(HttpClient client) {
        return new MetricsHttpClient(client, meterRegistry);
    }
    
    @Override
    public HttpClient release(HttpClient client) {
        return client;
    }
}

Use cases: Metrics, tracing, circuit breakers, logging proxies.

Learn more: references/graph-interceptor-reference.md — Complete interceptor patterns.

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Common Pitfalls

Problem	Solution
HTTP/GRPC server doesn't start	Mark server component with @Root
Kafka consumer not polling	Mark consumer with @Root
Component silently not created	Check it's used by @Root or marked @Root itself
Component restarted on config change	Use ValueOf<T> for lazy dependency
Ambiguous dependency error	Use @Tag to disambiguate
LifecycleWrapper factory returns wrong type	Return Wrapped<T>, use constructor (see Advanced DI)
Generic method in @Module causes implicit bindings	Move helpers to top-level private static methods (see Advanced DI)

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Checklist

- [ ] Component marked @Root if it must start automatically?
- [ ] Implements Lifecycle for init/release logic?
- [ ] Resources properly released in release()?
- [ ] Using @Tag to disambiguate multiple implementations?
- [ ] Using All<T> for untagged collections, @Tag(Tag.Any.class) List<T> for all components?
- [ ] Using ValueOf<T> for lazy dependencies?
- [ ] Using GraphInterceptor for component wrapping?

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Assets

Templates in assets/: Application, RootComponent, LifecycleComponent, LifecycleFactory, TaggedComponent, CollectionComponent, GraphInterceptor, ValueOfComponent, build configs.