Spring Boot

https://docs.spring.io/spring-boot/reference/
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### WebClient example in Kotlin

Source: https://docs.spring.io/spring-boot/reference/io/rest-client.html

Kotlin version of the WebClient example, using the same pre-configured WebClient.Builder bean and a similar GET request to retrieve a Mono<Details>.

```kotlin
import org.springframework.stereotype.Service
import org.springframework.web.reactive.function.client.WebClient
import reactor.core.publisher.Mono

@Service
class MyService(webClientBuilder: WebClient.Builder) {

	private val webClient: WebClient

	init {
		webClient = webClientBuilder.baseUrl("https://example.org").build()
	}

	fun someRestCall(name: String): Mono<Details> {
		return webClient.get().uri("/{name}/details", name)
				.retrieve().bodyToMono(Details::class.java)
	}

}
```

--------------------------------

### WebClient example in Java

Source: https://docs.spring.io/spring-boot/reference/io/rest-client.html

Inject the pre-configured WebClient.Builder bean and create a WebClient with a base URL. Use the WebClient to make a GET request and retrieve a Mono<Details>.

```java
import reactor.core.publisher.Mono;

import org.springframework.stereotype.Service;
import org.springframework.web.reactive.function.client.WebClient;

@Service
public class MyService {

	private final WebClient webClient;

	public MyService(WebClient.Builder webClientBuilder) {
		this.webClient = webClientBuilder.baseUrl("https://example.org").build();
	}

	public Mono<Details> someRestCall(String name) {
		return this.webClient.get().uri("/{name}/details", name).retrieve().bodyToMono(Details.class);
	}

}
```

--------------------------------

### Property ordering example 2 (Properties)

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Shows the same property ordering example with the import declared after the property. The result is the same as the first example.

```properties
my.property=value
spring.config.import=my.properties
```

--------------------------------

### Startup failure message for port 8080 in use

Source: https://docs.spring.io/spring-boot/reference/features/spring-application.html

Example output shown when a web application fails to start because port 8080 is already in use. The message includes a description and an action to resolve the issue.

```none
***************************
APPLICATION FAILED TO START
***************************

Description:

Embedded servlet container 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.
```

--------------------------------

### YAML example with only enabled

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

YAML equivalent of the properties example with only my.service.enabled. Demonstrates that when security property is not present, the Security instance is null.

```yaml
my:
  service:
    enabled: true
```

--------------------------------

### Remote client startup output

Source: https://docs.spring.io/spring-boot/reference/using/devtools.html

Example output from running RemoteSpringApplication, showing the Spring Boot banner, version, and startup logs. It indicates the LiveReload server runs on port 35729 and the application started successfully.

```none
  .   ____          _                                              __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _          ___               _      \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` |        | _ \___ _ __  ___| |_ ___ \ \ \ \
 \\/  ___)| |_)| | | | | || (_| []::::::[]   / -_) '  \/ _ \  _/ -_) ) ) ) )
  '  |____| .__|_| |_|_| |_\__, |        |_|_\___|_|_|_\___/\__\___|/ / / /
 =========|_|==============|___/===================================/_/_/_/
 :: Spring Boot Remote ::  (v4.1.1)

2026-08-17T10:51:38.751Z  INFO 11457 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Starting RemoteSpringApplication v4.1.1 using Java 25.0.4 with PID 11457 (/Users/myuser/.m2/repository/org/springframework/boot/spring-boot-devtools/4.1.1/spring-boot-devtools-4.1.1.jar started by myuser in /opt/apps/)
2026-08-17T10:51:38.769Z  INFO 11457 --- [           main] o.s.b.devtools.RemoteSpringApplication   : No active profile set, falling back to 1 default profile: "default"
2026-08-17T10:51:40.221Z  INFO 11457 --- [           main] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729
2026-08-17T10:51:40.337Z  INFO 11457 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Started RemoteSpringApplication in 3.297 seconds (process running for 4.891)
```

--------------------------------

### Environment variable content example

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Example content of an environment variable named MY_CONFIGURATION that can be imported using the 'env:' prefix.

```properties
my.name=Service1
my.cluster=Cluster1
```

--------------------------------

### Default Spring Boot startup output

Source: https://docs.spring.io/spring-boot/reference/features/spring-application.html

Example of the default startup output when a Spring Boot application starts. It shows the Spring Boot banner, version, and INFO log messages including the user, PID, active profile, Tomcat initialization, and graceful shutdown.

```none
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/

 :: Spring Boot ::                (v4.1.1)

2026-08-17T10:51:43.606Z  INFO 11484 --- [           main] o.s.b.d.f.logexample.MyApplication       : Starting MyApplication using Java 25.0.4 with PID 11484 (/opt/apps/myapp.jar started by myuser in /opt/apps/)
2026-08-17T10:51:43.627Z  INFO 11484 --- [           main] o.s.b.d.f.logexample.MyApplication       : No active profile set, falling back to 1 default profile: "default"
2026-08-17T10:51:48.207Z  INFO 11484 --- [           main] o.s.boot.tomcat.TomcatWebServer          : Tomcat initialized with port 8080 (http)
2026-08-17T10:51:48.266Z  INFO 11484 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2026-08-17T10:51:48.275Z  INFO 11484 --- [           main] o.apache.catalina.core.StandardEngine    : Starting Servlet engine: [Apache Tomcat/11.0.24]
2026-08-17T10:51:48.445Z  INFO 11484 --- [           main] b.w.c.s.WebApplicationContextInitializer : Root WebApplicationContext: initialization completed in 4488 ms
2026-08-17T10:51:50.163Z  INFO 11484 --- [           main] o.s.boot.tomcat.TomcatWebServer          : Tomcat started on port 8080 (http) with context path '/'
2026-08-17T10:51:50.190Z  INFO 11484 --- [           main] o.s.b.d.f.logexample.MyApplication       : Started MyApplication in 8.35 seconds (process running for 9.787)
2026-08-17T10:51:50.219Z  INFO 11484 --- [ionShutdownHook] o.s.boot.tomcat.GracefulShutdown         : Commencing graceful shutdown. Waiting for active requests to complete
2026-08-17T10:51:50.249Z  INFO 11484 --- [tomcat-shutdown] o.s.boot.tomcat.GracefulShutdown         : Graceful shutdown complete
```

--------------------------------

### YAML document with environments

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Example YAML configuration showing hierarchical environments with url and name properties.

```yaml
environments:
  dev:
    url: "https://dev.example.com"
    name: "Developer Setup"
  prod:
    url: "https://another.example.com"
    name: "My Cool App"
```

--------------------------------

### Property ordering example 1 (YAML)

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

YAML equivalent of the first property ordering example. The order of import declaration does not matter.

```yaml
spring:
  config:
    import: "my.properties"
my:
  property: "value"
```

--------------------------------

### YAML example with security username

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

YAML equivalent of the properties example, defining my.service.enabled and my.service.security.username. Demonstrates binding with @DefaultValue("USER") for roles.

```yaml
my:
  service:
    enabled: true
    security:
      username: admin
```

--------------------------------

### Custom info properties in application.yml

Source: https://docs.spring.io/spring-boot/reference/actuator/endpoints.html

YAML equivalent of the properties example, setting the same info.app.* properties.

```yaml
info:
  app:
    encoding: "UTF-8"
    java:
      source: "17"
      target: "17"
```

--------------------------------

### Expected output from the application

Source: https://docs.spring.io/spring-boot/reference/actuator/tracing.html

The output displayed in a web browser at localhost:8080 after starting the application.

```none
Hello World!
```

--------------------------------

### RSocketGraphQlClient example in Kotlin

Source: https://docs.spring.io/spring-boot/reference/web/spring-graphql.html

Kotlin version of the RSocketGraphQlClient example using constructor injection of the builder.

```kotlin
@Component
class RSocketGraphQlClientExample(private val builder: RSocketGraphQlClient.Builder<*>) {
```

--------------------------------

### Property ordering example 2 (YAML)

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

YAML equivalent of the second property ordering example. The order of import declaration does not matter.

```yaml
my:
  property: "value"
spring:
  config:
    import: "my.properties"
```

--------------------------------

### Kotlin @KafkaListener example

Source: https://docs.spring.io/spring-boot/reference/messaging/kafka.html

Kotlin version of the listener endpoint on the 'someTopic' topic. Uses the same annotation and auto-configuration behavior as the Java example.

```kotlin
import org.springframework.kafka.annotation.KafkaListener
import org.springframework.stereotype.Component

@Component
class MyBean {

	@KafkaListener(topics = ["someTopic"])
	fun processMessage(content: String?) {
		// ...
	}

}
```

--------------------------------

### Initialize OpenTelemetryAppender for Logback/Log4j (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/actuator/loggers.html

Kotlin equivalent of the OpenTelemetryAppender initializer. Installs the OpenTelemetry instance into the appender during startup.

```kotlin
import io.opentelemetry.api.OpenTelemetry
import io.opentelemetry.instrumentation.logback.appender.v1_0.OpenTelemetryAppender
import org.springframework.beans.factory.InitializingBean
import org.springframework.stereotype.Component

@Component
class OpenTelemetryAppenderInitializer(
	private val openTelemetry: OpenTelemetry
) : InitializingBean {

	override fun afterPropertiesSet() = OpenTelemetryAppender.install(openTelemetry)

}
```

--------------------------------

### Define ApplicationContextRunner with AutoConfigurations (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/features/developing-auto-configuration.html

Kotlin version of the base runner setup, using Kotlin class references.

```kotlin
	val contextRunner = ApplicationContextRunner()
		.withConfiguration(AutoConfigurations.of(MyServiceAutoConfiguration::class.java))
```

--------------------------------

### Main application code in Kotlin

Source: https://docs.spring.io/spring-boot/reference/actuator/tracing.html

The same example application written in Kotlin, with a REST endpoint that logs a message.

```kotlin
import org.apache.commons.logging.Log
import org.apache.commons.logging.LogFactory
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@SpringBootApplication
class MyApplication {

	private val logger: Log = LogFactory.getLog(MyApplication::class.java)

	@RequestMapping("/")
	fun home(): String {
		logger.info("home() has been called")
		return "Hello, World!"
	}

}

fun main(args: Array<String>) {
	runApplication<MyApplication>(*args)
}
```

--------------------------------

### Kotlin @PulsarReader example

Source: https://docs.spring.io/spring-boot/reference/messaging/pulsar.html

Kotlin version of the reader endpoint that starts reading from the beginning of 'someTopic'. Uses the same annotation and configuration as the Java example.

```kotlin
import org.springframework.pulsar.annotation.PulsarReader
import org.springframework.stereotype.Component

@Component
class MyBean {

	@PulsarReader(topics = ["someTopic"], startMessageId = "earliest")
	fun processMessage(content: String?) {
		// ...
	}

}
```

--------------------------------

### Create test application launcher with fromApplication (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/features/dev-services.html

Kotlin equivalent of the test launcher, using fromApplication<MyApplication>().run(*args).

```kotlin
import org.springframework.boot.fromApplication

fun main(args: Array<String>) {
	fromApplication<MyApplication>().run(*args)
}
```

--------------------------------

### Spring MVC @RestController example in Java

Source: https://docs.spring.io/spring-boot/reference/web/servlet.html

A typical @RestController that serves JSON data, with endpoints for getting a user, getting user customers, and deleting a user. Uses @RequestMapping, @GetMapping, @DeleteMapping, and @PathVariable.

```java
import java.util.List;

import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/users")
public class MyRestController {

	private final UserRepository userRepository;

	private final CustomerRepository customerRepository;

	public MyRestController(UserRepository userRepository, CustomerRepository customerRepository) {
		this.userRepository = userRepository;
		this.customerRepository = customerRepository;
	}

	@GetMapping("/{userId}")
	public User getUser(@PathVariable Long userId) {
		return this.userRepository.findById(userId).get();
	}

	@GetMapping("/{userId}/customers")
	public List<Customer> getUserCustomers(@PathVariable Long userId) {
		return this.userRepository.findById(userId).map(this.customerRepository::findByUser).get();
	}

	@DeleteMapping("/{userId}")
	public void deleteUser(@PathVariable Long userId) {
		this.userRepository.deleteById(userId);
	}

}
```

--------------------------------

### Configure Let's Encrypt PEM bundle with properties

Source: https://docs.spring.io/spring-boot/reference/features/ssl.html

Configure a PEM bundle for 'example.com' using Certbot-generated files. Enables reloading and sets the server SSL bundle to 'webserver'.

```properties
spring.ssl.bundle.pem.webserver.reload-on-update=true
spring.ssl.bundle.pem.webserver.keystore.certificate=file:/etc/letsencrypt/live/example.com/fullchain.pem
spring.ssl.bundle.pem.webserver.keystore.private-key=file:/etc/letsencrypt/live/example.com/privkey.pem
server.ssl.bundle=webserver
```

--------------------------------

### Spring MVC @RestController example in Kotlin

Source: https://docs.spring.io/spring-boot/reference/web/servlet.html

Kotlin version of the typical @RestController that serves JSON data, with endpoints for getting a user, getting user customers, and deleting a user. Uses @RequestMapping, @GetMapping, @DeleteMapping, and @PathVariable.

```kotlin
import org.springframework.web.bind.annotation.DeleteMapping
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController


@RestController
@RequestMapping("/users")
class MyRestController(private val userRepository: UserRepository, private val customerRepository: CustomerRepository) {

	@GetMapping("/{userId}")
	fun getUser(@PathVariable userId: Long): User {
		return userRepository.findById(userId).get()
	}

	@GetMapping("/{userId}/customers")
	fun getUserCustomers(@PathVariable userId: Long): List<Customer> {
		return userRepository.findById(userId).map(customerRepository::findByUser).get()
	}

	@DeleteMapping("/{userId}")
	fun deleteUser(@PathVariable userId: Long) {
		userRepository.deleteById(userId)
	}

}
```

--------------------------------

### Java @PulsarReader example

Source: https://docs.spring.io/spring-boot/reference/messaging/pulsar.html

Creates a reader endpoint that starts reading messages from the beginning of the 'someTopic' topic. The @PulsarReader annotation relies on a PulsarReaderFactory, which can be customized via spring.pulsar.reader.* properties or ReaderBuilderCustomizer beans.

```java
import org.springframework.pulsar.annotation.PulsarReader;
import org.springframework.stereotype.Component;

@Component
public class MyBean {

	@PulsarReader(topics = "someTopic", startMessageId = "earliest")
	public void processMessage(String content) {
		// ...
	}

}
```

--------------------------------

### Launch application with native image tracing agent

Source: https://docs.spring.io/spring-boot/reference/packaging/native-image/advanced-topics.html

Use this command to start the application with the GraalVM native image tracing agent attached. The agent writes hint files to the specified config output directory on application shutdown. After exercising the code paths, stop the application with ctrl-c, then copy the generated files into src/main/resources/META-INF/native-image/ for use in the next native image build.

```shell
$ java -Dspring.aot.enabled=true \
    -agentlib:native-image-agent=config-output-dir=/path/to/config-dir/ \
    -jar target/myproject-0.0.1-SNAPSHOT.jar
```

--------------------------------

### Typical RestClient usage with injected builder (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/io/rest-client.html

Kotlin version of the typical RestClient example. It injects the auto-configured RestClient.Builder, sets a base URL, and performs a GET request returning a Details object.

```kotlin
import org.springframework.boot.docs.io.restclient.restclient.ssl.Details
import org.springframework.stereotype.Service
import org.springframework.web.client.RestClient

@Service
class MyService(restClientBuilder: RestClient.Builder) {

	private val restClient: RestClient

	init {
		restClient = restClientBuilder.baseUrl("https://example.org").build()
	}

	fun someRestCall(name: String): Details {
		return restClient.get().uri("/{name}/details", name)
				.retrieve().body(Details::class.java)!!
	}

}
```

--------------------------------

### Typical Spring Boot project layout

Source: https://docs.spring.io/spring-boot/reference/using/structuring-your-code.html

Shows a recommended directory structure with the main application class in a root package and sub-packages for domain areas like customer and order.

```none
com
 +- example
     +- myapplication
         +- MyApplication.java
         |
         +- customer
         |   +- Customer.java
         |   +- CustomerController.java
         |   +- CustomerService.java
         |   +- CustomerRepository.java
         |
         +- order
             +- Order.java
             +- OrderController.java
             +- OrderService.java
             +- OrderRepository.java
```

--------------------------------

### Configure TCP readiness timeouts (Properties and YAML)

Source: https://docs.spring.io/spring-boot/reference/features/dev-services.html

Shows how to set connect and read timeouts for the TCP readiness check in both application.properties and application.yaml.

```properties
spring.docker.compose.readiness.tcp.connect-timeout=10s
spring.docker.compose.readiness.tcp.read-timeout=5s
```

```yaml
spring:
  docker:
    compose:
      readiness:
        tcp:
          connect-timeout: 10s
          read-timeout: 5s
```

--------------------------------

### Kotlin: Use @Testcontainers and @Container with Neo4jContainer

Source: https://docs.spring.io/spring-boot/reference/testing/testcontainers.html

Kotlin version of the Testcontainers JUnit extension example. The container is declared in a companion object with @Container and @JvmStatic, and starts before tests run. The container is stopped after the test class completes.

```kotlin
import org.junit.jupiter.api.Test;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;
import org.testcontainers.neo4j.Neo4jContainer;

import org.springframework.boot.test.context.SpringBootTest;

@Testcontainers
@SpringBootTest
class MyIntegrationTests {

	@Test
	fun myTest() {
		...
	}

	companion object {

		@Container
		@JvmStatic
		val neo4j = Neo4jContainer("neo4j:5");

	}
}
```

--------------------------------

### Typical Spring Boot main method (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin equivalent of the typical main method using runApplication.

```kotlin
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.docs.using.structuringyourcode.locatingthemainclass.MyApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class MyApplication

fun main(args: Array<String>) {
	runApplication<MyApplication>(*args)
}
```

--------------------------------

### Kotlin: Define a KStream bean with StreamsBuilder

Source: https://docs.spring.io/spring-boot/reference/messaging/kafka.html

Kotlin equivalent of the Java example, wiring a StreamsBuilder into a @Bean method to create a KStream that reads from 'ks1In', uppercases values, and writes to 'ks1Out'. Requires @EnableKafkaStreams and the kafka-streams dependency. The stream is started automatically unless spring.kafka.streams.auto-startup is set to false.

```kotlin
import org.apache.kafka.common.serialization.Serdes
import org.apache.kafka.streams.KeyValue
import org.apache.kafka.streams.StreamsBuilder
import org.apache.kafka.streams.kstream.KStream
import org.apache.kafka.streams.kstream.Produced
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.kafka.annotation.EnableKafkaStreams
import org.springframework.kafka.support.serializer.JacksonJsonSerde

@Configuration(proxyBeanMethods = false)
@EnableKafkaStreams
class MyKafkaStreamsConfiguration {

	@Bean
	fun kStream(streamsBuilder: StreamsBuilder): KStream<Int, String> {
		val stream = streamsBuilder.stream<Int, String>("ks1In")
		stream.map(this::uppercaseValue).to("ks1Out", Produced.with(Serdes.Integer(), JacksonJsonSerde()))
		return stream
	}

	private fun uppercaseValue(key: Int, value: String): KeyValue<Int, String> {
		return KeyValue(key, value.uppercase())
	}

}
```

--------------------------------

### Properties example with only enabled

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Properties file defining only my.service.enabled. Used to demonstrate that when security property is not present, the Security instance is null.

```properties
my.service.enabled=true
```

--------------------------------

### Kotlin test setup with @DataElasticsearchTest

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin equivalent of the Java example. Use this annotation to test Data Elasticsearch applications. It configures an ElasticsearchTemplate, scans for @Document classes, and configures Spring Data Elasticsearch repositories. Regular @Component and @ConfigurationProperties beans are not scanned; use @EnableConfigurationProperties to include them.

```kotlin
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.data.elasticsearch.test.autoconfigure.DataElasticsearchTest

@DataElasticsearchTest
class MyDataElasticsearchTests(@Autowired val repository: SomeRepository) {

	// ...

}
```

--------------------------------

### Custom ErrorWebExceptionHandler with AbstractErrorWebExceptionHandler (Java)

Source: https://docs.spring.io/spring-boot/reference/web/reactive.html

Java example of a custom error handler that extends AbstractErrorWebExceptionHandler. It routes requests that accept XML to a handler that returns an XML response. The constructor requires ErrorAttributes, WebProperties, ApplicationContext, and ServerCodecConfigurer. The handleErrorAsXml method is incomplete in the source, showing only the start of the response builder.

```java
import reactor.core.publisher.Mono;

import org.springframework.boot.autoconfigure.web.WebProperties;
import org.springframework.boot.webflux.autoconfigure.error.AbstractErrorWebExceptionHandler;
import org.springframework.boot.webflux.error.ErrorAttributes;
import org.springframework.context.ApplicationContext;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.codec.ServerCodecConfigurer;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.RouterFunctions;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import org.springframework.web.reactive.function.server.ServerResponse.BodyBuilder;

@Component
public class MyErrorWebExceptionHandler extends AbstractErrorWebExceptionHandler {

	public MyErrorWebExceptionHandler(ErrorAttributes errorAttributes, WebProperties webProperties,
			ApplicationContext applicationContext, ServerCodecConfigurer serverCodecConfigurer) {
		super(errorAttributes, webProperties.getResources(), applicationContext);
		setMessageReaders(serverCodecConfigurer.getReaders());
		setMessageWriters(serverCodecConfigurer.getWriters());
	}

	@Override
	protected RouterFunction<ServerResponse> getRoutingFunction(ErrorAttributes errorAttributes) {
		return RouterFunctions.route(this::acceptsXml, this::handleErrorAsXml);
	}

	private boolean acceptsXml(ServerRequest request) {
		return request.headers().accept().contains(MediaType.APPLICATION_XML);
	}

	public Mono<ServerResponse> handleErrorAsXml(ServerRequest request) {
		BodyBuilder builder = ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR);
		// ... additional builder calls
		return builder.build();
	}

}
```

--------------------------------

### Kotlin example using TestRestTemplate with @SpringBootTest and @AutoConfigureTestRestTemplate

Source: https://docs.spring.io/spring-boot/reference/testing/test-utilities.html

Use this snippet to write an integration test that makes HTTP requests to a running server. It injects TestRestTemplate via constructor, sends a GET request to /example, and asserts the response status is OK. The nested @TestConfiguration provides a custom RestTemplateBuilder with 1-second connect and read timeouts.

```kotlin
import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.boot.test.context.SpringBootTest.WebEnvironment
import org.springframework.boot.test.context.TestConfiguration
import org.springframework.boot.restclient.RestTemplateBuilder
import org.springframework.boot.resttestclient.TestRestTemplate
import org.springframework.boot.resttestclient.autoconfigure.AutoConfigureTestRestTemplate
import org.springframework.context.annotation.Bean
import org.springframework.http.HttpStatus
import java.time.Duration

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureTestRestTemplate
class MySpringBootTests(@Autowired val template: TestRestTemplate) {

	@Test
	fun testRequest() {
		val response = template.getForEntity("/example", String::class.java)
		assertThat(response.statusCode).isEqualTo(HttpStatus.OK)
		// Other assertions to verify the response
	}

	@TestConfiguration(proxyBeanMethods = false)
	internal class RestTemplateBuilderConfiguration {

		@Bean
		fun restTemplateBuilder(): RestTemplateBuilder {
			return RestTemplateBuilder().connectTimeout(Duration.ofSeconds(1))
				.readTimeout(Duration.ofSeconds(1))
		}

	}

}
```

--------------------------------

### Implement HelloWorld gRPC service in Java

Source: https://docs.spring.io/spring-boot/reference/io/grpc.html

Extends HelloWorldGrpc.HelloWorldImplBase and uses @GrpcService to expose the service. The sayHello method builds a HelloReply and sends it via the response observer.

```java
import io.grpc.stub.StreamObserver;

import org.springframework.grpc.server.service.GrpcService;

@GrpcService
public class MyHelloWorldService extends HelloWorldGrpc.HelloWorldImplBase {

	@Override
	public void sayHello(HelloRequest request, StreamObserver<HelloReply> responseObserver) {
		String message = "Hello '%s'".formatted(request.getName());
		HelloReply reply = HelloReply.newBuilder().setMessage(message).build();
		responseObserver.onNext(reply);
		responseObserver.onCompleted();
	}

}
```

--------------------------------

### Import extensionless file with explicit extension attribute (Properties)

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Shorthand for the previous example, using the 'extension=' attribute explicitly.

```properties
spring.config.import=file:/etc/config/myconfig[extension=.yaml]
```

--------------------------------

### OAuth2 Client Registration with Explicit Provider (YAML)

Source: https://docs.spring.io/spring-boot/reference/security/oauth2.html

YAML equivalent of the properties example. The 'my-client' registration sets the provider to 'example', while the 'example' registration relies on its ID as the provider.

```yaml
spring:
  security:
    oauth2:
      client:
        registration:
          my-client:
            client-id: "abcd"
            client-secret: "password"
            provider: "example"
          example:
            client-id: "abcd"
            client-secret: "password"
```

--------------------------------

### Configure both servers with SSL using different key stores (Properties)

Source: https://docs.spring.io/spring-boot/reference/actuator/monitoring.html

Use these properties to have both the main server and the management server use SSL but with different key stores. The main server uses main.jks and the management server uses management.jks.

```properties
server.port=8443
server.ssl.enabled=true
server.ssl.key-store=classpath:main.jks
server.ssl.key-password=secret
management.server.port=8080
management.server.ssl.enabled=true
management.server.ssl.key-store=classpath:management.jks
management.server.ssl.key-password=secret
```

--------------------------------

### runApplication<MyApplication>(*args) basic usage

Source: https://docs.spring.io/spring-boot/reference/features/kotlin.html

Idiomatic way to run a Spring Boot application in Kotlin. Drop-in replacement for SpringApplication.run(MyApplication::class.java, *args).

```kotlin
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class MyApplication

fun main(args: Array<String>) {
	runApplication<MyApplication>(*args)
}
```

--------------------------------

### @DataNeo4jTest example in Kotlin

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin version of the @DataNeo4jTest example. It uses constructor injection for the repository.

```kotlin
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.data.neo4j.test.autoconfigure.DataNeo4jTest

@DataNeo4jTest
class MyDataNeo4jTests(@Autowired val repository: SomeRepository) {

	// ...

}
```

--------------------------------

### @DataMongoTest example in Kotlin

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin version of the @DataMongoTest example. It uses constructor injection for MongoTemplate.

```kotlin
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.data.mongodb.test.autoconfigure.DataMongoTest
import org.springframework.data.mongodb.core.MongoTemplate

@DataMongoTest
class MyDataMongoDbTests(@Autowired val mongoTemplate: MongoTemplate) {

	// ...

}
```

--------------------------------

### Configure SSL bundle for Cassandra (Properties)

Source: https://docs.spring.io/spring-boot/reference/data/nosql.html

Applies custom SSL trust material from an SSL bundle named 'example' to the CqlSession.

```properties
spring.cassandra.keyspace-name=mykeyspace
spring.cassandra.contact-points=cassandrahost1,cassandrahost2
spring.cassandra.local-datacenter=datacenter1
spring.cassandra.ssl.bundle=example
```

--------------------------------

### Export Readiness State to File (Java)

Source: https://docs.spring.io/spring-boot/reference/features/spring-application.html

Listens for AvailabilityChangeEvent<ReadinessState> and creates or removes a file based on the state. Used for Kubernetes exec probes.

```java
import org.springframework.boot.availability.AvailabilityChangeEvent;
import org.springframework.boot.availability.ReadinessState;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
public class MyReadinessStateExporter {

	@EventListener
	public void onStateChange(AvailabilityChangeEvent<ReadinessState> event) {
		switch (event.getState()) {
			case ACCEPTING_TRAFFIC -> {
				// create file /tmp/healthy
			}
			case REFUSING_TRAFFIC -> {
				// remove file /tmp/healthy
			}
		}
	}

}
```

--------------------------------

### Configure Let's Encrypt PEM bundle with YAML

Source: https://docs.spring.io/spring-boot/reference/features/ssl.html

Configure a PEM bundle for 'example.com' using Certbot-generated files. Enables reloading and sets the server SSL bundle to 'webserver'.

```yaml
spring:
  ssl:
    bundle:
      pem:
        webserver:
          reload-on-update: true
          keystore:
            certificate: "file:/etc/letsencrypt/live/example.com/fullchain.pem"
            private-key: "file:/etc/letsencrypt/live/example.com/privkey.pem"
server:
  ssl:
    bundle: "webserver"
```

--------------------------------

### Show extract options with help command

Source: https://docs.spring.io/spring-boot/reference/packaging/efficient.html

Run this command to see all possible options for the extract command.

```shell
$ java -Djarmode=tools -jar my-app.jar help extract
```

--------------------------------

### @WebServiceServerTest Kotlin example

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin version of the @WebServiceServerTest example. It configures a MockWebServiceClient and tests a web service endpoint by sending a request and expecting a response payload.

```kotlin
import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.webservices.test.autoconfigure.server.WebServiceServerTest
import org.springframework.ws.test.server.MockWebServiceClient
import org.springframework.ws.test.server.RequestCreators
import org.springframework.ws.test.server.ResponseMatchers
import org.springframework.xml.transform.StringSource

@WebServiceServerTest(ExampleEndpoint::class)
class MyWebServiceServerTests(@Autowired val client: MockWebServiceClient) {

	@Test
	fun mockServerCall() {
		client
			.sendRequest(RequestCreators.withPayload(StringSource("<ExampleRequest/>")))
			.andExpect(ResponseMatchers.payload(StringSource("<ExampleResponse>42</ExampleResponse>")))
	}

}
```

--------------------------------

### @WebServiceClientTest Kotlin example

Source: https://docs.spring.io/spring-boot/reference/testing/spring-boot-applications.html

Kotlin version of the @WebServiceClientTest example. It configures a MockWebServiceServer and tests a web service client by expecting a request and responding with a payload, then asserting the response status.

```kotlin
import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.webservices.test.autoconfigure.client.WebServiceClientTest
import org.springframework.ws.test.client.MockWebServiceServer
import org.springframework.ws.test.client.RequestMatchers
import org.springframework.ws.test.client.ResponseCreators
import org.springframework.xml.transform.StringSource

@WebServiceClientTest(SomeWebService::class)
class MyWebServiceClientTests(
		@Autowired val server: MockWebServiceServer, @Autowired val someWebService: SomeWebService) {

	@Test
	fun mockServerCall() {
		server
			.expect(RequestMatchers.payload(StringSource("<request/>")))
			.andRespond(ResponseCreators.withPayload(StringSource("<response><status>200</status></response>")))
		assertThat(this.someWebService.test()).extracting(Response::status).isEqualTo(200)
	}

}
```

--------------------------------

### Kotlin @RabbitListener example

Source: https://docs.spring.io/spring-boot/reference/messaging/amqp.html

Kotlin equivalent of the Java @RabbitListener example, using a class with a method annotated to listen to 'someQueue' with the 'myFactory' container factory. The message content is received as a nullable String.

```kotlin
import org.springframework.amqp.rabbit.annotation.RabbitListener
import org.springframework.stereotype.Component

@Component
class MyBean {

	@RabbitListener(queues = ["someQueue"], containerFactory = "myFactory")
	fun processMessage(content: String?) {
		// ...
	}

}
```

--------------------------------

### Main application class with @SpringBootApplication (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/using/structuring-your-code.html

Kotlin example of the main class using @SpringBootApplication and runApplication. Place it in the root package to enable component scanning and entity scanning.

```kotlin
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class MyApplication

fun main(args: Array<String>) {
	runApplication<MyApplication>(*args)
}
```

--------------------------------

### SAML 2.0 Relying Party properties example

Source: https://docs.spring.io/spring-boot/reference/security/saml2.html

Example properties for configuring two relying party registrations under the spring.security.saml2.relyingparty prefix. Includes signing, decryption, single logout, and asserting party settings.

```properties
spring.security.saml2.relyingparty.registration.my-relying-party1.signing.credentials[0].private-key-location=path-to-private-key
spring.security.saml2.relyingparty.registration.my-relying-party1.signing.credentials[0].certificate-location=path-to-certificate
spring.security.saml2.relyingparty.registration.my-relying-party1.decryption.credentials[0].private-key-location=path-to-private-key
spring.security.saml2.relyingparty.registration.my-relying-party1.decryption.credentials[0].certificate-location=path-to-certificate
spring.security.saml2.relyingparty.registration.my-relying-party1.singlelogout.url=https://myapp/logout/saml2/slo
spring.security.saml2.relyingparty.registration.my-relying-party1.singlelogout.response-url=https://remoteidp2.slo.url
spring.security.saml2.relyingparty.registration.my-relying-party1.singlelogout.binding=POST
spring.security.saml2.relyingparty.registration.my-relying-party1.assertingparty.verification.credentials[0].certificate-location=path-to-verification-cert
spring.security.saml2.relyingparty.registration.my-relying-party1.assertingparty.entity-id=remote-idp-entity-id1
spring.security.saml2.relyingparty.registration.my-relying-party1.assertingparty.sso-url=https://remoteidp1.sso.url
spring.security.saml2.relyingparty.registration.my-relying-party2.signing.credentials[0].private-key-location=path-to-private-key
spring.security.saml2.relyingparty.registration.my-relying-party2.signing.credentials[0].certificate-location=path-to-certificate
spring.security.saml2.relyingparty.registration.my-relying-party2.decryption.credentials[0].private-key-location=path-to-private-key
spring.security.saml2.relyingparty.registration.my-relying-party2.decryption.credentials[0].certificate-location=path-to-certificate
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.verification.credentials[0].certificate-location=path-to-other-verification-cert
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.entity-id=remote-idp-entity-id2
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.sso-url=https://remoteidp2.sso.url
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.singlelogout.url=https://remoteidp2.slo.url
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.singlelogout.response-url=https://myapp/logout/saml2/slo
spring.security.saml2.relyingparty.registration.my-relying-party2.assertingparty.singlelogout.binding=POST
```

--------------------------------

### Project structure with trigger file

Source: https://docs.spring.io/spring-boot/reference/using/devtools.html

Example directory structure showing where the trigger file .reloadtrigger should be placed in the classpath (src/main/resources).

```none
src
+- main
   +- resources
      +- .reloadtrigger
```

--------------------------------

### Start Application with CDS Archive

Source: https://docs.spring.io/spring-boot/reference/packaging/aot-cache.html

Starts the extracted Spring Boot application using the previously generated CDS archive. The archive file must be used with the extracted form of the application, otherwise it has no effect.

```shell
$ java -XX:SharedArchiveFile=application.jsa -jar my-app.jar
```

--------------------------------

### YAML example for binding maps with bracket notation

Source: https://docs.spring.io/spring-boot/reference/features/external-config.html

Shows how to define map properties in YAML. Keys with brackets must be quoted to be parsed correctly. The example binds to a Map with keys '/key1', '/key2', and 'key3'.

```yaml
my:
  map:
    "[/key1]": "value1"
    "[/key2]": "value2"
    "/key3": "value3"
```

--------------------------------

### Export Readiness State to File (Kotlin)

Source: https://docs.spring.io/spring-boot/reference/features/spring-application.html

Kotlin version of the readiness state exporter, listening for AvailabilityChangeEvent<ReadinessState> and creating or removing a file based on the state.

```kotlin
import org.springframework.boot.availability.AvailabilityChangeEvent
import org.springframework.boot.availability.ReadinessState
import org.springframework.context.event.EventListener
import org.springframework.stereotype.Component

@Component
class MyReadinessStateExporter {

	@EventListener
	fun onStateChange(event: AvailabilityChangeEvent<ReadinessState>) {
		when (event.state) {
			ReadinessState.ACCEPTING_TRAFFIC -> {
				// create file /tmp/healthy
			}
			ReadinessState.REFUSING_TRAFFIC -> {
				// remove file /tmp/healthy
			}
		}
	}

}
```
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