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Marcin Grzejszczak
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docs/modules/ROOT/pages/functional.adoc
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docs/modules/ROOT/pages/functional.adoc
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Spring Cloud Function supports a "functional" style of bean declarations for small apps where you need fast startup. The functional style of bean declaration was a feature of Spring Framework 5.0 with significant enhancements in 5.1.
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== Comparing Functional with Traditional Bean Definitions
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Here's a vanilla Spring Cloud Function application from with the
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familiar `@Configuration` and `@Bean` declaration style:
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```java
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@SpringBootApplication
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public class DemoApplication {
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@Bean
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public Function<String, String> uppercase() {
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return value -> value.toUpperCase();
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}
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public static void main(String[] args) {
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SpringApplication.run(DemoApplication.class, args);
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}
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}
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```
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Now for the functional beans: the user application code can be recast into "functional"
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form, like this:
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```java
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@SpringBootConfiguration
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public class DemoApplication implements ApplicationContextInitializer<GenericApplicationContext> {
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public static void main(String[] args) {
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FunctionalSpringApplication.run(DemoApplication.class, args);
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}
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public Function<String, String> uppercase() {
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return value -> value.toUpperCase();
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}
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@Override
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public void initialize(GenericApplicationContext context) {
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context.registerBean("demo", FunctionRegistration.class,
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() -> new FunctionRegistration<>(uppercase())
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.type(FunctionTypeUtils.functionType(String.class, String.class)));
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}
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}
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```
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The main differences are:
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* The main class is an `ApplicationContextInitializer`.
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* The `@Bean` methods have been converted to calls to `context.registerBean()`
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* The `@SpringBootApplication` has been replaced with
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`@SpringBootConfiguration` to signify that we are not enabling Spring
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Boot autoconfiguration, and yet still marking the class as an "entry
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point".
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* The `SpringApplication` from Spring Boot has been replaced with a
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`FunctionalSpringApplication` from Spring Cloud Function (it's a
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subclass).
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The business logic beans that you register in a Spring Cloud Function app are of type `FunctionRegistration`.
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This is a wrapper that contains both the function and information about the input and output types. In the `@Bean`
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form of the application that information can be derived reflectively, but in a functional bean registration some of
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it is lost unless we use a `FunctionRegistration`.
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An alternative to using an `ApplicationContextInitializer` and `FunctionRegistration` is to make the application
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itself implement `Function` (or `Consumer` or `Supplier`). Example (equivalent to the above):
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```java
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@SpringBootConfiguration
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public class DemoApplication implements Function<String, String> {
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public static void main(String[] args) {
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FunctionalSpringApplication.run(DemoApplication.class, args);
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}
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@Override
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public String apply(String value) {
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return value.toUpperCase();
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}
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}
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```
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It would also work if you add a separate, standalone class of type `Function` and register it with
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the `SpringApplication` using an alternative form of the `run()` method. The main thing is that the generic
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type information is available at runtime through the class declaration.
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Suppose you have
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[source, java]
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----
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@Component
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public class CustomFunction implements Function<Flux<Foo>, Flux<Bar>> {
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@Override
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public Flux<Bar> apply(Flux<Foo> flux) {
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return flux.map(foo -> new Bar("This is a Bar object from Foo value: " + foo.getValue()));
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}
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}
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----
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You register it as such:
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[source, java]
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----
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@Override
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public void initialize(GenericApplicationContext context) {
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context.registerBean("function", FunctionRegistration.class,
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() -> new FunctionRegistration<>(new CustomFunction()).type(CustomFunction.class));
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}
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----
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== Limitations of Functional Bean Declaration
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Most Spring Cloud Function apps have a relatively small scope compared to the whole of Spring Boot,
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so we are able to adapt it to these functional bean definitions easily. If you step outside that limited scope,
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you can extend your Spring Cloud Function app by switching back to `@Bean` style configuration, or by using a hybrid
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approach. If you want to take advantage of Spring Boot autoconfiguration for integrations with external datastores,
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for example, you will need to use `@EnableAutoConfiguration`. Your functions can still be defined using the functional
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declarations if you want (i.e. the "hybrid" style), but in that case you will need to explicitly switch off the "full
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functional mode" using `spring.functional.enabled=false` so that Spring Boot can take back control.
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[[function_visualization]]
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= Function visualization and control
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Spring Cloud Function supports visualization of functions available in `FunctionCatalog` through Actuator endpoints as well as programmatic way.
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==== Programmatic way
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To see function available within your application context programmatically all you need is access to `FunctionCatalog`. There you can
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finds methods to get the size of the catalog, lookup functions as well as list the names of all the available functions.
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For example,
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[source,java]
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----
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FunctionCatalog functionCatalog = context.getBean(FunctionCatalog.class);
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int size = functionCatalog.size(); // will tell you how many functions available in catalog
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Set<String> names = functionCatalog.getNames(null); will list the names of all the Function, Suppliers and Consumers available in catalog
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. . .
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----
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==== Actuator
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Since actuator and web are optional, you must first add one of the web dependencies as well as add the actuator dependency manually.
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The following example shows how to add the dependency for the Web framework:
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[source,xml]
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----
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-web</artifactId>
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</dependency>
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----
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The following example shows how to add the dependency for the WebFlux framework:
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[source,xml]
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----
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-webflux</artifactId>
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</dependency>
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----
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You can add the Actuator dependency as follows:
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[source,xml]
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----
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-actuator</artifactId>
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</dependency>
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----
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You must also enable the `functions` actuator endpoints by setting the following property: `--management.endpoints.web.exposure.include=functions`.
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Access the following URL to see the functions in FunctionCatalog:
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`http://<host>:<port>/actuator/functions`
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For example,
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[source,text]
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----
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curl http://localhost:8080/actuator/functions
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----
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Your output should look something like this:
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[source,text]
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----
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{"charCounter":
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{"type":"FUNCTION","input-type":"string","output-type":"integer"},
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"logger":
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{"type":"CONSUMER","input-type":"string"},
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"functionRouter":
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{"type":"FUNCTION","input-type":"object","output-type":"object"},
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"words":
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{"type":"SUPPLIER","output-type":"string"}. . .
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----
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= Testing Functional Applications
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Spring Cloud Function also has some utilities for integration testing that will be very familiar to Spring Boot users.
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Suppose this is your application:
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[source, java]
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----
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@SpringBootApplication
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public class SampleFunctionApplication {
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public static void main(String[] args) {
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SpringApplication.run(SampleFunctionApplication.class, args);
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}
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@Bean
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public Function<String, String> uppercase() {
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return v -> v.toUpperCase();
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}
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}
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----
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Here is an integration test for the HTTP server wrapping this application:
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[source, java]
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----
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@SpringBootTest(classes = SampleFunctionApplication.class,
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webEnvironment = WebEnvironment.RANDOM_PORT)
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public class WebFunctionTests {
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@Autowired
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private TestRestTemplate rest;
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@Test
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public void test() throws Exception {
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ResponseEntity<String> result = this.rest.exchange(
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RequestEntity.post(new URI("/uppercase")).body("hello"), String.class);
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System.out.println(result.getBody());
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}
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}
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----
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or when function bean definition style is used:
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[source, java]
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----
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@FunctionalSpringBootTest
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public class WebFunctionTests {
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@Autowired
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private TestRestTemplate rest;
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@Test
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public void test() throws Exception {
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ResponseEntity<String> result = this.rest.exchange(
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RequestEntity.post(new URI("/uppercase")).body("hello"), String.class);
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System.out.println(result.getBody());
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}
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}
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----
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This test is almost identical to the one you would write for the `@Bean` version of the same app - the only difference
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is the `@FunctionalSpringBootTest` annotation, instead of the regular `@SpringBootTest`. All the other pieces,
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like the `@Autowired` `TestRestTemplate`, are standard Spring Boot features.
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And to help with correct dependencies here is the excerpt from POM
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[source, xml, subs=attributes+]
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----
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<parent>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-parent</artifactId>
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<version>{spring-boot-version}</version>
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<relativePath/> <!-- lookup parent from repository -->
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</parent>
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. . . .
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<dependency>
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<groupId>org.springframework.cloud</groupId>
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<artifactId>spring-cloud-function-web</artifactId>
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<version>{project-version}</version>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter</artifactId>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-web</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-test</artifactId>
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<scope>test</scope>
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</dependency>
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----
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Or you could write a test for a non-HTTP app using just the `FunctionCatalog`. For example:
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[source, java]
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----
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@FunctionalSpringBootTest
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public class FunctionalTests {
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@Autowired
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private FunctionCatalog catalog;
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@Test
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public void words() {
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Function<String, String> function = catalog.lookup(Function.class,
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"uppercase");
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assertThat(function.apply("hello")).isEqualTo("HELLO");
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}
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}
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----
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