The annotation @RestController cannot be resolved when only adding spring-boot-starter-webflux as maven dependency:
@RestController
public class MyController {
}
pom.xml:
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.6.6</version>
<relativePath/>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
</dependencies>
What is missing here? According to many resources out there (eg https://medium.com/javarevisited/basic-introduction-to-spring-webflux-eb155f501b17), the webflux dependency should be sufficient for a webflux-webservice in spring-boot.
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Answer
RestController annotation is part of org.springframework:spring-web: dependency
org.springframework:spring-web dependency is part of org.springframework.boot:spring-boot-starter-webflux jar so it should get resolved. You can check the complete dependency hierarchy by running mvn dependency:tree command from the terminal.
[INFO] +- org.springframework.boot:spring-boot-starter-webflux:jar:2.6.6:compile [INFO] | +- org.springframework.boot:spring-boot-starter:jar:2.6.6:compile [INFO] | | +- org.springframework.boot:spring-boot:jar:2.6.6:compile [INFO] | | | - org.springframework:spring-context:jar:5.3.18:compile [INFO] | | | +- org.springframework:spring-aop:jar:5.3.18:compile [INFO] | | | - org.springframework:spring-expression:jar:5.3.18:compile [INFO] | | +- org.springframework.boot:spring-boot-autoconfigure:jar:2.6.6:compile [INFO] | | +- org.springframework.boot:spring-boot-starter-logging:jar:2.6.6:compile [INFO] | | | +- ch.qos.logback:logback-classic:jar:1.2.11:compile [INFO] | | | | - ch.qos.logback:logback-core:jar:1.2.11:compile [INFO] | | | +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.17.2:compile [INFO] | | | | - org.apache.logging.log4j:log4j-api:jar:2.17.2:compile [INFO] | | | - org.slf4j:jul-to-slf4j:jar:1.7.36:compile [INFO] | | +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:compile [INFO] | | - org.yaml:snakeyaml:jar:1.29:compile [INFO] | +- org.springframework.boot:spring-boot-starter-json:jar:2.6.6:compile [INFO] | | +- com.fasterxml.jackson.core:jackson-databind:jar:2.13.2.2:compile [INFO] | | | +- com.fasterxml.jackson.core:jackson-annotations:jar:2.13.2:compile [INFO] | | | - com.fasterxml.jackson.core:jackson-core:jar:2.13.2:compile [INFO] | | +- com.fasterxml.jackson.datatype:jackson-datatype-jdk8:jar:2.13.2:compile [INFO] | | +- com.fasterxml.jackson.datatype:jackson-datatype-jsr310:jar:2.13.2:compile [INFO] | | - com.fasterxml.jackson.module:jackson-module-parameter-names:jar:2.13.2:compile [INFO] | +- org.springframework.boot:spring-boot-starter-reactor-netty:jar:2.6.6:compile [INFO] | | - io.projectreactor.netty:reactor-netty-http:jar:1.0.17:compile [INFO] | | +- io.netty:netty-codec-http:jar:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-common:jar:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-buffer:jar:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-transport:jar:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-codec:jar:4.1.75.Final:compile [INFO] | | | - io.netty:netty-handler:jar:4.1.75.Final:compile [INFO] | | +- io.netty:netty-codec-http2:jar:4.1.75.Final:compile [INFO] | | +- io.netty:netty-resolver-dns:jar:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-resolver:jar:4.1.75.Final:compile [INFO] | | | - io.netty:netty-codec-dns:jar:4.1.75.Final:compile [INFO] | | +- io.netty:netty-resolver-dns-native-macos:jar:osx-x86_64:4.1.75.Final:compile [INFO] | | | - io.netty:netty-resolver-dns-classes-macos:jar:4.1.75.Final:compile [INFO] | | +- io.netty:netty-transport-native-epoll:jar:linux-x86_64:4.1.75.Final:compile [INFO] | | | +- io.netty:netty-transport-native-unix-common:jar:4.1.75.Final:compile [INFO] | | | - io.netty:netty-transport-classes-epoll:jar:4.1.75.Final:compile [INFO] | | - io.projectreactor.netty:reactor-netty-core:jar:1.0.17:compile [INFO] | | - io.netty:netty-handler-proxy:jar:4.1.75.Final:compile [INFO] | | - io.netty:netty-codec-socks:jar:4.1.75.Final:compile [INFO] | +- org.springframework:spring-web:jar:5.3.18:compile [INFO] | | - org.springframework:spring-beans:jar:5.3.18:compile [INFO] | - org.springframework:spring-webflux:jar:5.3.18:compile
If you are starting a new project you can use the spring initializer – https://start.spring.io/