IK.AM

Let's try using Spring MVC's Functional Endpoints, commonly known as WebMvc.fn, on Embedded Tomcat without Spring Boot.

First, we define the Functional Endpoints. We will write a lambda expression for an endpoint that outputs the classic "Hello World".

package com.example;

import org.springframework.web.servlet.function.RouterFunction;
import org.springframework.web.servlet.function.RouterFunctions;
import org.springframework.web.servlet.function.ServerResponse;

public class Routing {
    public RouterFunction<ServerResponse> routes() {
        return RouterFunctions.route()
                .GET("/", request -> ServerResponse.ok().body("Hello World!"))
                .build();
    }
}

The class to start the Application will look like this. It only includes the definition of DispatcherServlet and the setup of Embedded Tomcat. Although it has a bit of boilerplate, it is quite simple.

package com.example;

import java.util.List;
import java.util.Optional;

import org.apache.catalina.Context;
import org.apache.catalina.startup.Tomcat;

import org.springframework.http.converter.ByteArrayHttpMessageConverter;
import org.springframework.http.converter.StringHttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
import org.springframework.web.context.support.StaticWebApplicationContext;
import org.springframework.web.servlet.DispatcherServlet;
import org.springframework.web.servlet.HandlerMapping;
import org.springframework.web.servlet.function.RouterFunction;
import org.springframework.web.servlet.function.ServerResponse;
import org.springframework.web.servlet.function.support.RouterFunctionMapping;

public class Application {

    public static void main(String[] args) throws Exception {
        startTomcat();
    }

    static StaticWebApplicationContext applicationContext(RouterFunction<ServerResponse> routes) {
        StaticWebApplicationContext applicationContext = new StaticWebApplicationContext();
        applicationContext.registerBean(DispatcherServlet.HANDLER_MAPPING_BEAN_NAME,
                HandlerMapping.class, () -> {
                    RouterFunctionMapping mapping = new RouterFunctionMapping(routes);
                    mapping.setMessageConverters(List.of(
                            new StringHttpMessageConverter(),
                            new ByteArrayHttpMessageConverter(),
                            new AllEncompassingFormHttpMessageConverter(),
                            new MappingJackson2HttpMessageConverter()
                    ));
                    return mapping;
                });
        return applicationContext;
    }


    static void startTomcat() throws Exception {
        Tomcat tomcat = new Tomcat();
        int port = Optional.ofNullable(System.getenv("PORT")).map(Integer::parseInt).orElse(8080);
        tomcat.getConnector().setPort(port);
        Context context = tomcat.addContext("", System.getProperty("java.io.tmpdir"));
        DispatcherServlet dispatcherServlet = new DispatcherServlet(applicationContext(new Routing().routes()));
        Tomcat.addServlet(context, "dispatcherServlet", dispatcherServlet).addMapping("/");
        tomcat.start();
        tomcat.getServer().await();
    }

}

The pom.xml will look like this. We use Spring Boot for dependency management, but we don't need to use Boot in the application.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.1.1</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>vanilla-mvcfn</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>vanilla-mvcfn</name>
    <description>vanilla-mvcfn</description>
    <properties>
        <java.version>17</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
        <dependency>
            <groupId>org.apache.tomcat.embed</groupId>
            <artifactId>tomcat-embed-core</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

Now, you can run the Application class or create an executable jar with ./mvnw clean package and run it. Accessing http://localhost:8080 will return "Hello World!".

$ curl localhost:8080
Hello World!

You can write tests using MockMvc as follows. As of Spring 6.0, you cannot use MockMvc standalone like with @Controller, so you need to go through WebApplicationContext, which is a bit verbose. Once https://github.com/spring-projects/spring-framework/issues/30477 is addressed, it will be easier to write tests.

package com.example;

import org.junit.jupiter.api.Test;

import org.springframework.mock.web.MockServletContext;
import org.springframework.test.web.servlet.MockMvc;
import org.springframework.test.web.servlet.setup.MockMvcBuilders;
import org.springframework.web.context.support.StaticWebApplicationContext;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

class ApplicationTests {
    MockMvc mockMvc = initMockMvc();

    MockMvc initMockMvc() {
        StaticWebApplicationContext applicationContext = Application.applicationContext(new Routing().routes());
        applicationContext.setServletContext(new MockServletContext());
        applicationContext.refresh();
        // https://github.com/spring-projects/spring-framework/issues/30477
        return MockMvcBuilders.webAppContextSetup(applicationContext).build();
    }

    @Test
    void hello() throws Exception {
        this.mockMvc.perform(get("/"))
                .andExpect(status().isOk())
                .andExpect(content().string("Hello World!"));
    }

}

The source code is available at https://github.com/making/vanilla-mvcfn.

By the way, if you use Spring Boot, you can simplify the code as follows.

package com.example;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.function.RouterFunction;
import org.springframework.web.servlet.function.RouterFunctions;
import org.springframework.web.servlet.function.ServerResponse;

@Configuration
public class Routing {

    @Bean
    public RouterFunction<ServerResponse> routes() {
        return RouterFunctions.route()
                .GET("/", request -> ServerResponse.ok().body("Hello World!"))
                .build();
    }
}

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}

Changes to pom.xml and test code are omitted.

You can check the full diff at https://github.com/making/vanilla-mvcfn/compare/boot.