Create a WebFlux application from Scratch

@Data
@ToString
@Builder
@NoArgsConstructor
@AllArgsConstructor
class Post {
    
    private Long id;
    private String title;
    private String content;
    
}

@Component
class PostRepository {

    private static final Map<Long, Post> DATA = new HashMap<>();
    private static long ID_COUNTER = 1L;

    static {
        Arrays.asList("First Post", "Second Post")
            .stream()
            .forEach((java.lang.String title) -> {
                long id = ID_COUNTER++;
                DATA.put(Long.valueOf(id), Post.builder().id(id).title(title).content("content of " + title).build());
            }
            );
    }

    Flux<Post> findAll() {
        return Flux.fromIterable(DATA.values());
    }

    Mono<Post> findById(Long id) {
        return Mono.just(DATA.get(id));
    }

    Mono<Post> createPost(Post post) {
        long id = ID_COUNTER++;
        post.setId(id);
        DATA.put(id, post);
        return Mono.just(post);
    }

}

@RestController
@RequestMapping(value = "/posts")
class PostController {
    
    private final PostRepository posts;

    public PostController(PostRepository posts) {
        this.posts = posts;
    }

    @GetMapping(value = "")
    public Flux<Post> all() {
        return this.posts.findAll();
    }

    @GetMapping(value = "/{id}")
    public Mono<Post> get(@PathVariable(value = "id") Long id) {
        return this.posts.findById(id);
    }
    
    @PostMapping(value = "")
    public Mono<Post> create(Post post) {
        return this.posts.createPost(post);
    }
   
}

@Configuration
@ComponentScan
@EnableWebFlux
class WebConfig {
    
}

@Configuration
@ComponentScan
@PropertySource(value = "classpath:application.properties", ignoreResourceNotFound = true)
public class Application {

    @Value("${server.port:8080}")
    private int port = 8080;

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(Application.class);  // (1)

        Tomcat tomcatServer = context.getBean(Tomcat.class);
        tomcatServer.start();

        System.out.println("Tomcat server is running at port:"
                           + tomcatServer.getConnector().getLocalPort());
    	//System.in.read();
    }

    @Bean
    @Profile("default")
    public Tomcat embeddedTomcatServer(ApplicationContext context) {
        HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();

        Servlet servlet = new TomcatHttpHandlerAdapter(handler);
        Tomcat tomcat = new Tomcat();

        File base = new File(System.getProperty("java.io.tmpdir"));
        Context rootContext = tomcat.addContext("", base.getAbsolutePath());
        Tomcat.addServlet(rootContext, "main", servlet).setAsyncSupported(true);
        rootContext.addServletMappingDecoded("/", "main");

        tomcat.setHostname("localhost");
        tomcat.setPort(this.port);
        tomcat.setBaseDir(System.getProperty("java.io.tmpdir"));

        return tomcat;
    }

}

@Configuration
@ComponentScan
@PropertySource(value = "classpath:application.properties", ignoreResourceNotFound = true)
public class Application {

    @Value("${server.port:8080}")
    private int port = 8080;

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(Application.class);  // (1)
        Server server = context.getBean(Server.class);
        server.start();
        server.join();

        System.out.println("Press ENTER to exit.");
        System.in.read();
    }

    @Bean
    public Server jettyServer(ApplicationContext context) throws Exception {
        HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();
        Servlet servlet = new JettyHttpHandlerAdapter(handler);

        Server server = new Server();
        ServletContextHandler contextHandler = new ServletContextHandler(server, "");
        contextHandler.addServlet(new ServletHolder(servlet), "/");
        contextHandler.start();

        ServerConnector connector = new ServerConnector(server);
        connector.setHost("localhost");
        connector.setPort(port);
        server.addConnector(connector);

        return server;
    }

}

@Configuration
@ComponentScan
@PropertySource(value = "classpath:application.properties", ignoreResourceNotFound = true)
public class Application {

    @Value("${server.port:8080}")
    private int port = 8080;

    public static void main(String[] args) throws Exception {
        try (AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
                Application.class)) {
            context.getBean(HttpServer.class)
                    .bindUntilJavaShutdown(Duration.ofSeconds(60), null);
        }
    }

    @Bean
    public HttpServer httpServer(ApplicationContext context) {
        HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();
        ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
        return  HttpServer.create()
                .host("localhost")
                .port(this.port)
                .handle(adapter);
    }

}

@Configuration
@ComponentScan
@PropertySource(value = "classpath:application.properties", ignoreResourceNotFound = true)
public class Application {

    @Value("${server.port:8080}")
    private int port = 8080;

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(Application.class);  // (1)
        Undertow server = context.getBean(Undertow.class);
        server.start();
        System.out.println("Press ENTER to exit.");
        System.in.read();
    }

    @Bean
    public Undertow undertowServer(ApplicationContext context) {
        HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();  // (2)

        // Undertow
        UndertowHttpHandlerAdapter undertowAdapter = new UndertowHttpHandlerAdapter(handler);
        Undertow server = Undertow.builder()
            .addHttpListener(port, "localhost")
            .setHandler(undertowAdapter)
            .build();

        return server;
    }

}

public class AppInitializer extends AbstractReactiveWebInitializer {

    @Override
    protected Class<?>[] getConfigClasses() {
        return new Class[]{
            WebConfig.class,
            SecurityConfig.class
        };
    }
}

public class Application {

    public static void main(String[] args) throws Exception {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
        PostRepository posts = new PostRepository();
        PostHandler postHandler = new PostHandler(posts);
        Routes routesBean = new Routes(postHandler);

        context.registerBean(PostRepository.class, () -> posts);
        context.registerBean(PostHandler.class, () -> postHandler);
        context.registerBean(Routes.class, () -> routesBean);
        context.registerBean(WebHandler.class, () -> RouterFunctions.toWebHandler(routesBean.routes(), HandlerStrategies.builder().build()));
        context.refresh();

        nettyServer(context).onDispose().block();
    }

    public static DisposableServer nettyServer(ApplicationContext context) {
        HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context).build();
        ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
        HttpServer httpServer = HttpServer.create().host("localhost").port(8080);
        return httpServer.handle(adapter).bindNow();
    }

}