Polish and update reactive getting started reference

This commit updates the instructions on getting started with
Spring Web Reactive and also updates constructors and setters to
streamline the getting started procedure.

Issue: SPR-14640

spring-web-reactive/src/main/java/org/springframework/web/reactive/DispatcherHandler.java

@@ -71,6 +71,14 @@ public class DispatcherHandler implements WebHandler, ApplicationContextAware {
 	private List<HandlerResultHandler> resultHandlers;
 
 
+	public DispatcherHandler() {
+	}
+
+	public DispatcherHandler(ApplicationContext applicationContext) {
+		initStrategies(applicationContext);
+	}
+
+
 	@Override
 	public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
 		initStrategies(applicationContext);

spring-web-reactive/src/test/java/org/springframework/web/reactive/DispatcherHandlerErrorTests.java

@@ -87,8 +87,7 @@ public class DispatcherHandlerErrorTests {
 		appContext.register(TestConfig.class);
 		appContext.refresh();
 
-		this.dispatcherHandler = new DispatcherHandler();
-		this.dispatcherHandler.setApplicationContext(appContext);
+		this.dispatcherHandler = new DispatcherHandler(appContext);
 
 		this.request = new MockServerHttpRequest(HttpMethod.GET, new URI("/"));
 		MockServerHttpResponse response = new MockServerHttpResponse();

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/SimpleUrlHandlerMappingIntegrationTests.java

@@ -61,10 +61,7 @@ public class SimpleUrlHandlerMappingIntegrationTests extends AbstractHttpHandler
 		wac.register(WebConfig.class);
 		wac.refresh();
 
-		DispatcherHandler dispatcherHandler = new DispatcherHandler();
-		dispatcherHandler.setApplicationContext(wac);
-
-		return WebHttpHandlerBuilder.webHandler(dispatcherHandler)
+		return WebHttpHandlerBuilder.webHandler(new DispatcherHandler(wac))
 				.exceptionHandlers(new ResponseStatusExceptionHandler())
 				.build();
 	}

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/AbstractRequestMappingIntegrationTests.java

@@ -44,9 +44,9 @@ public abstract class AbstractRequestMappingIntegrationTests extends AbstractHtt
 	@Override
 	protected HttpHandler createHttpHandler() {
 		this.applicationContext = initApplicationContext();
-		DispatcherHandler handler = new DispatcherHandler();
-		handler.setApplicationContext(this.applicationContext);
-		return WebHttpHandlerBuilder.webHandler(handler).build();
+		return WebHttpHandlerBuilder
+				.webHandler(new DispatcherHandler(this.applicationContext))
+				.build();
 	}
 
 	protected abstract ApplicationContext initApplicationContext();

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/SseIntegrationTests.java

@@ -65,10 +65,7 @@ public class SseIntegrationTests extends AbstractHttpHandlerIntegrationTests {
 		this.wac.register(TestConfiguration.class);
 		this.wac.refresh();
 
-		DispatcherHandler webHandler = new DispatcherHandler();
-		webHandler.setApplicationContext(this.wac);
-
-		return WebHttpHandlerBuilder.webHandler(webHandler).build();
+		return WebHttpHandlerBuilder.webHandler(new DispatcherHandler(this.wac)).build();
 	}
 
 	@Test