Update MockMvcConfigurer support

This is a follow-up on the commit introducing MockMvcConfigurer:
https://github.com/spring-projects/spring-framework/commit/c2b0fac852dd9f865699fb374ad78543767fec05

This commit refines the MockMvcConfigurer contract to use (the new)
ConfigurableMockMvcBuilder hence not requiring downcasting to
AbstractMockMvcBuilder.

The same also no longer passes the "default" RequestBuilder which would
also require a downcast, but rather allows a RequestPostProcessor to be
returned so that a 3rd party framework or application can modify any
property of every performed MockHttpServletRequest.

To make this possible the new SmartRequestBuilder interface separates
request building from request post processing while the new
ConfigurableSmartRequestBuilder allows adding a RequestPostProcessor
to a MockMvcBuilder.

Issue: SPR-11497

spring-test/src/test/java/org/springframework/test/web/servlet/samples/standalone/FrameworkExtensionTests.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.test.web.servlet.samples.standalone;
 
+import static org.mockito.Mockito.mock;
 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;
@@ -28,56 +29,72 @@ import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.stereotype.Controller;
 import org.springframework.test.web.servlet.MockMvc;
 import org.springframework.test.web.servlet.request.RequestPostProcessor;
+import org.springframework.test.web.servlet.setup.ConfigurableMockMvcBuilder;
+import org.springframework.test.web.servlet.setup.MockMvcConfigurerAdapter;
+import org.springframework.util.Assert;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.ResponseBody;
+import org.springframework.web.context.WebApplicationContext;
+
+import java.security.Principal;
 
 /**
- * Demonstrates how to implement and plug in a custom {@link RequestPostProcessor}.
+ * Demonstrates use of SPI extension points:
+ * <ul>
+ * 	<li> {@link org.springframework.test.web.servlet.request.RequestPostProcessor}
+ * 	for extending request building with custom methods.
+ * 	<li> {@link org.springframework.test.web.servlet.setup.MockMvcConfigurer
+ * 	MockMvcConfigurer} for extending MockMvc building with some automatic setup.
+ * </ul>
  *
  * @author Rossen Stoyanchev
  */
-public class RequestBuilderTests {
+public class FrameworkExtensionTests {
 
 	private MockMvc mockMvc;
 
+
 	@Before
 	public void setup() {
-		this.mockMvc = standaloneSetup(new SampleController())
-				.defaultRequest(get("/").accept(MediaType.TEXT_PLAIN))
-				.alwaysExpect(status().isOk()).build();
+		this.mockMvc = standaloneSetup(new SampleController()).apply(defaultSetup()).build();
 	}
 
+
 	@Test
 	public void fooHeader() throws Exception {
-		this.mockMvc.perform(get("/").with(headers().foo("a=b"))).andExpect(
-				content().string("Foo"));
+		this.mockMvc.perform(get("/").with(headers().foo("a=b"))).andExpect(content().string("Foo"));
 	}
 
 	@Test
 	public void barHeader() throws Exception {
-		this.mockMvc.perform(get("/").with(headers().bar("a=b"))).andExpect(
-				content().string("Bar"));
+		this.mockMvc.perform(get("/").with(headers().bar("a=b"))).andExpect(content().string("Bar"));
 	}
 
-	private static HeaderRequestPostProcessor headers() {
-		return new HeaderRequestPostProcessor();
+
+	private static TestMockMvcConfigurer defaultSetup() {
+		return new TestMockMvcConfigurer();
 	}
 
+	private static TestRequestPostProcessor headers() {
+		return new TestRequestPostProcessor();
+	}
+
+
 
 	/**
-	 * Implementation of {@code RequestPostProcessor} with additional request
-	 * building methods.
+	 * Test {@code RequestPostProcessor}.
 	 */
-	private static class HeaderRequestPostProcessor implements RequestPostProcessor {
+	private static class TestRequestPostProcessor implements RequestPostProcessor {
 
 		private HttpHeaders headers = new HttpHeaders();
 
-		public HeaderRequestPostProcessor foo(String value) {
+
+		public TestRequestPostProcessor foo(String value) {
 			this.headers.add("Foo", value);
 			return this;
 		}
 
-		public HeaderRequestPostProcessor bar(String value) {
+		public TestRequestPostProcessor bar(String value) {
 			this.headers.add("Bar", value);
 			return this;
 		}
@@ -91,19 +108,40 @@ public class RequestBuilderTests {
 		}
 	}
 
+	/**
+	 * Test {@code MockMvcConfigurer}.
+	 */
+	private static class TestMockMvcConfigurer extends MockMvcConfigurerAdapter {
+
+		@Override
+		public void afterConfigurerAdded(ConfigurableMockMvcBuilder<?> builder) {
+			builder.alwaysExpect(status().isOk());
+		}
+
+		@Override
+		public RequestPostProcessor beforeMockMvcCreated(ConfigurableMockMvcBuilder<?> builder, WebApplicationContext context) {
+			return request -> {
+				request.setUserPrincipal(mock(Principal.class));
+				return request;
+			};
+		}
+	}
+
 	@Controller
 	@RequestMapping("/")
 	private static class SampleController {
 
 		@RequestMapping(headers = "Foo")
 		@ResponseBody
-		public String handleFoo() {
+		public String handleFoo(Principal principal) {
+			Assert.isTrue(principal != null);
 			return "Foo";
 		}
 
 		@RequestMapping(headers = "Bar")
 		@ResponseBody
-		public String handleBar() {
+		public String handleBar(Principal principal) {
+			Assert.isTrue(principal != null);
 			return "Bar";
 		}
 	}