Add debug support for MvcResult

This commit adds a debug() shortcut to print the detail of a MvcResult
to ease debugging when a request did not work as expected.

Closes gh-33059

Author: snicoll

spring-test/src/main/java/org/springframework/test/web/servlet/assertj/MvcTestResultAssert.java

@@ -17,6 +17,7 @@
 package org.springframework.test.web.servlet.assertj;
 
 import java.io.BufferedReader;
+import java.io.OutputStream;
 import java.io.PrintWriter;
 import java.io.StringReader;
 import java.io.StringWriter;
@@ -37,6 +38,7 @@
 import org.springframework.test.web.servlet.MvcResult;
 import org.springframework.test.web.servlet.ResultHandler;
 import org.springframework.test.web.servlet.ResultMatcher;
+import org.springframework.test.web.servlet.result.MockMvcResultHandlers;
 import org.springframework.web.servlet.ModelAndView;
 
 /**
@@ -136,6 +138,26 @@ public ObjectAssert<Object> asyncResult() {
 		return Assertions.assertThat(getMvcResult().getAsyncResult()).as("Async result");
 	}
 
+	/**
+	 * Print {@link MvcResult} details to {@code System.out}.
+	 * <p>You must call it <b>before</b> calling the assertion otherwise it is ignored
+	 * as the failing assertion breaks the chained call by throwing an
+	 * AssertionError.
+	 */
+	public MvcTestResultAssert debug() {
+		return debug(System.out);
+	}
+
+	/**
+	 * Print {@link MvcResult} details to the supplied {@link OutputStream}.
+	 * <p>You must call it <b>before</b> calling the assertion otherwise it is ignored
+	 * as the failing assertion breaks the chained call by throwing an
+	 * AssertionError.
+	 */
+	public MvcTestResultAssert debug(OutputStream stream) {
+		return apply(MockMvcResultHandlers.print(stream));
+	}
+
 	/**
 	 * Verify that the request has failed.
 	 */

spring-test/src/test/java/org/springframework/test/web/servlet/assertj/MockMvcTesterIntegrationTests.java

@@ -16,6 +16,9 @@
 
 package org.springframework.test.web.servlet.assertj;
 
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -30,6 +33,8 @@
 import jakarta.servlet.http.HttpServletResponse;
 import jakarta.validation.Valid;
 import jakarta.validation.constraints.Size;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 
@@ -275,6 +280,44 @@ void isInvokedOn() {
 		}
 	}
 
+	@Nested
+	class DebugTests {
+
+		private final PrintStream standardOut = System.out;
+
+		private final ByteArrayOutputStream capturedOut = new ByteArrayOutputStream();
+
+		@BeforeEach
+		public void setUp() {
+			System.setOut(new PrintStream(capturedOut));
+		}
+
+		@AfterEach
+		public void tearDown() {
+			System.setOut(standardOut);
+		}
+
+		@Test
+		void debugUsesSystemOutByDefault() {
+			assertThat(mvc.get().uri("/greet")).debug().hasStatusOk();
+			assertThat(capturedOut()).contains("MockHttpServletRequest:", "MockHttpServletResponse:");
+		}
+
+		@Test
+		void debugCanPrintToCustomOutputStream() {
+			ByteArrayOutputStream out = new ByteArrayOutputStream();
+			assertThat(mvc.get().uri("/greet")).debug(out).hasStatusOk();
+			assertThat(out.toString(StandardCharsets.UTF_8))
+					.contains("MockHttpServletRequest:", "MockHttpServletResponse:");
+			assertThat(capturedOut()).isEmpty();
+		}
+
+		private String capturedOut() {
+			return this.capturedOut.toString(StandardCharsets.UTF_8);
+		}
+
+	}
+
 	@Nested
 	class ExceptionTests {