Support Servlet error message in MockMvc assertions

Prior to this commit, `MockMvc` would support checking for the Servlet
error message as the "response status reason". While this error message
can be driven with the `@ResponseStatus` annotation, this message is not
technically the HTTP status reason listed on the response status line.

This message is provided by the Servlet container in the error page when
the `response.sendError(int, String)` method is used.

This commit adds the missing
`mvc.get().uri("/error/message")).hasErrorMessage("error message")`
assertion to check for this Servlet error message.

Closes gh-34016

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

@@ -22,6 +22,7 @@ import org.assertj.core.api.AbstractByteArrayAssert;
 import org.assertj.core.api.AbstractStringAssert;
 import org.assertj.core.api.Assertions;
 import org.assertj.core.api.ByteArrayAssert;
+import org.assertj.core.api.StringAssert;
 
 import org.springframework.lang.Nullable;
 import org.springframework.mock.web.MockHttpServletResponse;
@@ -163,4 +164,16 @@ public abstract class AbstractMockHttpServletResponseAssert<SELF extends Abstrac
 		return this.myself;
 	}
 
+	/**
+	 * Verify that the {@link jakarta.servlet.http.HttpServletResponse#sendError(int, String)} Servlet error message}
+	 * is equal to the given value.
+	 * @param errorMessage the expected Servlet error message (can be null)
+	 * @since 6.2.1
+	 */
+	public SELF hasErrorMessage(@Nullable String errorMessage) {
+		new StringAssert(getResponse().getErrorMessage())
+				.as("Servlet error message").isEqualTo(errorMessage);
+		return this.myself;
+	}
+
 }

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

@@ -110,6 +110,13 @@ public class AbstractMockHttpServletResponseAssertTests {
 				.withMessageContainingAll("Redirected URL", redirectedUrl, "another");
 	}
 
+	@Test
+	void hasServletErrorMessage() throws Exception{
+		MockHttpServletResponse response = new MockHttpServletResponse();
+		response.sendError(403, "expected error message");
+		assertThat(fromResponse(response)).hasErrorMessage("expected error message");
+	}
+
 
 	private MockHttpServletResponse createResponse(String body) {
 		try {

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

@@ -67,6 +67,7 @@ import org.springframework.web.bind.annotation.PutMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RequestPart;
+import org.springframework.web.bind.annotation.ResponseStatus;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.bind.annotation.SessionAttributes;
 import org.springframework.web.context.WebApplicationContext;
@@ -596,6 +597,13 @@ public class MockMvcTesterIntegrationTests {
 					result -> assertThat(result).hasRedirectedUrl("test"));
 		}
 
+		@Test
+		void assertErrorMessageWithUnresolvedException() {
+			assertThatExceptionOfType(AssertionError.class)
+					.isThrownBy(() -> assertThat(mvc.get().uri("/error/message")).hasErrorMessage("invalid"))
+					.withMessageContainingAll("[Servlet error message]", "invalid", "expected error message");
+		}
+
 		@Test
 		void assertRequestWithUnresolvedException() {
 			testAssertionFailureWithUnresolvableException(
@@ -798,6 +806,13 @@ public class MockMvcTesterIntegrationTests {
 		public String validation(@PathVariable @Size(max = 4) String id) {
 			return "Hello " + id;
 		}
+
+		@GetMapping("/error/message")
+		@ResponseStatus(code = HttpStatus.BAD_REQUEST, reason = "expected error message")
+		public void errorMessage() {
+
+		}
+
 	}
 
 }