Improve MediaTypeAssert to-string-equality assertions

This commit introduces a `isNotEqualTo(String)` assertion in order to
avoid false negatives when using the default Object-based assertion.

This is a risk since MediaTypeAssert has a `isEqualTo(String)` method
that overrides the base object method and parses the provided String
into a MediaType. Users may thus be tempted to use the reverse assertion
and expect the same parsing behaviour.

This commit also adds tests around the String parsing and the isNotEqual
cases.

Closes gh-32756

Author: simonbasle

spring-test/src/main/java/org/springframework/test/http/MediaTypeAssert.java

@@ -49,10 +49,21 @@ public MediaTypeAssert(@Nullable MediaType mediaType) {
 	/**
 	 * Verify that the actual media type is equal to the given string
 	 * representation.
-	 * @param expected the expected media type
+	 * @param mediaType the expected media type, as a String to be parsed
+	 * into a MediaType
+*/
+	public MediaTypeAssert isEqualTo(String mediaType) {
+		return isEqualTo(parseMediaType(mediaType));
+	}
+
+	/**
+	 * Verify that the actual media type is not equal to the given string
+	 * representation.
+	 * @param mediaType the given media type, as a String to be parsed
+	 * into a MediaType
 	 */
-	public MediaTypeAssert isEqualTo(String expected) {
-		return isEqualTo(parseMediaType(expected));
+	public MediaTypeAssert isNotEqualTo(String mediaType) {
+		return isNotEqualTo(parseMediaType(mediaType));
 	}
 
 	/**
@@ -84,7 +95,8 @@ public MediaTypeAssert isCompatibleWith(MediaType mediaType) {
 	 * // Check that actual is compatible with "text/plain"
 	 * assertThat(mediaType).isCompatibleWith("text/plain");
 	 * </code></pre>
-	 * @param mediaType the media type with which to compare
+	 * @param mediaType the media type with which to compare, as a String
+	 * to be parsed into a MediaType
 	 */
 	public MediaTypeAssert isCompatibleWith(String mediaType) {
 		return isCompatibleWith(parseMediaType(mediaType));

spring-test/src/test/java/org/springframework/test/http/MediaTypeAssertTests.java

@@ -43,19 +43,35 @@ void actualStringCanBeNull() {
 	}
 
 	@Test
-	void isEqualWhenSameShouldPass() {
+	void isEqualWhenActualIsNullStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(null).isEqualTo("text/html"))
+				.withMessageContaining("Media type");
+	}
+
+	@Test
+	void isEqualWhenSameStringShouldPass() {
 		assertThat(mediaType("application/json")).isEqualTo("application/json");
 	}
 
 	@Test
-	void isEqualWhenDifferentShouldFail() {
+	void isEqualWhenDifferentStringShouldFail() {
 		assertThatExceptionOfType(AssertionError.class)
 				.isThrownBy(() -> assertThat(mediaType("application/json")).isEqualTo("text/html"))
 				.withMessageContaining("Media type");
 	}
 
 	@Test
-	void isEqualWhenActualIsNullShouldFail() {
+	void isEqualInvalidStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isEqualTo("example of a bad value"))
+				.withMessageContaining("[Media type]")
+				.withMessageEndingWith("To be a valid media type but got:\n" +
+						"  \"Invalid mime type \"example of a bad value\": does not contain '/'\"\n");
+	}
+
+	@Test
+	void isEqualWhenActualIsNullTypeShouldFail() {
 		assertThatExceptionOfType(AssertionError.class)
 				.isThrownBy(() -> assertThat(null).isEqualTo(MediaType.APPLICATION_JSON))
 				.withMessageContaining("Media type");
@@ -73,6 +89,49 @@ void isEqualWhenDifferentTypeShouldFail() {
 				.withMessageContaining("Media type");
 	}
 
+	@Test
+	void isNotEqualWhenActualIsNullStringShouldPass() {
+		assertThat(null).isNotEqualTo("application/json");
+	}
+
+	@Test
+	void isNotEqualWhenDifferentStringShouldPass() {
+		assertThat(mediaType("application/json")).isNotEqualTo("text/html");
+	}
+
+	@Test
+	void isNotEqualWhenSameStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo("application/json"))
+				.withMessageContaining("Media type");
+	}
+
+	@Test
+	void isNotEqualInvalidStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo("example of a bad value"))
+				.withMessageContaining("[Media type]")
+				.withMessageEndingWith("To be a valid media type but got:\n" +
+						"  \"Invalid mime type \"example of a bad value\": does not contain '/'\"\n");
+	}
+
+	@Test
+	void isNotEqualWhenActualIsNullTypeShouldPass() {
+		assertThat(null).isNotEqualTo(MediaType.APPLICATION_JSON);
+	}
+
+	@Test
+	void isNotEqualWhenDifferentTypeShouldPass() {
+		assertThat(mediaType("application/json")).isNotEqualTo(MediaType.TEXT_HTML);
+	}
+
+	@Test
+	void isNotEqualWhenSameTypeShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo(MediaType.APPLICATION_JSON))
+				.withMessageContaining("Media type");
+	}
+
 	@Test
 	void isCompatibleWhenSameShouldPass() {
 		assertThat(mediaType("application/json")).isCompatibleWith("application/json");