Update Date/Time formatting tests for pre/post JDK 20

This commit updates our Date/Time formatting/printing tests to
demonstrate that the use of fallback patterns can help mitigate
locale-based parsing/formatting issues beginning with JDK 20.

The documentation within the tests is intentionally rather thorough for
two reasons:

1. We need to understand exactly what it is we are testing and why the
   tests are written that way.

2. We may re-use parts of the documentation and examples in forthcoming
   documentation that we will provide to users.

See gh-33151

Author: sbrannen

spring-context/src/test/java/org/springframework/format/datetime/DateFormattingTests.java

@@ -339,6 +339,70 @@ void isoDate(String propertyValue) {
 			assertThat(bindingResult.getFieldValue(propertyName)).isEqualTo("2021-03-02");
 		}
 
+		/**
+		 * {@link SimpleDateBean#styleDateTimeWithFallbackPatternsForPreAndPostJdk20}
+		 * configures "SS" as the date/time style to use. Thus, we have to be aware
+		 * of the following if we do not configure fallback patterns for parsing.
+		 *
+		 * <ul>
+		 * <li>JDK &le; 19 requires a standard space before the "PM".
+		 * <li>JDK &ge; 20 requires a narrow non-breaking space (NNBSP) before the "PM".
+		 * </ul>
+		 *
+		 * <p>To avoid compatibility issues between JDK versions, we have configured
+		 * two fallback patterns which emulate the "SS" style: <code>"MM/dd/yy h:mm a"</code>
+		 * matches against a standard space before the "PM", and <code>"MM/dd/yy h:mm&#92;u202Fa"</code>
+		 * matches against a narrow non-breaking space (NNBSP) before the "PM".
+		 *
+		 * <p>Thus, the following should theoretically be supported on any JDK (or at least
+		 * JDK 17 - 23, where we have tested it).
+		 *
+		 * @see #patternDateTime(String)
+		 */
+		@ParameterizedTest(name = "input date: {0}")  // gh-33151
+		@ValueSource(strings = {"10/31/09, 12:00 PM", "10/31/09, 12:00\u202FPM"})
+		void styleDateTime_PreAndPostJdk20(String propertyValue) {
+			String propertyName = "styleDateTimeWithFallbackPatternsForPreAndPostJdk20";
+			MutablePropertyValues propertyValues = new MutablePropertyValues();
+			propertyValues.add(propertyName, propertyValue);
+			binder.bind(propertyValues);
+			BindingResult bindingResult = binder.getBindingResult();
+			assertThat(bindingResult.getErrorCount()).isEqualTo(0);
+			String value = binder.getBindingResult().getFieldValue(propertyName).toString();
+			// Since the "SS" style is always used for printing and the underlying format
+			// changes depending on the JDK version, we cannot be certain that a normal
+			// space is used before the "PM". Consequently we have to use a regular
+			// expression to match against any Unicode space character (\p{Zs}).
+			assertThat(value).startsWith("10/31/09").matches(".+?12:00\\p{Zs}PM");
+		}
+
+		/**
+		 * To avoid the use of Locale-based styles (such as "MM") for
+		 * {@link SimpleDateBean#patternDateTimeWithFallbackPatternForPreAndPostJdk20}, we have configured a
+		 * primary pattern (<code>"MM/dd/yy h:mm a"</code>) that matches against a standard space
+		 * before the "PM" and a fallback pattern (<code>"MM/dd/yy h:mm&#92;u202Fa"</code> that matches
+		 * against a narrow non-breaking space (NNBSP) before the "PM".
+		 *
+		 * <p>Thus, the following should theoretically be supported on any JDK (or at least
+		 * JDK 17 - 23, where we have tested it).
+		 *
+		 * @see #styleDateTime(String)
+		 */
+		@ParameterizedTest(name = "input date: {0}")  // gh-33151
+		@ValueSource(strings = {"10/31/09 3:45 PM", "10/31/09 3:45\u202FPM"})
+		void patternDateTime_PreAndPostJdk20(String propertyValue) {
+			String propertyName = "patternDateTimeWithFallbackPatternForPreAndPostJdk20";
+			MutablePropertyValues propertyValues = new MutablePropertyValues();
+			propertyValues.add(propertyName, propertyValue);
+			binder.bind(propertyValues);
+			BindingResult bindingResult = binder.getBindingResult();
+			assertThat(bindingResult.getErrorCount()).isEqualTo(0);
+			String value = binder.getBindingResult().getFieldValue(propertyName).toString();
+			// Since the "MM/dd/yy h:mm a" primary pattern is always used for printing, we
+			// can be certain that a normal space is used before the "PM".
+			assertThat(value).matches("10/31/09 3:45 PM");
+		}
+
 		@Test
 		void patternDateWithUnsupportedPattern() {
 			String propertyValue = "210302";
@@ -389,12 +453,23 @@ private static class SimpleDateBean {
 		@DateTimeFormat(style = "S-", fallbackPatterns = { "yyyy-MM-dd", "yyyyMMdd", "yyyy.MM.dd" })
 		private Date styleDateWithFallbackPatterns;
 
+		// "SS" style matches either a standard space or a narrow non-breaking space (NNBSP) before AM/PM,
+		// depending on the version of the JDK.
+		// Fallback patterns match a standard space OR a narrow non-breaking space (NNBSP) before AM/PM.
+		@DateTimeFormat(style = "SS", fallbackPatterns = { "M/d/yy, h:mm a", "M/d/yy, h:mm\u202Fa" })
+		private Date styleDateTimeWithFallbackPatternsForPreAndPostJdk20;
+
 		@DateTimeFormat(pattern = "M/d/yy h:mm")
 		private Date patternDate;
 
 		@DateTimeFormat(pattern = "yyyy-MM-dd", fallbackPatterns = { "M/d/yy", "yyyyMMdd", "yyyy.MM.dd" })
 		private Date patternDateWithFallbackPatterns;
 
+		// Primary pattern matches a standard space before AM/PM.
+		// Fallback pattern matches a narrow non-breaking space (NNBSP) before AM/PM.
+		@DateTimeFormat(pattern = "MM/dd/yy h:mm a", fallbackPatterns = "MM/dd/yy h:mm\u202Fa")
+		private Date patternDateTimeWithFallbackPatternForPreAndPostJdk20;
+
 		@DateTimeFormat(iso = ISO.DATE)
 		private Date isoDate;
 
@@ -459,6 +534,14 @@ public void setStyleDateWithFallbackPatterns(Date styleDateWithFallbackPatterns)
 			this.styleDateWithFallbackPatterns = styleDateWithFallbackPatterns;
 		}
 
+		public Date getStyleDateTimeWithFallbackPatternsForPreAndPostJdk20() {
+			return this.styleDateTimeWithFallbackPatternsForPreAndPostJdk20;
+		}
+
+		public void setStyleDateTimeWithFallbackPatternsForPreAndPostJdk20(Date styleDateTimeWithFallbackPatternsForPreAndPostJdk20) {
+			this.styleDateTimeWithFallbackPatternsForPreAndPostJdk20 = styleDateTimeWithFallbackPatternsForPreAndPostJdk20;
+		}
+
 		public Date getPatternDate() {
 			return this.patternDate;
 		}
@@ -475,6 +558,14 @@ public void setPatternDateWithFallbackPatterns(Date patternDateWithFallbackPatte
 			this.patternDateWithFallbackPatterns = patternDateWithFallbackPatterns;
 		}
 
+		public Date getPatternDateTimeWithFallbackPatternForPreAndPostJdk20() {
+			return this.patternDateTimeWithFallbackPatternForPreAndPostJdk20;
+		}
+
+		public void setPatternDateTimeWithFallbackPatternForPreAndPostJdk20(Date patternDateTimeWithFallbackPatternForPreAndPostJdk20) {
+			this.patternDateTimeWithFallbackPatternForPreAndPostJdk20 = patternDateTimeWithFallbackPatternForPreAndPostJdk20;
+		}
+
 		public Date getIsoDate() {
 			return this.isoDate;
 		}

spring-context/src/test/java/org/springframework/format/datetime/standard/DateTimeFormattingTests.java

@@ -605,6 +605,41 @@ private void styleLocalTime(String propertyValue) {
 			assertThat(bindingResult.getFieldValue(propertyName)).asString().matches("12:00:00\\p{Zs}PM");
 		}
 
+		/**
+		 * {@link DateTimeBean#styleLocalTimeWithFallbackPatternsForPreAndPostJdk20}
+		 * configures "-M" as the time style to use. Thus, we have to be aware
+		 * of the following if we do not configure fallback patterns for parsing.
+		 *
+		 * <ul>
+		 * <li>JDK &le; 19 requires a standard space before the "PM".
+		 * <li>JDK &ge; 20 requires a narrow non-breaking space (NNBSP) before the "PM".
+		 * </ul>
+		 *
+		 * <p>To avoid compatibility issues between JDK versions, we have configured
+		 * two fallback patterns which emulate the "-M" style: <code>"HH:mm:ss a"</code>
+		 * matches against a standard space before the "PM", and <code>"HH:mm:ss&#92;u202Fa"</code>
+		 * matches against a narrow non-breaking space (NNBSP) before the "PM".
+		 *
+		 * <p>Thus, the following should theoretically be supported on any JDK (or at least
+		 * JDK 17 - 23, where we have tested it).
+		 */
+		@ParameterizedTest(name = "input date: {0}")  // gh-33151
+		@ValueSource(strings = { "12:00:00 PM", "12:00:00\u202FPM" })
+		void styleLocalTime_PreAndPostJdk20(String propertyValue) {
+			String propertyName = "styleLocalTimeWithFallbackPatternsForPreAndPostJdk20";
+			MutablePropertyValues propertyValues = new MutablePropertyValues();
+			propertyValues.add(propertyName, propertyValue);
+			binder.bind(propertyValues);
+			BindingResult bindingResult = binder.getBindingResult();
+			assertThat(bindingResult.getErrorCount()).isEqualTo(0);
+			String value = binder.getBindingResult().getFieldValue(propertyName).toString();
+			// Since the "-M" style is always used for printing and the underlying format
+			// changes depending on the JDK version, we cannot be certain that a normal
+			// space is used before the "PM". Consequently we have to use a regular
+			// expression to match against any Unicode space character (\p{Zs}).
+			assertThat(value).matches("12:00:00\\p{Zs}PM");
+		}
+
 		@ParameterizedTest(name = "input date: {0}")
 		@ValueSource(strings = {"2021-03-02T12:00:00", "2021-03-02 12:00:00", "3/2/21 12:00"})
 		void isoLocalDateTime(String propertyValue) {
@@ -682,6 +717,12 @@ public static class DateTimeBean {
 		@DateTimeFormat(style = "-M", fallbackPatterns = {"HH:mm:ss", "HH:mm"})
 		private LocalTime styleLocalTimeWithFallbackPatterns;
 
+		// "-M" style matches either a standard space or a narrow non-breaking space (NNBSP) before AM/PM,
+		// depending on the version of the JDK.
+		// Fallback patterns match a standard space OR a narrow non-breaking space (NNBSP) before AM/PM.
+		@DateTimeFormat(style = "-M", fallbackPatterns = {"HH:mm:ss a", "HH:mm:ss\u202Fa"})
+		private LocalTime styleLocalTimeWithFallbackPatternsForPreAndPostJdk20;
+
 		private LocalDateTime localDateTime;
 
 		@DateTimeFormat(style = "MM")
@@ -782,6 +823,15 @@ public void setStyleLocalTimeWithFallbackPatterns(LocalTime styleLocalTimeWithFa
 			this.styleLocalTimeWithFallbackPatterns = styleLocalTimeWithFallbackPatterns;
 		}
 
+		public LocalTime getStyleLocalTimeWithFallbackPatternsForPreAndPostJdk20() {
+			return this.styleLocalTimeWithFallbackPatternsForPreAndPostJdk20;
+		}
+
+		public void setStyleLocalTimeWithFallbackPatternsForPreAndPostJdk20(
+				LocalTime styleLocalTimeWithFallbackPatternsForPreAndPostJdk20) {
+			this.styleLocalTimeWithFallbackPatternsForPreAndPostJdk20 = styleLocalTimeWithFallbackPatternsForPreAndPostJdk20;
+		}
+
 		public LocalDateTime getLocalDateTime() {
 			return this.localDateTime;
 		}