(Re)introduce CronExpression::isValidExpression

This commit introduces CronExpression::isValidExpression, which was also
present on the deprecated CronSequenceGenerator.

Closes: gh-26996

spring-context/src/main/java/org/springframework/scheduling/support/CronExpression.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -202,6 +202,25 @@ public final class CronExpression {
 		}
 	}
 
+	/**
+	 * Determine whether the given string represents a valid cron expression.
+	 * @param expression the expression to evaluate
+	 * @return {@code true} if the given expression is a valid cron expression
+	 * @since 5.3.8
+	 */
+	public static boolean isValidExpression(@Nullable String expression) {
+		if (expression == null) {
+			return false;
+		}
+		try {
+			parse(expression);
+			return true;
+		}
+		catch (IllegalArgumentException ex) {
+			return false;
+		}
+	}
+
 
 	private static String resolveMacros(String expression) {
 		expression = expression.trim();

spring-context/src/test/java/org/springframework/scheduling/support/CronExpressionTests.java

@@ -51,6 +51,16 @@ class CronExpressionTests {
 		}
 	};
 
+	@Test
+	public void isValidExpression() {
+		assertThat(CronExpression.isValidExpression(null)).isFalse();
+		assertThat(CronExpression.isValidExpression("")).isFalse();
+		assertThat(CronExpression.isValidExpression("*")).isFalse();
+		assertThat(CronExpression.isValidExpression("* * * * *")).isFalse();
+		assertThat(CronExpression.isValidExpression("* * * * * * *")).isFalse();
+
+		assertThat(CronExpression.isValidExpression("* * * * * *")).isTrue();
+	}
 
 	@Test
 	void matchAll() {