新增: CronUtil.isValidExpression方法，验证传入的Cron表达式是否合法；完善PatternParser.parseGroupPattern逻辑，加入对传入表达式为null和空的校验。

hutool-cron/src/main/java/org/dromara/hutool/cron/CronUtil.java

@@ -20,6 +20,7 @@ package org.dromara.hutool.cron;
 import org.dromara.hutool.core.exception.HutoolException;
 import org.dromara.hutool.core.io.resource.NoResourceException;
 import org.dromara.hutool.cron.pattern.CronPattern;
+import org.dromara.hutool.cron.pattern.parser.PatternParser;
 import org.dromara.hutool.cron.task.Task;
 import org.dromara.hutool.setting.Setting;
 import org.dromara.hutool.setting.SettingException;
@@ -211,4 +212,20 @@ public class CronUtil {
 		scheduler.stop(true);
 	}
 
+	/**
+	 * 验证是否为合法的Cron表达式
+	 */
+	public static boolean isValidExpression(String expression) {
+		if (expression == null) {
+			return false;
+		} else {
+			try {
+				PatternParser.parse(expression);
+				return true;
+			} catch (RuntimeException e) {
+				return false;
+			}
+		}
+	}
+
 }

hutool-cron/src/main/java/org/dromara/hutool/cron/pattern/parser/PatternParser.java

@@ -63,6 +63,7 @@ public class PatternParser {
 	 * @return {@link List}
 	 */
 	private static List<PatternMatcher> parseGroupPattern(final String groupPattern) {
+		Assert.notBlank(groupPattern, "Cron expression must not be empty!");
 		final List<String> patternList = SplitUtil.splitTrim(groupPattern, "|");
 		final List<PatternMatcher> patternMatchers = new ArrayList<>(patternList.size());
 		for (final String pattern : patternList) {

hutool-cron/src/test/java/org/dromara/hutool/cron/demo/CronTest.java

@@ -96,4 +96,15 @@ public class CronTest {
 		CronUtil.setMatchSecond(true);
 		CronUtil.start();
 	}
+
+	@Test
+	@Disabled
+	public void isValidExpressionTest() {
+		Console.log(CronUtil.isValidExpression("5 * * * *"));
+		Console.log(CronUtil.isValidExpression("3-18 5 * * * *"));
+		Console.log(CronUtil.isValidExpression(""));
+		Console.log(CronUtil.isValidExpression(null));
+		Console.log(CronUtil.isValidExpression("A B C D E F"));
+
+	}
 }