Introduce CronExpression

This commit introduces CronExpression, a new for representing cron
expressions, and a direct replacement for CronSequenceGenerator.

Author: poutsma

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

@@ -0,0 +1,177 @@
+/*
+ * Copyright 2002-2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.scheduling.support;
+
+import java.time.temporal.ChronoUnit;
+import java.time.temporal.Temporal;
+import java.util.Arrays;
+
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
+
+/**
+ * Representation of a
+ * <a href="https://www.manpagez.com/man/5/crontab/">crontab expression</a>
+ * that can calculate the next time it matches.
+ *
+ * <p>{@code CronExpression} instances are created through
+ * {@link #parse(String)}; the next match is determined with
+ * {@link #next(Temporal)}.
+ *
+ * @author Arjen Poutsma
+ * @since 5.3
+ * @see CronTrigger
+ */
+public final class CronExpression {
+
+	static final int MAX_ATTEMPTS = 366;
+
+
+	private final CronField[] fields;
+
+	private final String expression;
+
+
+	private CronExpression(
+			CronField seconds,
+			CronField minutes,
+			CronField hours,
+			CronField daysOfMonth,
+			CronField months,
+			CronField daysOfWeek,
+			String expression) {
+
+		// to make sure we end up at 0 nanos, we add an extra field
+		this.fields = new CronField[]{daysOfWeek, months, daysOfMonth, hours, minutes, seconds, CronField.zeroNanos()};
+		this.expression = expression;
+	}
+
+
+	/**
+	 * Parse the given expression string into a {@code CronExpression}.
+	 * The string has six single space-separated fields, representing
+	 * second, minute, hour, day, month, weekday. Month and weekday names can be
+	 * given as the first three letters of the English names.
+	 *
+	 * <p>Example expressions:
+	 * <ul>
+	 * <li>{@code "0 0 * * * *"} = the top of every hour of every day.</li>
+	 * <li><code>"*&#47;10 * * * * *"</code> = every ten seconds.</li>
+	 * <li>{@code "0 0 8-10 * * *"} = 8, 9 and 10 o'clock of every day.</li>
+	 * <li>{@code "0 0 6,19 * * *"} = 6:00 AM and 7:00 PM every day.</li>
+	 * <li>{@code "0 0/30 8-10 * * *"} = 8:00, 8:30, 9:00, 9:30, 10:00 and 10:30 every day.</li>
+	 * <li>{@code "0 0 9-17 * * MON-FRI"} = on the hour nine-to-five weekdays</li>
+	 * <li>{@code "0 0 0 25 12 ?"} = every Christmas Day at midnight</li>
+	 * </ul>
+	 *
+	 * @param expression the expression string to parse
+	 * @return the parsed {@code CronExpression} object
+	 * @throws IllegalArgumentException in the expression does not conform to
+	 * the cron format
+	 */
+	public static CronExpression parse(String expression) {
+		Assert.hasLength(expression, "Expression string must not be empty");
+
+		String[] fields = StringUtils.tokenizeToStringArray(expression, " ");
+		if (fields.length != 6) {
+			throw new IllegalArgumentException(String.format(
+					"Cron expression must consist of 6 fields (found %d in \"%s\")", fields.length, expression));
+		}
+		try {
+			CronField seconds = CronField.parseSeconds(fields[0]);
+			CronField minutes = CronField.parseMinutes(fields[1]);
+			CronField hours = CronField.parseHours(fields[2]);
+			CronField daysOfMonth = CronField.parseDaysOfMonth(fields[3]);
+			CronField months = CronField.parseMonth(fields[4]);
+			CronField daysOfWeek = CronField.parseDaysOfWeek(fields[5]);
+
+			return new CronExpression(seconds, minutes, hours, daysOfMonth, months, daysOfWeek, expression);
+		}
+		catch (IllegalArgumentException ex) {
+			String msg = ex.getMessage() + " in cron expression \"" + expression + "\"";
+			throw new IllegalArgumentException(msg, ex);
+		}
+	}
+
+
+	/**
+	 * Calculate the next {@link Temporal} that matches this expression.
+	 * @param temporal the seed value
+	 * @param <T> the type of temporal
+	 * @return the next temporal that matches this expression, or {@code null}
+	 * if no such temporal can be found
+	 */
+	@Nullable
+	public <T extends Temporal> T next(T temporal) {
+		return nextOrSame(ChronoUnit.NANOS.addTo(temporal, 1));
+	}
+
+
+	@Nullable
+	private <T extends Temporal> T nextOrSame(T temporal) {
+		for (int i = 0; i < MAX_ATTEMPTS; i++) {
+			T result = nextOrSameInternal(temporal);
+			if (result == null || result.equals(temporal)) {
+				return result;
+			}
+			temporal = result;
+		}
+		return null;
+	}
+
+	@Nullable
+	private <T extends Temporal> T nextOrSameInternal(T temporal) {
+		for (CronField field : this.fields) {
+			temporal = field.nextOrSame(temporal);
+			if (temporal == null) {
+				return null;
+			}
+		}
+		return temporal;
+	}
+
+
+	@Override
+	public int hashCode() {
+		return Arrays.hashCode(this.fields);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o) {
+			return true;
+		}
+		if (o instanceof CronExpression) {
+			CronExpression other = (CronExpression) o;
+			return Arrays.equals(this.fields, other.fields);
+		}
+		else {
+			return false;
+		}
+	}
+
+	/**
+	 * Return the expression string used to create this {@code CronExpression}.
+	 * @return the expression string
+	 */
+	@Override
+	public String toString() {
+		return this.expression;
+	}
+
+}