Eagerly initialize ZERO_NANOS constant

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

@@ -29,17 +29,15 @@ import org.springframework.util.StringUtils;
  * Created using the {@code parse*} methods.
  *
  * @author Arjen Poutsma
+ * @author Juergen Hoeller
  * @since 5.3
  */
 final class BitsCronField extends CronField {
 
+	public static final BitsCronField ZERO_NANOS = forZeroNanos();
+
 	private static final long MASK = 0xFFFFFFFFFFFFFFFFL;
 
-
-	@Nullable
-	private static BitsCronField zeroNanos = null;
-
-
 	// we store at most 60 bits, for seconds and minutes, so a 64-bit long suffices
 	private long bits;
 
@@ -48,16 +46,14 @@ final class BitsCronField extends CronField {
 		super(type);
 	}
 
+
 	/**
 	 * Return a {@code BitsCronField} enabled for 0 nanoseconds.
 	 */
-	public static BitsCronField zeroNanos() {
-		if (zeroNanos == null) {
-			BitsCronField field = new BitsCronField(Type.NANO);
-			field.setBit(0);
-			zeroNanos = field;
-		}
-		return zeroNanos;
+	private static BitsCronField forZeroNanos() {
+		BitsCronField field = new BitsCronField(Type.NANO);
+		field.setBit(0);
+		return field;
 	}
 
 	/**
@@ -108,7 +104,6 @@ final class BitsCronField extends CronField {
 		return result;
 	}
 
-
 	private static BitsCronField parseDate(String value, BitsCronField.Type type) {
 		if (value.equals("?")) {
 			value = "*";
@@ -174,6 +169,7 @@ final class BitsCronField extends CronField {
 		}
 	}
 
+
 	@Nullable
 	@Override
 	public <T extends Temporal & Comparable<? super T>> T nextOrSame(T temporal) {
@@ -217,7 +213,6 @@ final class BitsCronField extends CronField {
 		else {
 			return -1;
 		}
-
 	}
 
 	private void setBits(ValueRange range) {

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

@@ -29,7 +29,7 @@ import org.springframework.util.StringUtils;
 
 /**
  * Single field in a cron pattern. Created using the {@code parse*} methods,
- * main and only entry point is {@link #nextOrSame(Temporal)}.
+ * the main and only entry point is {@link #nextOrSame(Temporal)}.
  *
  * <p>Supports a Quartz day-of-month/week field with an L/# expression. Follows
  * common cron conventions in every other respect, including 0-6 for SUN-SAT
@@ -60,7 +60,7 @@ abstract class CronField {
 	 * Return a {@code CronField} enabled for 0 nanoseconds.
 	 */
 	public static CronField zeroNanos() {
-		return BitsCronField.zeroNanos();
+		return BitsCronField.ZERO_NANOS;
 	}
 
 	/**
@@ -186,7 +186,6 @@ abstract class CronField {
 		MONTH(ChronoField.MONTH_OF_YEAR, ChronoUnit.YEARS, ChronoField.DAY_OF_MONTH, ChronoField.HOUR_OF_DAY, ChronoField.MINUTE_OF_HOUR, ChronoField.SECOND_OF_MINUTE, ChronoField.NANO_OF_SECOND),
 		DAY_OF_WEEK(ChronoField.DAY_OF_WEEK, ChronoUnit.WEEKS, ChronoField.HOUR_OF_DAY, ChronoField.MINUTE_OF_HOUR, ChronoField.SECOND_OF_MINUTE, ChronoField.NANO_OF_SECOND);
 
-
 		private final ChronoField field;
 
 		private final ChronoUnit higherOrder;

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

@@ -75,8 +75,9 @@ final class QuartzCronField extends CronField {
 	}
 
 	/**
-	 * Parse the given value into a days of months {@code QuartzCronField}, the fourth entry of a cron expression.
-	 * Expects a "L" or "W" in the given value.
+	 * Parse the given value into a days of months {@code QuartzCronField},
+	 * the fourth entry of a cron expression.
+	 * <p>Expects a "L" or "W" in the given value.
 	 */
 	public static QuartzCronField parseDaysOfMonth(String value) {
 		int idx = value.lastIndexOf('L');