TimeMachine.java

package com.tanapoln.utils;


import java.time.Clock;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;

/**
 * {@code TimeMachine} provide a central method for accessing system time with an ease on testing.<br>
 * You can set system clock to be at a specific time. When your class using this class to retrieve time,
 * it will break a coupling with real clock which cannot be tested.
 */
public class TimeMachine {
    private static Clock INSTANCE = Clock.systemDefaultZone();

    /**
     * Prevent from create {@code TimeMachine} instance.
     */
    private TimeMachine() {
    }

    /**
     * Set {@code Clock} instance.
     *
     * @param clock clock instance
     */
    public static void set(Clock clock) {
        INSTANCE = clock;
    }

    /**
     * Get current {@code Clock} instance.
     *
     * @return a clock instance
     */
    public static Clock get() {
        return INSTANCE;
    }

    /**
     * Fixed {@code Clock} instance at number of millisecond(s) since epoch time with default system zone.
     *
     * @param millis time in millisecond(s)
     */
    public static void fixedMillisecondDefaultZone(long millis) {
        INSTANCE = Clock.fixed(Instant.ofEpochMilli(millis), ZoneId.systemDefault());
    }

    /**
     * Fixed {@code Clock} instance at number of millisecond(s) since epoch time with a specific zone.
     *
     * @param millis time in millisecond(s)
     * @param zone   zone id
     */
    public static void fixedMillisecond(long millis, ZoneId zone) {
        INSTANCE = Clock.fixed(Instant.ofEpochMilli(millis), zone);
    }

    /**
     * Reset {@code Clock} instance to be real system time with default system zone.
     */
    public static void resetToSystemDefaultZone() {
        INSTANCE = Clock.systemDefaultZone();
    }

    /**
     * Reset {@code Clock} instance to be real system time with a specific zone.
     *
     * @param zone zone
     */
    public static void resetToSystem(ZoneId zone) {
        INSTANCE = Clock.system(zone);
    }

    /**
     * Create {@code LocalDateTime} instance with a {@code Clock} instance in this {@code TimeMachine}.
     *
     * @return local date time of current clock
     */
    public static LocalDateTime now() {
        return LocalDateTime.now(INSTANCE);
    }

    /**
     * @see Clock#instant()
     */
    public static Instant instant() {
        return INSTANCE.instant();
    }

    /**
     * @see Clock#millis()
     */
    public static long millis() {
        return INSTANCE.millis();
    }

    /**
     * @see Clock#getZone()
     */
    public static ZoneId getZone() {
        return INSTANCE.getZone();
    }
}