返回的是64位long型数值，末尾12位是自增序列，最大支持4096，逻辑上是每毫秒的id最大生成数量；末尾12到22位的10bit是机器id；前面43位是时间戳（ms）左移22位得到的；最前面1位是保留位（因为整形的二进制首位默认就是表示正负用的）。

/*
 * Copyright 2016-2018 shardingsphere.io.
 * <p>
 * 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
 *
 *     http://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.
 * </p>
 */
package io.shardingsphere.core.keygen;
import com.google.common.base.Preconditions;
import lombok.Setter;
import lombok.SneakyThrows;
import java.util.Calendar;
/**
 * Default distributed primary key generator.
 * 
 * <p>
 * Use snowflake algorithm. Length is 64 bit.
 * </p>
 * 
 * <pre>
 * 1bit   sign bit.
 * 41bits timestamp offset from 2016.11.01(ShardingSphere distributed primary key published data) to now.
 * 10bits worker process id.
 * 12bits auto increment offset in one mills
 * </pre>
 * 
 * <p>
 * Call @{@code DefaultKeyGenerator.setWorkerId} to set worker id, default value is 0.
 * </p>
 * 
 * <p>
 * Call @{@code DefaultKeyGenerator.setMaxTolerateTimeDifferenceMilliseconds} to set max tolerate time difference milliseconds, default value is 0.
 * </p>
 * 
 * @author gaohongtao
 */
public final class DefaultKeyGenerator implements KeyGenerator {
    public static final long EPOCH;
    private static final long SEQUENCE_BITS = 12L;
    private static final long WORKER_ID_BITS = 10L;
    private static final long SEQUENCE_MASK = (1 << SEQUENCE_BITS) - 1;
    private static final long WORKER_ID_LEFT_SHIFT_BITS = SEQUENCE_BITS;
    private static final long TIMESTAMP_LEFT_SHIFT_BITS = WORKER_ID_LEFT_SHIFT_BITS + WORKER_ID_BITS;
    private static final long WORKER_ID_MAX_VALUE = 1L << WORKER_ID_BITS;
    @Setter
    private static TimeService timeService = new TimeService();
    private static long workerId;
    private static int maxTolerateTimeDifferenceMilliseconds = 10;
    static {
        Calendar calendar = Calendar.getInstance();
        calendar.set(2016, Calendar.NOVEMBER, 1);
        calendar.set(Calendar.HOUR_OF_DAY, 0);
        calendar.set(Calendar.MINUTE, 0);
        calendar.set(Calendar.SECOND, 0);
        calendar.set(Calendar.MILLISECOND, 0);
        EPOCH = calendar.getTimeInMillis();
    }
    private byte sequenceOffset;
    private long sequence;
    private long lastMilliseconds;
    /**
     * Set work process id.
     * 
     * @param workerId work process id
     */
    public static void setWorkerId(final long workerId) {
        Preconditions.checkArgument(workerId >= 0L && workerId < WORKER_ID_MAX_VALUE);
        DefaultKeyGenerator.workerId = workerId;
    }
    /**
     * Set max tolerate time difference milliseconds.
     *
     * @param maxTolerateTimeDifferenceMilliseconds max tolerate time difference milliseconds
     */
    public static void setMaxTolerateTimeDifferenceMilliseconds(final int maxTolerateTimeDifferenceMilliseconds) {
        DefaultKeyGenerator.maxTolerateTimeDifferenceMilliseconds = maxTolerateTimeDifferenceMilliseconds;
    }
    /**
     * Generate key.
     * 
     * @return key type is @{@link Long}.
     */
    @Override
    public synchronized Number generateKey() {
        long currentMilliseconds = timeService.getCurrentMillis();
        if (waitTolerateTimeDifferenceIfNeed(currentMilliseconds)) {
            currentMilliseconds = timeService.getCurrentMillis();
        }
        if (lastMilliseconds == currentMilliseconds) {
            if (0L == (sequence = (sequence + 1) & SEQUENCE_MASK)) {
                currentMilliseconds = waitUntilNextTime(currentMilliseconds);
            }
        } else {
            vibrateSequenceOffset();
            sequence = sequenceOffset;
        }
        lastMilliseconds = currentMilliseconds;
        return ((currentMilliseconds - EPOCH) << TIMESTAMP_LEFT_SHIFT_BITS) | (workerId << WORKER_ID_LEFT_SHIFT_BITS) | sequence;
    }
    @SneakyThrows
    private boolean waitTolerateTimeDifferenceIfNeed(final long currentMilliseconds) {
        if (lastMilliseconds <= currentMilliseconds) {
            return false;
        }
        long timeDifferenceMilliseconds = lastMilliseconds - currentMilliseconds;
        Preconditions.checkState(timeDifferenceMilliseconds < maxTolerateTimeDifferenceMilliseconds, 
                "Clock is moving backwards, last time is %d milliseconds, current time is %d milliseconds", lastMilliseconds, currentMilliseconds);
        Thread.sleep(timeDifferenceMilliseconds);
        return true;
    }
    private long waitUntilNextTime(final long lastTime) {
        long result = timeService.getCurrentMillis();
        while (result <= lastTime) {
            result = timeService.getCurrentMillis();
        }
        return result;
    }
    private void vibrateSequenceOffset() {
        sequenceOffset = (byte) (~sequenceOffset & 1);
    }
}