| /* |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you 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. |
| * |
| */ |
| |
| package org.apache.hadoop.hdds.utils; |
| |
| import org.apache.hadoop.hdds.HddsUtils; |
| |
| /** |
| * This class uses system current time milliseconds to generate unique id. |
| */ |
| public final class UniqueId { |
| /* |
| * When we represent time in milliseconds using 'long' data type, |
| * the LSB bits are used. Currently we are only using 44 bits (LSB), |
| * 20 bits (MSB) are not used. |
| * We will exhaust this 44 bits only when we are in year 2525, |
| * until then we can safely use this 20 bits (MSB) for offset to generate |
| * unique id within millisecond. |
| * |
| * Year : Mon Dec 31 18:49:04 IST 2525 |
| * TimeInMillis: 17545641544247 |
| * Binary Representation: |
| * MSB (20 bits): 0000 0000 0000 0000 0000 |
| * LSB (44 bits): 1111 1111 0101 0010 1001 1011 1011 0100 1010 0011 0111 |
| * |
| * We have 20 bits to run counter, we should exclude the first bit (MSB) |
| * as we don't want to deal with negative values. |
| * To be on safer side we will use 'short' data type which is of length |
| * 16 bits and will give us 65,536 values for offset. |
| * |
| */ |
| |
| private static volatile short offset = 0; |
| |
| /** |
| * Private constructor so that no one can instantiate this class. |
| */ |
| private UniqueId() {} |
| |
| /** |
| * Calculate and returns next unique id based on System#currentTimeMillis. |
| * |
| * @return unique long value |
| */ |
| public static synchronized long next() { |
| long utcTime = HddsUtils.getUtcTime(); |
| if ((utcTime & 0xFFFF000000000000L) == 0) { |
| return utcTime << Short.SIZE | (offset++ & 0x0000FFFF); |
| } |
| throw new RuntimeException("Got invalid UTC time," + |
| " cannot generate unique Id. UTC Time: " + utcTime); |
| } |
| } |