java 生成数字主键

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生成数字主键的方法介绍

在Java编程中，生成数字主键是一个常见的需求。数字主键通常用于唯一标识数据记录或实体，在数据库表的设计中起着至关重要的作用。通过生成数字主键，可以确保每个数据记录都有唯一的标识符，避免数据冲突和重复。

本文将介绍几种在Java中生成数字主键的常用方法，并提供代码示例进行演示。

UUID

UUID（Universally Unique Identifier）是一种生成全局唯一ID的方法。它是一个128位长度的数字，在Java中可以通过java.util.UUID类来生成。UUID通常以32个十六进制数字表示，形式为8-4-4-4-12的组合。

import java.util.UUID;

public class UUIDGenerator {
    public static void main(String[] args) {
        UUID uuid = UUID.randomUUID();
        System.out.println(uuid.toString().replace("-", ""));
    }
}

自增序列

自增序列是一种简单有效的生成数字主键的方法。通过数据库的自增字段或者程序中的计数器来实现，保证每次生成的主键值都是递增的。

public class IncrementGenerator {
    private static int counter = 0;

    public static int generate() {
        return ++counter;
    }

    public static void main(String[] args) {
        System.out.println(generate());
        System.out.println(generate());
    }
}

Snowflake算法

Snowflake算法是Twitter开源的一种分布式ID生成算法，可以生成唯一、有序、分布式的ID。它的核心思想是将64位的ID拆分成不同的部分，包括时间戳、机器ID、序列号等。

public class SnowflakeGenerator {
    private final long datacenterId;
    private final long machineId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;
    private final static long twepoch = 1288834974657L;
    private final static long datacenterIdBits = 5L;
    private final static long machineIdBits = 5L;
    private final static long sequenceBits = 12L;
    private final static long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private final static long maxMachineId = -1L ^ (-1L << machineIdBits);
    private final static long sequenceMask = -1L ^ (-1L << sequenceBits);
    private long datacenterIdShift = sequenceBits + machineIdBits;
    private long machineIdShift = sequenceBits;
    private long timestampLeftShift = sequenceBits + machineIdBits + datacenterIdBits;

    public SnowflakeGenerator(long datacenterId, long machineId) {
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException("Datacenter ID must be between 0 and " + maxDatacenterId);
        }
        if (machineId > maxMachineId || machineId < 0) {
            throw new IllegalArgumentException("Machine ID must be between 0 and " + maxMachineId);
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    public synchronized long generate() {
        long timestamp = System.currentTimeMillis();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id for " + (lastTimestamp - timestamp) + " milliseconds");
        }

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift)
                | (datacenterId << datacenterIdShift)
                | (machineId << machineIdShift)
                | sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }

    public static void main(String[] args) {
        SnowflakeGenerator snowflakeGenerator = new SnowflakeGenerator(1, 1);
        System.out.println(snowflakeGenerator.generate());
        System.out.println(snowflakeGenerator.generate());
    }
}