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揭秘Java并发高效取序号技巧，告别锁冲突，轻松应对高并发场景

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在Java并发编程中，高效地生成唯一序号是一个常见的需求，尤其是在高并发场景下。正确的实现方式可以显著提高系统的性能和稳定性，避免锁冲突和资源竞争。本文将详细介绍Java中高效取序号的技巧，帮助开发者告别锁冲突，轻松应对高并发场景。

1. 引言

在高并发环境下，生成唯一序号通常需要考虑以下问题：

  • 原子性：确保生成的序号是唯一的，不被其他线程干扰。
  • 高效性：减少锁的使用，提高并发性能。
  • 可扩展性：随着系统规模的扩大，序号生成策略应能保持高效。

2. 传统序列号生成方法

2.1 使用锁

最简单的方法是使用synchronized关键字或ReentrantLock来保证线程安全。例如：

public class SequenceNumberGenerator {
    private int number = 0;

    public synchronized int getNextNumber() {
        return number++;
    }
}

这种方法简单易用，但缺点是效率低下，在高并发场景下会导致严重的性能瓶颈。

2.2 使用原子类

Java提供了AtomicInteger等原子类，可以保证操作的原子性。例如：

import java.util.concurrent.atomic.AtomicInteger;

public class SequenceNumberGenerator {
    private AtomicInteger number = new AtomicInteger(0);

    public int getNextNumber() {
        return number.incrementAndGet();
    }
}

这种方法比使用锁更高效，但仍然存在线程竞争。

3. 高效序列号生成技巧

3.1 使用Snowflake算法

Snowflake算法是由Twitter开源的分布式唯一ID生成算法，可以生成64位的长整型ID。该算法将时间戳、数据中心ID、机器ID和序列号组合在一起，确保了ID的唯一性。

public class SnowflakeIdGenerator {
    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long twepoch = 1288834974657L;
    private long workerIdBits = 5L;
    private long datacenterIdBits = 5L;
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private long sequenceBits = 12L;

    private long workerIdShift = sequenceBits;
    private long datacenterIdShift = sequenceBits + workerIdBits;
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private long sequenceMask = -1L ^ (-1L << sequenceBits);

    private long lastTimestamp = -1L;

    public SnowflakeIdGenerator(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

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

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    private long timeGen() {
        return System.currentTimeMillis();
    }
}

3.2 使用TwitterId算法

TwitterId算法与Snowflake算法类似，也是基于时间戳、数据中心ID和机器ID生成唯一ID。

public class TwitterIdGenerator {
    private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long twepoch = 1288834974657L;
    private long workerIdBits = 5L;
    private long datacenterIdBits = 5L;
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private long sequenceBits = 12L;

    private long workerIdShift = sequenceBits;
    private long datacenterIdShift = sequenceBits + workerIdBits;
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private long sequenceMask = -1L ^ (-1L << sequenceBits);

    private long lastTimestamp = -1L;

    public TwitterIdGenerator(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

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

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    private long timeGen() {
        return System.currentTimeMillis();
    }
}

4. 总结

本文介绍了Java中高效取序号的技巧，包括使用锁、原子类、Snowflake算法和TwitterId算法。在实际应用中，应根据具体场景选择合适的算法，以提高系统的性能和稳定性。

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