贴一段代码，一个id的生成器，求解

Question

[code="java"]
/*

• This file is part of aion-unique . *
• aion-unique is free software: you can redistribute it and/or modify
• it under the terms of the GNU General Public License as published by
• the Free Software Foundation, either version 3 of the License, or
• (at your option) any later version. *
• aion-unique is distributed in the hope that it will be useful,
• but WITHOUT ANY WARRANTY; without even the implied warranty of
• MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
• GNU General Public License for more details. *
• You should have received a copy of the GNU General Public License
• along with aion-unique. If not, see http://www.gnu.org/licenses/. */ package com.aionemu.chatserver.utils;

import java.util.BitSet;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.ReentrantLock;

/**

• Simplified version of idfactory

• @author ATracer
  */
  public class IdFactory
  {
  private final BitSet idList = new BitSet();
  private final ReentrantLock lock = new ReentrantLock();
  private AtomicInteger nextMinId = new AtomicInteger(1);

  protected static IdFactory instance = new IdFactory();

  public int nextId()
  {
  try
  {
  lock.lock();
  int id = idList.nextClearBit(nextMinId.intValue());
  idList.set(id);
  nextMinId.incrementAndGet();
  return id;
  }
  finally
  {
  lock.unlock();
  }
  }

  public static IdFactory getInstance()
  {
  return instance;
  }

  public static void main(String[] xiaoe)
  {
  BitSet idList = new BitSet();
  for (int i = 0; i < 100; i++)
  System.out.println(idList.nextClearBit(i));
  }

}

[/code]
我觉得有两个问题：
1. 这里既然已经用锁同步了，就可以不用atomic类
2. BitSet有点画蛇添足的味道

不知道作者为何要这么设计，求解。谁能指点下这么写的好处。

Answer 1

[code="java"]public class IdFactory {
private final ReentrantLock lock = new ReentrantLock();
private volatile int id = 0;
private static final IdFactory instance = new IdFactory();

private IdFactory() {
}

public int nextId() {
    try {
        lock.lock();
        id++;
        return id;
    } finally {
        lock.unlock();
    }
}

public static IdFactory getInstance() {
    return instance;
}

static class TestThread extends Thread {
    ArrayList<Integer> idList = new ArrayList<Integer>();

    public void run() {
        IdFactory fac = IdFactory.getInstance();
        for (int i = 0; i < 10000; i++) {
            idList.add(fac.nextId());
        }
    }
}

public static void main(String[] arg) throws Exception {
    TestThread t1 = new TestThread();
    TestThread t2 = new TestThread();
    t1.start();
    t2.start();
    t1.join();
    t2.join();
    System.out.println(t1.idList.size());
    System.out.println(t2.idList.size());
    t1.idList.removeAll(t2.idList);
    System.out.println(t1.idList.size());
    System.out.println(t2.idList.size());
}

}[/code]
我这样写，效果应该一样吧！