java 多线程方法加锁获取自增变量重复问题

Question

/**

• 测试多线程并发获取唯一子增长的值

• @author Administrator
  *
  */
  public class BB {
  private int increment = 0;

  final static Set set = new HashSet();
  final static List list = new ArrayList();
  public synchronized int getauto() {
  return increment++;
  }
  public static void main(String[] args) throws Exception {
  final BB b = new BB();
  Vector workers = new Vector();
  for (int i = 0; i < 2; i++) {
  final Thread t = new Thread(new Runnable() {

          @Override
          public void run() {
              synchronized (this) {
                  for (int j = 0; j < 10000; j++) {
                      int num = b.getauto();
                      System.out.println(String.valueOf(num));
                      set.add(num);
                      list.add(num);
                  }
              }
  
          }
      });
      workers.add(t);
      t.setName("thread-" + i);
      t.start();
  }
  for (Thread t : workers) {
      t.join();
  }
  System.out.println("set  size :" + set.size());
  System.out.println("list  size :" + list.size());
  

  }
  }
  list和set的size不同

Answer 1

System.out.println("set size :" + set.size());
System.out.println("list size :" + list.size());
这两句可没有在锁区域内，一个线程完毕后，在这两句执行的任意时刻，另一个线程可能已经开始执行，并改写了list