Java での AtomicInteger クラスの使用の概要

public class Sample1 {
  private static Integer count = 0;
  synchronized public static void increment() {
    count++;
  }
}

public class Sample2 {
  private static AtomicInteger count = new AtomicInteger(0);
  public static void increment() {
    count.getAndIncrement();
  }
}

public class AtomicInteger extends Number implements java.io.Serializable {
  private static final long serialVersionUID = 6214790243416807050L;
  // setup to use Unsafe.compareAndSwapInt for updates
  private static final Unsafe unsafe = Unsafe.getUnsafe();
  private static final long valueOffset;
  static {
    try {
      valueOffset = unsafe.objectFieldOffset
        (AtomicInteger.class.getDeclaredField("value"));
    } catch (Exception ex) { throw new Error(ex); }
  }
  private volatile int value;

 public class AtomicTest {
  static long randomTime() {
    return (long) (Math.random() * 1000);
  }
  public static void main(String[] args) {
    // 阻塞队列，能容纳100个文件
    final BlockingQueue<File> queue = new LinkedBlockingQueue<File>(100);
    // 线程池
    final ExecutorService exec = Executors.newFixedThreadPool(5);
    final File root = new File("D:\\ISO");
    // 完成标志
    final File exitFile = new File("");
    // 原子整型，读个数
    // AtomicInteger可以在并发情况下达到原子化更新，避免使用了synchronized，而且性能非常高。
    final AtomicInteger rc = new AtomicInteger();
    // 原子整型，写个数
    final AtomicInteger wc = new AtomicInteger();
    // 读线程
    Runnable read = new Runnable() {
      public void run() {
        scanFile(root);
        scanFile(exitFile);
      }
      public void scanFile(File file) {
        if (file.isDirectory()) {
          File[] files = file.listFiles(new FileFilter() {
            public boolean accept(File pathname) {
              return pathname.isDirectory() || pathname.getPath().endsWith(".iso");
            }
          });
          for (File one : files)
            scanFile(one);
        } else {
          try {
            // 原子整型的incrementAndGet方法，以原子方式将当前值加 1，返回更新的值
            int index = rc.incrementAndGet();
            System.out.println("Read0: " + index + " " + file.getPath());
            // 添加到阻塞队列中
            queue.put(file);
          } catch (InterruptedException e) {
          }
        }
      }
    };
    // submit方法提交一个 Runnable 任务用于执行，并返回一个表示该任务的 Future。
    exec.submit(read);
    // 四个写线程
    for (int index = 0; index < 4; index++) {
      // write thread
      final int num = index;
      Runnable write = new Runnable() {
        String threadName = "Write" + num;
        public void run() {
          while (true) {
            try {
              Thread.sleep(randomTime());
              // 原子整型的incrementAndGet方法，以原子方式将当前值加 1，返回更新的值
              int index = wc.incrementAndGet();
              // 获取并移除此队列的头部，在元素变得可用之前一直等待（如果有必要）。
              File file = queue.take();
              // 队列已经无对象
              if (file == exitFile) {
                // 再次添加"标志"，以让其他线程正常退出
                queue.put(exitFile);
                break;
              }
              System.out.println(threadName + ": " + index + " " + file.getPath());
            } catch (InterruptedException e) {
            }
          }
        }
      };
      exec.submit(write);
    }
    exec.shutdown();
  }
}