java - 如何保证AtomicLong addAndGet结果的正确性

Question

我想计算我的多线程下载程序中的当前百分比。但是有一个奇怪的问题。第二次下载时的lastDownloadSize必须是lastDown的write和lastDownloadSize之和。 example

这是我的代码

private long getDownloadSize() {
    synchronized (this) {
        final AtomicLong totalWriteCount = new AtomicLong(0);
        final AtomicLong lastDownloadSize = new AtomicLong(0);
        for (DownloadTask task : downloadTasks) {
            final long writeCount = task.getWriteCount();
            totalWriteCount.addAndGet(writeCount);
            final long downloadSize = task.getPosition().getDownloadSize();
            lastDownloadSize.addAndGet(downloadSize);
        }
        System.out.println("=====  writeCount : " + totalWriteCount + "lastDownloadSize : " + lastDownloadSize);
        return totalWriteCount.addAndGet(lastDownloadSize.get());
    }
}

Answer 1

您的 totalWriteCount 和 lastDownloadSize 变量是 getDownloadSize() 方法的局部变量。在这种情况下，使用 AtomicLong 没有意义，因为只有单个线程可以访问它们。

您的意思可能是让 totalWriteCount 和 lastDownloadSize 成为您的类的成员:

class SomeClass {
    // ...
    final AtomicLong totalWriteCount = new AtomicLong(0);
    final AtomicLong lastDownloadSize = new AtomicLong(0);
    // ...

    private long getDownloadSize() {
        synchronized (this) {
            for (DownloadTask task : downloadTasks) {
                final long writeCount = task.getWriteCount();
                totalWriteCount.addAndGet(writeCount);
                final long downloadSize = task.getPosition().getDownloadSize();
                lastDownloadSize.addAndGet(downloadSize);
            }
            System.out.println("=====  writeCount : " + totalWriteCount + "lastDownloadSize : " + lastDownloadSize);
            return totalWriteCount.addAndGet(lastDownloadSize.get());
        }
    }
}

但是，同样在这种情况下，如果仅从 synchronized(this) block 内访问它们，则不需要使用 AtomicLong，因为同步块(synchronized block)已经确保它们只能同时由单个线程访问。