我应该使用原子整数还是同步

Question

我对在我的代码中使用 atomic/volatile/sync 有点困惑。假设我在一家书店有一个图书信息对象，例如，可能会发生两个线程想要拿走同一本书，而库存中的数量只有 1，我怎么能保证只有一个线程会拿走这本书？我必须使用同步吗？图书库存信息：

package bgu.spl.mics.application.passiveObjects;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * Passive data-object representing a information about a certain book in the inventory.
 * 
 * <p>
 *
 */
public class BookInventoryInfo {

    //The title of the book, his amount in the inventory and the price
    private String bookTitle;
    private AtomicInteger amountInInventory;
    private int price;

    public BookInventoryInfo(String bookTitle, int amountInInventory, int price) {
        this.bookTitle = bookTitle;
        this.price = price;
        this.amountInInventory = new AtomicInteger(amountInInventory);
    }


    /**
     * Retrieves the title of this book.
     * <p>
     * @return The title of this book.   
     */
    public String getBookTitle() {
        return this.bookTitle;
    }

    /**
     * Retrieves the amount of books of this type in the inventory.
     * <p>
     * @return amount of available books.      
     */
    public int getAmountInInventory() {
        return this.amountInInventory.get();
    }

    /**
     * Retrieves the price for  book.
     * <p>
     * @return the price of the book.
     */
    public int getPrice() {
        return this.price;
    }

    public void reduceAmountInInventory() {
        this.amountInInventory.decrementAndGet();
    }
}

我想要的取书方式：

if(book.getAmountInInventory > 0)
{
    book.amountInInventory--
}

Answer 1

您应该使用 synchronized，因为使用 AtomicInteger 并不像乍看起来那么简单。虽然 AtomicInteger 上的单个操作是线程安全的，但使用多个操作可能不是。你的例子很好。说你有

// say it start at 1
Thread1: if(book.getAmountInInventory > 0)
Thread2: if(book.getAmountInInventory > 0)
Thread1: book.amountInInventory--
Thread2: book.amountInInventory--

金额现在是-1。

如果你使用synchronized，那么在整个操作中持有锁会简单得多

synchronized (book) {
    if(book.getAmountInInventory > 0) // no chance for a race condition here.
    {
        book.amountInInventory--
    }

Answer 2

这里 AtomicInteger 是不够的。虽然它允许您以原子方式减少库存中的副本数量，但这还不够——您不需要仅以原子方式减少它，您还需要添加一些自定义逻辑。

我会使用普通的旧 int，并使用显式 synchronized 块或方法保护其修改：

public class BookInventoryInfo {

    private String bookTitle;
    private int amountInInventory;
    private int price;

    public synchronized void checkOut() {
        if (amountInInventory <= 0) {
            throw new BookCheckoutException("No book in inventory");
        }
        amountInInventory--;
    }

    // All the other methods...
}

Answer 3

作为同步的替代方法，您还可以使用 compareAndSet:

int a = book.amountInventory.get();
if (a > 0) {
  boolean updated = book.amountInInventory.compareAndSet(a, a - 1);
}

这将只减少amountInInventory的值，如果它的值仍然是a，当你来更新它时。 compareAndSet 的 return 值表示该值是否已更改。

您可以将其包装在一个循环中，例如：

while (true) {
  int a = book.amountInventory.get();
  if (a == 0) {
    return false;  // To mean "no book in inventory";
  }
  if (book.amountInInventory.compareAndSet(a, a - 1)) {
    return true;  // To mean "there was a book in inventory, and it was removed".
  }
}

我应该使用原子整数还是同步

Should I use atomic integer or sync

java

concurrency

multithreading

synchronization

atomic