我应该使用原子整数还是同步

Question

我在代码中使用原子/易失性/同步有点困惑.假设我在书店中有书信息的对象,例如,可能会发生两个线程想要同一本书,而库存中的金额只有1,我怎么能保证只有一个线程会拿这本书？我必须使用同步吗？BookInventoryInfo:

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();
    }
}

我想拿这本书的方式:

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

Answer 1

您应该使用synchronizedAtomicInteger并不像初看起来那么简单.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.你的榜样很好.说你有

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

金额现在为-1.

如果使用synchronized它,可以更轻松地保持整个操作的锁定

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