java - 如何保护单例类方法在 Java 中是线程安全的？

Question

我有一个线程安全的双重检查单例类，它在单例类中包含一个带有 get/set/size 方法的 LinkedList。然后我有一个简单的池类，它使用这个 Singleton 类来管理对象池。

我的问题是如何在不使用同步方法的情况下保护单例和池类中的 get/set 方法。这是我的代码

public class SingletonDoubleCheckedLockingPattern {

    private static SingletonDoubleCheckedLockingPattern s = new SingletonDoubleCheckedLockingPattern();

    private LinkedList<Object> linkedList;

    public int GetListObjectCount() {
        return linkedList.size();
    }

    public Object GetObjectFromList() {
        return linkedList.poll();
    }

    public void SetObjectFromList(Object ee) {
        linkedList.add(ee);
    }

    private SingletonDoubleCheckedLockingPattern() {

        linkedList = new LinkedList<Object>();

    }

    /**
     * SingletonHolder is loaded on the first execution of
     * Singleton.getInstance() or the first access to SingletonHolder.INSTANCE,
     * not before.
     */
    private static class SingletonHolder {
        public static final SingletonDoubleCheckedLockingPattern INSTANCE = new SingletonDoubleCheckedLockingPattern();
    }

    public static SingletonDoubleCheckedLockingPattern getInstance() {
        return SingletonHolder.INSTANCE;
    }

    // avoid cloning
    public final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

}

public class SingletonObjectPool  {

    private int maxlistValue = 10;


    public Object GetObject() 
    {
        int listCount = SingletonDoubleCheckedLockingPattern.getInstance().GetListObjectCount();
        if(listCount > 0)
        {
            return SingletonDoubleCheckedLockingPattern.getInstance().GetObjectFromList();
        }
        return null;
    }


    public void SetObject() 
    {
        int listCount = SingletonDoubleCheckedLockingPattern.getInstance().GetListObjectCount();
        if(listCount < maxlistValue)
        {
            SingletonDoubleCheckedLockingPattern.getInstance().SetObjectFromList(new Object());
        }

    }


}

Answer 1

您可以使用线程安全的 BlockingQueue。在尝试删除元素之前，您不需要检查集合是否为空，集合有执行此操作的方法。

要简化您的代码并使其成为线程安全的，您可以这样做。

public class SingletonObjectPool  {
    private static final int maxlistValue = 10;
    private static final BlockingQueue queue 
        = new ArrayBlockingQueue(maxListValue);

    public static Object getObject() {
        return queue.poll();
    }


    public static void addObjectAsRequired() {
        queue.offer(new Object());
    }
}