一次只允许一个线程使用一个对象

Question

I have the following class:

public class MyClass{

    private List<Integer> ints = new LinkedList<Integer>();

    public List<Integer> getInts(){
        return ints;
    }

    public synchronized void doAction(){ 
        //Do some with the list
    }
}

I need to allow only one thread at the time having acces to the List . I would do that as follows:

public class MyClass{

    private List<Integer> ints = new LinkedList<Integer>();
    private static final Semaphore s = new Semaphore(1);

    public List<Integer> getInts(){
        s.acquire();
        return ints;
    }

    public void release(){
        s.release();
    }

    public synchronized void doAction(){ 
        s.acquire();
        //Do some with the list
        s.release();
    }
}

But the implementaion is obviously not reliable, because if the client request the List through the getter for adding some elements into it and forget to call release() we'll get into troubles if try to invoke the doAction method.

What is the solution for the problem?

Answer 1

Don't allow the client to get the reference. Put all the methods that work on the list to MyClass and synchronize them.

You can allow the users to get a snapshot copy of the list however.

Answer 2

您可以使用同步列表：

private List<Integer> ints = Collections.synchronizedList(new LinkedList<Integer>());