Java同步调查

Question

Here's some code I saw once. Can you see what's wrong with it?

[updated]

public class ResourceManager1
{
    private final String mutex = "";
    Object resource = null;

    public Object getResource()
    {
        synchronized (mutex)
        {
            if (resource == null)
            {
                resource = new Object();
            }
        }

        return resource;
    }
}

public class ResourceManager2
{
    private final String mutex = "";
    Object resource = null;

    public Object getResource()
    {
        synchronized (mutex)
        {
            if (resource == null)
            {
                resource = new Object();
            }
        }

        return resource;
    }
}

Answer 1

Never synchronize on strings, particularly string literals which are interned. You've basically just got a single lock.

In general, never synchronize on any reference that might be visible outside your class (including "this") unless the purpose of the external visibility is precisely for locking purposes. I usually use a private final variable created solely for the purpose of locking.

Answer 2

对于两个类，您都使用相同的String作为互斥体，因此一次只能使用一个同步块，这似乎并不是代码的目的。

Answer 3

mutex is not final and resource is not private.

Besides, you need a getResource method that returns resource, but I suppose this is just a typo.