Is this thread-safe?

I want to make my class thread-safe without large overhead. The instances will be seldom used concurrently, but it may happen. Most of the class is immutable, there's only one mutable member used as a cache:

private volatile SoftReference<Map<String, Something>> cache
    = new SoftReference(null);

which gets assigned in the constructor (not shared) like

Map<String, Something> tmp = new HashMap<String, Something>();
tmp.put("a", new Something("a");
tmp.put("b开发者_开发技巧", new Something("b");
cache = new SoftReference(tmp);

After the assignment, the map gets never modified.

It's no problem, when two threads compute the cache in parallel, since the value will be the same. The additional overhead of the word done twice is acceptable.

When a thread wouldn't see the value computed by another tread, it'd compute it unnecessary, and this is acceptable. This wouldn't happen because of volatile. When a thread sees value computed by another tread, it's fine.

The only possible problem would be a thread seeing inconsistent state (e.g. a partly filled map). Can this happen?

Notes:

1. I really want the whole map being softly referenced, there's no use for a map using soft keys or values here.

2. I know about ConcurrentHashMap and will maybe use it anyway, but I'm curious, if using volatile only works.

The only possible problem would be a thread seeing inconsistent state (e.g. a partly filled map). Can this happen?

No. Actions performed within a thread must be performed as if they had been executed in order. Writing a volatile variable happens-before any read of that value. Hence, initialization of the map happens-before any thread reading the reference to the map from the field.

The problem with using a soft reference is that you can lose the whole map/cache after a GC. This means the performance of your application can be hit very hard. You are better off using a cache with an eviction policy so that you never have this problem.

The volatile doesn't make any operation safe here.

You haven't shown all your code, perhaps we could offer some suggestion on how you could improve your code e.g. your sample code should compile ;)