CODEWITHSUNDEEP
javaunsafe
Jin Kwon
Jin Kwon

Reputation: 21978

Is sun.misc.Unsafe thread safe?

Is sun.misc.Unsafe or the theUnsafe instance thread safe?

Upvotes: 1

Views: 1079

Answers (2)

Steves
Steves

Reputation: 3224

The accepted answer is at least misleading if not wrong.

On the Java level, Unsafe does not hold any instance state. Most of its methods call native code, which either does not work with any shared mutable state or it calls to the native allocator to allocate/free memory and those operations are thread safe (usually you don't synchronize calls to new C++, for example). OTOH, sun.misc.Unsafe is internal API and its contract is not documented and may change.

Do you need to synchronize calls to methods on the Unsafe instance? That depends on whether you may be passing in the same addresses as the arguments to those methods. In the same way as calls to the addItem method of the following Java class need to be synchronized if you are passing to it the same list from different threads.

class ListUtils {
  public static void addItem(List<Object> list, Object obj) { list.add(obj); }
}

// Good:
// thread1:
List<Object> l1 = new List<>();
ListUtils.addItem(l1, 1);
// thread2:
List<Object> l2 = new List<>();
ListUtils.addItem(l2, 2);

// Needs synchronization:
class SharedState {
   public static List<Object> sharedList = new List<>();
}
// thread1:
ListUtils.addItem(SharedState.sharedList, 1);
// thread2:
ListUtils.addItem(SharedState.sharedList, 2);

I would not say that ListUtils is not thread-safe. It does not carry any mutable and potentially shared state. Yes, synchronizing the addItem method would remove the need for synchronization in the second example, but that would unnecessarily synchronize operations that are working with completely different data.

Upvotes: 4

Brian
Brian

Reputation: 892

The methods of Unsafe are not thread safe. You need to synchronize access to data you want to manipulate yourself - as usual. Accessing the instance theUnsafe however is, because the field gets initialized when the class is being loaded (it's static).

Upvotes: 4

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