CODEWITHSUNDEEP
javamultithreadingthread-safety
Vivek
Vivek

Reputation: 2020

Thread safety issues in reference assignment using method call

I have the following code:

class SomeClass {
    private Map<String, String> someMap = null;

    public String getValue(String key) {
        if (someMap == null) {
            someMap = initialize(); // initialize() is some method which returns a proper map.
        }
        return someMap.get(key);
    }
}

Assuming I don't care about someMap getting initialized more than once, are there any other thread-safety issues that I need to be concerned about here.

As per what-operations-in-java-are-considered-atomic , Reference assignment is certainly atmoic. Is the assignment to someMap guaranteed to happen after the method call to initialize() (seems logical to me). Is there any possibility that a thread can see a partially constructed someMap. Does it matter if instead of a map, I have some other type of object.

Upvotes: 1

Views: 71

Answers (2)

Siva Kumar
Siva Kumar

Reputation: 2006

Its not a Thread Safe even if you put volatile because if two threads are called getValue method at a same time, the following scenario can happen

1) Thread 1 & Thread 2 check if (someMap == null) , both will pass and try to reinitialise.

2) Both will reinitialize the reference. Thread 1 return the old reference which one is override by thread 2 .

3) So please refer the link Double Checked Locking in Singleton

Upvotes: 0

nosid
nosid

Reputation: 50044

The code is not thread-safe. If two threads invoke the method getValue on the same object, it is possible, that one thread sees a partially created someMap.

In order to avoid this problem, you have to remove the data races. The simplest solution is to declare someMap as volatile. The simple rule to remember is: If the code contains no data races, then all executions will appear to be sequentially consistent.

Upvotes: 4

Related Questions