在多线程程序线程中执行类的静态方法是否安全？

Question

I have a static method in a class that runs some calcs, here is code snippet:

public MyClass {

MyClass(){}

public static float runCalcs()
{
    float sum;
    Float[] floatArray = map.entrySet()
                        .stream()
                        .map(key -> key.getKey().getPrice())
                        .toArray(size -> new Float[size]);

    for(int i=0; i<floatArray.length; i++) {
        sum += floatArray[i];
    }   

    return sum;
}

}

My question is when I execute this method from a thread like so:

float retVal = MyClass.runCalcs();

Is this a thread safe execution of static method or do I need to synchronize or lock MyClass.RunCalcs() to make it thread safe.
In my application, multiple threads will be executing this method simultaneously.
Please let me know.
Thanks in advance!

Answer 1

Thread safety is not a questions about methods or classes. Thread safety is a question about shared data .

Your shared data is not safe if a method running in one thread could temporarily put the data into a state that you would not want the same method or any other method running in a different thread to see.

Your runCalcs() method looks at map which presumably is a Map instance, and which presumably is shared with some other thread.

Is there any way that the other thread could change the contents of the map while runCalcs() is iterating over the map's entrySet() ? If so, then your code is not safe.

The way to make it safe is to change runCalcs() lock a lock while iterating, and also, change every method that could update map in a different thread to lock the same lock while changing map .