When do you have to worry about thread safety?

Question

What are some situations where you need to worry about whether a static method is thread safe?

For example, if I have static utility function that doesn't touch any static class level variables, is that method already thread safe? If I have a static method that does touch static class variables, is that method potentially not thread safe?

Thanks in advance.

Answer 1

If I have static utility function that doesn't touch any static class level variables, is that method already thread safe?

Most of the time - what matters is if your method is reentrant. Something like this is reentrant because everything is local variables and each thread receives its own copy:

static int add(int a, int b)
{
    return a + b;
}

There are still some things to be careful of. If you pass in an object to the method and the method mutates the object and before the method completes you call the same method again on a different thread but with the same object, then you have a problem.

If I have a static method that does touch static class variables, is that method potentially not thread safe?

The main issue again is if the variables are mutable. If you are only reading from immutable objects then there may not be any problem.

If you call a method and you are not sure if it is reentrant and the documentation doesn't say, it's best to assume that it isn't.