Race conditions "check-then-act" and "read-modify-write"

Question

Anyone can explain me about what is race condition, how to avoid it, and how to find it out in java codes?

Okay, I just know "race condition" several days, I have two examples, maybe they are not good enough, that's why I need your help:) Hope any of you can explain it for me.

example1: check then act:

if(vector.contains(e))//check
{
vector.remove(e)
}

if there are 2 threads can access, thread1 suspends after check vector contains e, and e does in vector, then thread2 access to check and then remove e from vector, then thread1 comes back and do remove action, error will occur, because e is already removed by thread2.

example2: read modify write:

assume we have a counter variable in a method, once the method is called, counter increase 1,

counter++

this is not a atomic operation, it has 3 steps: 1. get the value 2. increase the value 3. assign to the value

What I know about race condition is all here, hope you can share your knowledge with me:)

thanks

Answer 1

What is a race condition? Check this stack-overflow question.

There are primarily two scenarios for race-condition: read-modify-write and check-then-act.

For read-modify-write classical example is of counter++ which is not an atomic operation so leads to race condition.

For check-then-act there are multiple examples. One example is when you check for key existence in ConcurrentHashMap and then do some work in if-case. Another example is singleton class code:

public Singleton getInstance()
{
   if(_instance == null)
   { 
      _instance = new Singleton();
   }
}

You can read more about them on internet. One good book on concurrency is Java Concurrency in Practice by Brian Goetz. You can also find this article helpful.