Race conditions

Let's start by looking at some concurrency bugs. Here is a simple example of incrementing a counter:

public class Counter {
  private int counter;
    
  public int incrementAndGet() {
    ++counter;
    return counter;
}

This code is not thread-safe. If two threads are running using the same object concurrently, the sequence of counter values each gets is essentially unpredictable.  The reason for this is the ++counter operation. This simple-looking statement is actually made up of three distinct operations, namely read the new value,  modify the new value, and store the new value:

         

As shown in the preceding diagram, the thread executions are oblivious of each other, and hence interfere unknowingly, thereby introducing a lost update. 

The following code illustrates the ...