When you have multiple threads executing at the same time, there is a possibility that two or more threads may access the same variable or another resource at the same time and cause a problem.
For this reason, you should carefully consider how to make your code thread safe.
The simplest mechanism for implementing thread safety is to use an object variable as a flag or traffic light to indicate when a shared resource has an exclusive lock applied.
Note
In William Golding's Lord of the Flies, Piggy and Ralph spot a conch shell and use it to call a meeting. The boys impose a "rule of the conch" on themselves, deciding that no one can speak unless he's holding the conch. I like to name the object variable I use the "conch." When a thread has the conch, no other thread can access the shared resource(s) represented by that conch.
We will explore some types that can be used to synchronize access to resources, as shown in the following diagram:
