In concurrent programming, there are two basic units of execution:

• processes
• threads

A process has a self-contained execution environment.A process generally has a complete private set of basic run-time resources; inparticular; each process has its own memory space.

Threads are sometimes called lightweight processes.Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process.

• Each thread is associated with an instance of the class Thread
• Two best stratergies for using Threads:
  • To directly control thread creation and management, simply instantiate Thread each time the application needs to initiate an asunchronous task.
  • To abstract thread managment from the rest of your application, pass the application's tasks to an executor.

Threads communicate primarily by sharing access to fields and the objects reference fields refer to. This form of communication is extremely efficient, but makes two kinds of errors possible: thread interference and memory consistency errors. The tool needed to prevent these errors is synchronization.