Java Multithreading Overview

• A Thread is a single sequential flow control within a program.
• Multi Threading  means that you could have multiple threads of execution inside the same program.
• Creating and Starting Java Threads
• By implementing Runnable Interface
• By implementing Callable Interface
• By Extending Thread Class
• Race Conditions and Critical Sections
• The problems arise when multiple threads access the same resources.
• In fact, problems only arise if one or more of the threads write to same resource.
• Thread Safety and Shared Resources
• Code that is safe to call by multiple threads simultaneously is called thread safe.
• Race condition only occur when multiple threads update shared resources.Therefore it is important to know what resources Java threads share when executing.
• Local Variables
• Local variables are stored in each thread's own stack. That means that local variables are never shared between threads and it is thread safe.
• That also means that all local primitive variables are thread safe.
• Local Object References
• All objects are stored in the shared heap.
• If an object created locally never escapes the method it was created in, it is thread safe.
• Object Members
• Object members are stored on the heap along with the object.
• if two threads call a method on the same object instance and this method updates object members, the method is not thread safe.