Many Java programmers confused themselves like hell while writing multi-threaded Java programs like where to synchronized? Which Lock to use? What Lock to use etc. I often receive a request to explain how to use Locks and ReentrantLock in Java, so I thought to write a simple Java program, which is multi-threaded and uses a rather new Lock interface and ReentrantLock to lock critical section. Remember Lock is your tool to guard shared resources which can be anything like a database, File system, a Prime number Generator, or a Message processor. Before using Locks in the Java program, it’s also better to learn some basics. Lock is an interface from java.util.concurrent package. It was introduced in JDK 1.5 release as an alternative to the synchronized keyword.