Java Synchronization is a way where we want to allow only one thread to access the shared resource. So when a thread is already using a resource, preventing any other thread from using on the same resource is called thread synchronization or thread safe. The synchronization is mainly used to prevent the thread interference and to prevent the consistency problem. The object on which the threads are synchronized is called synchronized object. Basically there are three ways that a thread can be synchronized such as:

• • by synchronized method
• • by synchronized block
• • by static synchronization

Synchronization is built around an internal entity known as the lock or monitor. Every object has an lock associated with it. By convention, a thread that needs consistent access to an object's fields has to acquire the object's lock before accessing them, and then release the lock when it's done with them. From Java 5 the package java.util.concurrent.locks contains several lock implementations.