1. Synchronized keyword in Java is used to provide mutually exclusive access to a shared resource with multiple threads in Java. Synchronization in Java guarantees that no two threads can execute a synchronized method which requires the same lock simultaneously or concurrently.

How synchronized keyword works internally?

It can be roughly described as each synchronization block having invisible instructions at the entry and exit, the first one saying "read everything from the shared memory to be as up-to-date as possible" and the last one saying "now flush whatever you've been doing there to the shared memory".

How do you synchronize variables in Java?

Use the synchronized keyword. Using the synchronized keyword on the methods will require threads to obtain a lock on the instance of sample . Thus, if any one thread is in newmsg() , no other thread will be able to get a lock on the instance of sample , even if it were trying to invoke getmsg() .