The key difference between synchronized and volatile lies in their purpose and usage:

  • synchronized:
    • Used to control access to a block of code or method by multiple threads, ensuring that only one thread can execute it at a time.
    • Provides mutual exclusion (locks) and can be used to protect data integrity during concurrent access.
    • Can be used for both reading and writing to variables.
  • volatile:
    • Used to ensure that changes to a variable are immediately visible to all threads, avoiding caching issues.
    • Does not provide mutual exclusion or synchronization for method/block access.
    • Primarily used for simple flags or state variables that are read and written by multiple threads.

In summary, use synchronized for managing thread access to critical sections of code and volatile for ensuring visibility of changes to a variable across threads.