Java: Volatile vs Synchronization

• The values of volatile variables will never be cached
• All read & writes will directly be done from main memory
• Adding volatile variable is another way of making class thread-safe

Volatile is useful when there are below problems when volatile variables are not there

 

1. Memory Visibility: When one thread modifies the value of a value it stored in a write buffer or cache. Processor will push these buffered/cached values 'all at once' to main memory. This might lead to data inconsistency, if there is not good 'cache coherence'.

2. Reordering: Reordering is an optimization technique for performance improvement by processor or compiler. For example, reader thread will see the updates to variables in any order other than the actual program order

 

Using volatile keyword, we tell the runtime & processor to not reorder these variables and immediately flush any updates to these variables.

 

Volatile vs Synchronization:

Synchronized methods & blocks provide 'Mutual Exclusion' and 'Visibility' at the cost of application performance.

• Mutual Exclusion: Only one thread executes a critical section at a time
• Visibility: Changes made by one thread to shared data are visible to other threads to maintain data consistency 

Volatile keyword ensures Visibility without Mutual Exclusion, thus it is useful when we are fine with multiple threads accessing a block code parallel but we need to ensure data visibility.