Programming Concurrency on the JVM

Ensure Visibility

The memory barrier has to be crossed at the appropriate time. Crossing it in the constructor is not good, and we fixed that problem in the example. However, we must ensure that other methods that access the shared mutable variable level cross the memory barrier. See ​What’s This Memory Barrier?​ to refresh your memory about why to cross the memory barrier.

If we consider race conditions only, we may argue against synchronizing getter methods; we may get convinced that the slightly old copy of a variable’s value is adequate. The motivation to synchronize or lock getters is as much about visibility as race conditions—if we fail to cross the memory barrier, our thread is not guaranteed to see the change for an unpredictable duration ...