Role of volatile keyword

Once a shared variable （ Member variables of class 、 Static member variable of class ） By volatile After retouching , So there are two levels of semantics ：

It ensures the visibility of different threads when they operate on this variable , That is, a thread changes the value of a variable , This new value is for other

Is immediately visible to threads .

Disables instruction reordering .

volatile The essence is to tell jvm Current variable in register （ The working memory ） The value in is uncertain , Need to read from main memory ;

synchronized Lock the current variable , Only the current thread can access this variable , Other threads blocked .

volatile Can only be used at variable level ;synchronized You can use the 、 Method 、 And class level .

volatile Only variable modification visibility can be achieved , There is no guarantee of atomicity ;synchronized The modification of variables can be guaranteed

Observability and atomicity .

volatile No thread blocking ;synchronized May cause thread blocking .

volatile Tagged variables are not optimized by the compiler ;synchronized Tagged variables can be optimized by the compiler .

From a practical point of view ,volatile One of the most important functions of is and CAS combination , It guarantees atomicity , For details, please refer to

java.util.concurrent.atomic Class under package , such as AtomicInteger .