Three concurrent features 1- visibility

visibility ： When a thread changes the value of a shared variable , Other threads can see the modified value

The bottom layer is how to ensure visibility ？
1. adopt volatile Keywords guarantee visibility .
2. adopt Memory barrier ensures visibility .
3. adopt synchronized Keywords guarantee visibility .
4. adopt Lock Guaranteed visibility .
5. adopt final Keywords guarantee visibility

How to interact data between main memory and working memory ？

adopt 8 Big atom operation ：

lock（ lock ）： A variable acting on main memory , Identifies a variable as a thread exclusive state .
unlock（ Unlock ）： Operates on the main memory variable , Release a variable that is locked , Release
Later variables can be locked by other threads .
read（ Read ）： Operates on the main memory variable , Transfer a variable value from main memory to the thread's working memory
in , For subsequent load The action to use
load（ load ）： A variable acting on working memory , It is the read The operation puts the variable value obtained from the main memory
Into a copy of the variable in working memory
use（ Use ）： A variable acting on working memory , Pass the value of a variable in the working memory to the execution reference
Qing Dynasty , This is done whenever the vm gets a bytecode instruction that requires the value of the variable to be used .
assign（ assignment ）： A variable acting on working memory , It assigns a value received from the execution engine to
Working memory variables , This is done whenever the vm receives a bytecode instruction that assigns a value to a variable .
store（ Storage ）： A variable acting on working memory , Transfer the value of a variable in the working memory to the main
In the memory , For subsequent write The operation of .
write（ write in ）： A variable acting on main memory , It is the store Operate on the value of a variable from working memory
To a variable in main memory .
 

volatile Characteristics of
visibility ： To a volatile Variable reading , Always see （ Arbitrary thread ） For this volatile Variable most
Write after .
Atomicity ： For any single volatile Variable reading / It's atomic , But similar to volatile++ Such complex
The combination operation is not atomic
Orderliness ： Yes volatile Various specific memory barriers are added before and after the read and write operations of modified variables to prohibit referring to
Reorder to ensure order
 

volatile stay hotspot The implementation of the
Bytecode interpreter implementation

Conclusion ：x86 In the processor lock Achieve the effect similar to memory barrier

Assembly level volatile The implementation of the

Add the following jvm Parameter visibility before viewing Demo Assembly instructions for

‐XX

:

+UnlockDiagnosticVMOptions

‐XX

:

+PrintAssembly

‐Xcomp