[JUC] concurrent keyword volatile

Concurrent keywords you need to know volatile

1. keyword volatile What is it? ？

volatile yes Java Lightweight synchronization mechanism provided by virtual machine .
Its three characteristics are ：
1> visibility
2> There is no guarantee of atomicity
3> Prohibit command rearrangement
The detailed explanation of specific features will not be repeated here ;

1.1 visibility

volatile The variable visibility of decoration is mainly reflected in the following aspects ：

• Any thread pair volatile The write of modified variables will be synchronized to the main memory immediately ;
• Other threads will read the latest value when reading this variable , That is to say, the latest content ;
  

1.2 There is no guarantee of atomicity

This happens in the case of concurrency ：

1.3 Prohibit command rearrangement

 When the computer is executing the program ,  To improve performance ,  Compilers and processors reorder instructions .
1. Single threaded environment ,  Reordering can ensure that the final result of the program is consistent with the result of sequential execution 
2. The processor processing reordering must consider the data dependency between instructions ( Such as b rely on a,  Can't be b Sort in a Before execution )
3. During multithread scheduling ,  Due to reordering ,  The variables of two threads cannot guarantee consistency 

3. keyword volatile What scenarios are applicable to ？

volatile It can be understood as lightweight synchronized, Most suitable for a thread to write , When multiple threads read .
We are java.util.concurrent.atomic Found in the atomic class of , Use volatile Keywords are atomic ;

4. Suggestions for practical use

If you use volatile Modifying variables , Be sure to pay attention to volatile Whether the feature of is applicable in the business scenario .