Explain the volatile keyword

One 、 brief introduction

1.Volatile Memory semantics

1. When you write a volatile variable ,JMM The shared variable value in the local memory corresponding to the thread will be immediately refreshed back to the main memory .
2. When reading a volatile variable ,JMM Will set the local memory corresponding to this thread to be invalid , Go back to main memory and read the latest shared variable .
3. therefore volatile The write memory semantics of is to refresh directly into main memory , The memory semantics of reading is to read directly from main memory .

2. Two characteristics

1. visibility ： Flush back to main memory immediately after writing and send notification in time , You can go to the main memory to get the latest version , The previous changes are visible to all subsequent threads ！
2. Prohibit command rearrangement ： Reordering is a means by which compiler and processor reorder instruction sequences in order to optimize program performance , Sometimes it will change the sequence of program statements . There is no data dependency , You can reorder ; There is a data dependency , No reordering . But the rearranged instructions can never change the original serial semantics ! This must be taken into account in concurrent design !

3. The above prohibition of instruction rearrangement is based on memory barrier ：

Roughly divided into two kinds
1. Reading barrier ： Insert a read barrier before reading instructions , Let working memory or CPU The cached data in the cache is invalid , Go back to jade memory to get the latest data .
2. Write barriers ： Insert a write barrier after writing instructions , Force data written to the buffer to be flushed back to main memory .

stay Java Level of Unsafe Classes have the following native Method ：