"Ask every day" what is volatile

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Memory semantics

volatile Write ： When you write a volatile Variable time ,JMM All shared variables in the local memory of this thread will be flushed to the main memory . Be careful , It's all , Not just volatile Variable
volatile read ： When reading a volatile Variable time ,JMM The local memory of this thread will be set as invalid , Then read the shared variables from main memory .

If the above two steps are taken together , The reading thread is reading a volatile Variable time , The writing thread is writing this volatile All operations on shared variables before variables will immediately become visible to the reader thread .

Implementation of memory semantics

I said before , Reordering is divided into compiler and processor rearrangement . In order to achieve volatile What about the semantics of ,JMM Will limit these two reordering .

For compilers ：

1. For the latter operation volatile When writing , No matter what the previous operation is , Can't reorder .
2. The previous operation is volatile When reading , No matter what the latter operation is , Can't reorder .
3. Previous operation volatile Write , The latter operation volatile read , Cannot reorder .

For processors ：
When the compiler generates bytecode , A memory barrier is inserted in the instruction sequence to prevent a particular type of processor reordering .

Recommended reading ：Java Theory and practice ： Use... Correctly Volatile Variable