[multithreading] non atomic agreement of long and double

long and double The nonatomic agreement of

Java Memory model requirements lock、unlock、read、load、assign、use、store、write this 8 All operations are atomic , But for 64 Bit data type （double、long） Defines relatively loose regulations ： Allowing virtual machines will not be volatile Embellished 64 The read-write operation of bit data is divided into two operations 32 Bit operation , That is, the virtual machine can be allowed without guarantee 64 Bit data type load、store、read and write Atomicity of operation .

Possible problems caused by non atomic agreements
If there are multiple threads sharing an undeclared volatile Of long or double Variable of type , And read and modify it at the same time , It is possible that a thread will read " Half a variable " Or half correct and half wrong failure data .

Solution in practical application
Because of the problems that may be caused by the above , It's bound to be right long and double Type variables should be operated with volatile keyword , In fact, it is as follows ：

1、64 Bit java This problem does not exist in virtual machines , Can operate 64 A data

2、 Currently commercial JVM Basically, they will 64 The operation of bit data is implemented as an atomic operation

So we usually don't need to write code with long and double Variables are specifically declared as volatile