在32位的机器上对long/Double型变量进行加减操作存在并发隐患

在32位的机器上对long型变量进行加减操作存在并发隐患的说法是正确的。

原因就是：线程切换带来的原子性问题。

非volatile类型的long和double型变量是8字节64位的，32位机器读或写这个变量时得把人家咔嚓分成两个32位操作，可能一个线程读了某个值的高32位，低32位已经被另一个线程改了。所以官方推荐最好把long\double 变量声明为volatile或是同步加锁synchronize以避免并发问题。

 

贴一段java文档说明：

https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html#jls-17.7

17.7. Non-Atomic Treatment of double and long

For the purposes of the Java programming language memory model, a single write to a non-volatile long or double value is treated as two separate writes: one to each 32-bit half. This can result in a situation where a thread sees the first 32 bits of a 64-bit value from one write, and the second 32 bits from another write.

Writes and reads of volatile long and double values are always atomic.

Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values.

Some implementations may find it convenient to divide a single write action on a 64-bit long or double value into two write actions on adjacent 32-bit values. For efficiency's sake, this behavior is implementation-specific; an implementation of the Java Virtual Machine is free to perform writes to long and double values atomically or in two parts.

Implementations of the Java Virtual Machine are encouraged to avoid splitting 64-bit values where possible. Programmers are encouraged to declare shared 64-bit values as volatile or synchronize their programs correctly to avoid possible complications.