java atomic volatile,关于volatile和Atomic类的一些疑问？

i am going thru Java threads book. I came across this statement

Statement 1:- "volatile variables can be safely used only for single load or store operation and can't be

applied to long or double variales. These restrictions make the use of volatile variables uncommon"

I did not get what does single load or store operation mean here? why volatile can't be

applied to long or double variales?

Statement 2:- "A Volatile integer can not be used with the ++ operator because ++ operator contains

multiple instructions.The AtomicInteger class has a method that allows the integer it holds to be

incremented atomically."

Why Volatile integer can not be used with the ++ operator and how AtomicInteger addresses it?

解决方案

Statement 1:- "volatile variables can be safely used only for single load or store operation and can't be applied to long or double variales. These restrictions make the use of volatile variables uncommon"

What?! I believe this is simply flat-out wrong. Maybe your book is out of date.

Statement 2:- "A Volatile integer can not be used with the ++ operator because ++ operator contains multiple instructions.The AtomicInteger class has a method that allows the integer it holds to be incremented atomically."

Exactly what it says. The ++ operator actually translates to machine code like this (in Java-like pseudocode):

sync_CPU_caches();

int processorRegister = variable;

processorRegister = processorRegister + 1;

variable = processorRegister;

sync_CPU_caches();

This is not thread-safe, because even though it has a memory barrier, and reads atomically, and writes atomically, it is not guaranteed that you won't get a thread switch in the middle, and processor registers are local to a CPU core (think of them as like "local variables" inside the CPU core). But an AtomicInteger is thread-safe - it probably is implemented using special machine code instructions such as compare-and-swap.