zoukankan      html  css  js  c++  java
  • 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longdouble 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.

  • 相关阅读:
    2019-01-28 dubbo安装并启动测试
    JAVAWEB项目获取服务器、项目相关路径方法
    Hibernate中的查询方式
    Hibernate在eclipse中的简单运用
    JQuery Tips(2)----关于$()包装集你不知道的
    JQuery Tips(3)----关于$()包装集内元素的改变
    JQuery Tips(4)----一些关于提高JQuery性能的Tips
    利用FireBug使JQuery的学习更加轻松愉快
    asp.net core2 mvc 基础教程--中间件
    asp.net core2 mvc 基础教程--服务注册和管道
  • 原文地址:https://www.cnblogs.com/bbsh/p/11648970.html
Copyright © 2011-2022 走看看