什么时候在 Java 中使用 volatile boolean 而不是 AtomicBoolean 更可取? [英] When is it preferable to use volatile boolean in Java rather than AtomicBoolean?

问题描述

我查看了 SO 中的其他 volatile 与 Atomicxxxx 问题(包括这个) 并已阅读 java.util.current.atomic 的描述，我对其中的细微差别不太满意.

I've looked at the other volatile vs. Atomicxxxx questions in SO (including this one) and have read the description of java.util.current.atomic, and I am not quite satisfied with the nuances.

如果我试图在使用 volatile boolean 和 AtomicBoolean 之间做出决定，除了 AtomicBoolean 提供的原子读-修改-写操作之外，还有实际区别吗?(例如 compareAndSet() 和 getAndSet())

If I'm trying to decide between using volatile boolean and AtomicBoolean, are there practical differences besides the atomic read-modify-write operations offered by AtomicBoolean? (e.g. compareAndSet() and getAndSet())

假设我有

volatile boolean flag;

然后一个或多个线程设置标志(但不清除它).如果我有一个线程读取标志，如果设置，则执行一个操作，然后清除标志，volatile 是否足够?

Then one or more threads set the flag (but not clear it). If I have one thread that reads the flag, and if set, does an action, and then clears the flag, is volatile adequate?

AtomicBoolean 的成本是否比 volatile boolean 更高，就

Is there a higher cost to AtomicBoolean than volatile boolean, in terms of

• 内存空间
• 性能下降(volatile boolean 似乎需要内存防护，AtomicBoolean 似乎需要内存防护 + 根据 java.util.current 对 CAS 操作进行一些小锁定.原子描述)

• memory space
• performance hit (volatile boolean appears to require memory fencing, AtomicBoolean appears to require memory fencing + some minor locking on CAS operations as per the java.util.current.atomic description)

我的直觉是只使用 AtomicBoolean 并保持安全，但我想了解是否有使用 volatile boolean 代替的情况(例如，如果我有数千个实例和性能是一个问题).

My gut call is to just go with AtomicBoolean and be safe, but I want to understand if there's ever a situation to use volatile boolean instead (e.g. if I had thousands of instances of them and performance were an issue).

推荐答案

基本上所有 AtomicBoolean 都是对象中的 volatile boolean.

Essentially all AtomicBoolean is a volatile boolean in an object.

每个对象会有少量开销.可能微不足道，但可能有更多内存可以进入缓存.

There will be a small overhead per object. Probably insignificant, but possibly more memory to get into cache.

如果你需要使用AtomicBooleanFieldUpdater，那么性能开销会很大.如果你不经常使用也没关系(如attach在蔚来).

If you need to use AtomicBooleanFieldUpdater then there is quite a lot of performance overhead.It can be okay if you aren't going to do it often (as attach in NIO).

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