什么时候在 Java 中使用 AtomicReference? [英] When to use AtomicReference in Java?
问题描述
我们什么时候使用 AtomicReference?
When do we use AtomicReference?
所有的多线程程序都需要创建对象吗?
Is it needed to create objects in all multithreaded programs?
提供一个应该使用 AtomicReference 的简单示例.
Provide a simple example where AtomicReference should be used.
推荐答案
原子引用应该用在你需要做简单的原子(即线程安全，非平凡)对引用的操作，基于监视器的同步不适合这种操作.假设您要检查特定字段是否仅当对象的状态保持为您上次检查时:
Atomic reference should be used in a setting where you need to do simple atomic (i.e. thread-safe, non-trivial) operations on a reference, for which monitor-based synchronization is not appropriate. Suppose you want to check to see if a specific field only if the state of the object remains as you last checked:
AtomicReference<Object> cache = new AtomicReference<Object>();
Object cachedValue = new Object();
cache.set(cachedValue);
//... time passes ...
Object cachedValueToUpdate = cache.get();
//... do some work to transform cachedValueToUpdate into a new version
Object newValue = someFunctionOfOld(cachedValueToUpdate);
boolean success = cache.compareAndSet(cachedValue,cachedValueToUpdate);
由于原子引用语义，即使 cache 对象在线程之间共享，您也可以这样做，而无需使用 synchronized.一般来说，除非您知道自己在做什么，否则最好使用同步器或 java.util.concurrent 框架而不是纯粹的 Atomic*.
Because of the atomic reference semantics, you can do this even if the cache object is shared amongst threads, without using synchronized. In general, you're better off using synchronizers or the java.util.concurrent framework rather than bare Atomic* unless you know what you're doing.
两个优秀的死树参考文献将向您介绍这个主题:
Two excellent dead-tree references which will introduce you to this topic:
注意(我不知道这是否一直都是真的)reference 赋值(即 =)本身是原子的(更新 primitive 64 位类型如 long 或 double 可能不是原子的；但更新 reference 总是原子的，即使它是 64 位的)没有显式使用 Atomic*.
请参阅 Java 语言规范 3ed，第 17.7 节.
Note that (I don't know if this has always been true) reference assignment (i.e. =) is itself atomic (updating primitive 64-bit types like long or double may not be atomic; but updating a reference is always atomic, even if it's 64 bit) without explicitly using an Atomic*.
See the Java Language Specification 3ed, Section 17.7.
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