同步和Java内存模型(四)有序性

原文链接: http://gee.cs.oswego.edu/dl/cpj/jmm.html

作者:Doug lea 译者:杜建雄  校对者:欧振聪,方腾飞

有序性 

有序性规则表现在以下两种场景: 线程内和线程间

  •  从某个线程的角度看方法的执行,指令会按照一种叫“串行”(as-if-serial)的方式执行,此种方式已经应用于顺序编程语言。
  •  这个线程“观察”到其他线程并发地执行非同步的代码时,任何代码都有可能交叉执行。唯一起作用的约束是:对于同步方法,同步块以及volatile字段的操作仍维持相对有序。

 

再次提醒,这些仅是最小特性的规则。具体到任何一个程序或平台上,可能存在更严格的有序性规则。所以你不能依赖它们,因为即使你的代码遵循了这些更严格的规则,仍可能在不同特性的JVM上运行失败,而且测试非常困难。

需要注意的是,线程内部的观察视角被JLS [1] 中其他的语义的讨论所采用。例如,算术表达式的计算在线程内看来是从左到右地执行操作(JLS 15.6章节),而这种执行效果是没有必要被其他线程观察到的。

仅当某一时刻只有一个线程操作变量时,线程内的执行表现为串行。出现上述情景,可能是因为使用了同步,互斥体[2] 或者纯属巧合。当多线程同时运行在非同步的代码里进行公用字段的读写时,会形成一种执行模式。在这种模式下,代码会任意交叉执行,原子性和可见性会失效,以及产生竞态条件。这时线程执行不再表现为串行。

尽管JLS列出了一些特定的合法和非法的重排序,如果碰到所列范围之外的问题,会降低以下这条实践保证 :运行结果反映了几乎所有的重排序产生的代码交叉执行的情况。所以,没必要去探究这些代码的有序性。

译注:

【1】JLS:Java Language Specification ,Java语言规范

【2】互斥体:原文为structural exclusion,译者认为意同 mutual exclusion ,详见 互斥体

原文

Synchronization and the Java Memory Model

Ordering

Ordering rules fall under two cases, within-thread and between-thread:

  • From the point of view of the thread performing the actions in a method, instructions proceed in the normal as-if-serial manner that applies in sequential programming languages.
  • From the point of view of other threads that might be “spying” on this thread by concurrently running unsynchronized methods, almost anything can happen. The only useful constraint is that the relative orderings of synchronized methods and blocks, as well as operations on volatile fields, are always preserved.

Again, these are only the minimal guaranteed properties. In any given program or platform, you may find stricter orderings. But you cannot rely on them, and you may find it difficult to test for code that would fail on JVM implementations that have different properties but still conform to the rules.

Note that the within-thread point of view is implicitly adopted in all other discussions of semantics in JLS. For example, arithmetic expression evaluation is performed in left-to-right order (JLS section 15.6) as viewed by the thread performing the operations, but not necessarily as viewed by other threads.

The within-thread as-if-serial property is helpful only when only one thread at a time is manipulating variables, due to synchronization, structural exclusion, or pure chance. When multiple threads are all running unsynchronized code that reads and writes common fields, then arbitrary interleavings, atomicity failures, race conditions, and visibility failures may result in execution patterns that make the notion of as-if-serial just about meaningless with respect to any given thread.

Even though JLS addresses some particular legal and illegal reorderings that can occur, interactions with these other issues reduce practical guarantees to saying that the results may reflect just about any possible interleaving of just about any possible reordering. So there is no point in trying to reason about the ordering properties of such code.

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杜 建雄

2012年毕业,先后在UC北京分部和广州UC工作。主要参与后端程序的java开发以及php web开发。对java多线程编程感兴趣,期望用编程技能做些与自身爱好相结合的事情。stay hungry,stay foolish(像外行一样思考,像专家一样实践)。2015年从心出发

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