【翻譯三】什麼是JSR 133

JSR 133 - The Java(tm) Memory Model and Thread Specification Revision

Since 1997, several serious flaws have been discovered in the Java Memory Model as defined in Chapter 17 of the Java Language Specification. These flaws allowed for confusing behaviors (such as final fields being observed to change their value) and undermined the compiler's ability to perform common optimizations.

1997年，Java語言規範中，第17章Threads and Locks，暴露了JMM的一些嚴重漏洞。這些漏洞引發混亂的行爲，好比final字段的值會發生變化，同時也會破壞編譯器的普通的優化能力。

The Java Memory Model was an ambitious undertaking; it was the first time that a programming language specification attempted to incorporate a memory model which could provide consistent semantics for concurrency across a variety of architectures. Unfortunately, defining a memory model which is both consistent and intuitive proved far more difficult than expected. JSR 133 defines a new memory model for the Java language which fixes the flaws of the earlier memory model. In order to do this, the semantics of final and volatile needed to change.

JMM是編程語言規範中的第一次，試圖在不一樣處理器架構下，經過加入語義一致的多線程內存模型，來支持併發的，JMM，是一個野心勃勃的事業。不幸的是，定義一個一致並直觀的內存模型遠比指望的要困難。JSR 133規範定義了一個新的內存規範來修復早先的漏洞，同時也改變了final和volatile的語義。

The full semantics are available at http://www.cs.umd.edu/users/pugh/java/memoryModel, but the formal semantics are not for the timid. It is surprising, and sobering, to discover how complicated seemingly simple concepts like synchronization really are. Fortunately, you need not understand the details of the formal semantics -- the goal of JSR 133 was to create a set of formal semantics that provides an intuitive framework for how volatile, synchronized, and final work.

可在以上網址看到全部的語義，正式的語義是很清楚的。但使人吃驚的是，像synchronized這樣看似簡單的概念事實上卻很複雜。。。幸運的是，你不須要理解這些正式語義的細節，JSR 133的目的，就是爲了建立一組正式的語義，用直觀的框架來解釋volatile、synchronized、final是怎麼工做的。

The goals of JSR 133 include:

JSR 133的目的在於：

• Preserving existing safety guarantees, like type-safety, and strengthening others. For example, variable values may not be created "out of thin air": each value for a variable observed by some thread must be a value that can reasonably be placed there by some thread.

• 維護現有的安全保證，如類型安全，並強化其餘組件。例如，變量的值不能憑空產生，一些線程若是能看到一個變量的值，那這個變量必須是由其它線程設置的。

• The semantics of correctly synchronized programs should be as simple and intuitive as possible.

• 使程序言簡意賅，正確同步的程序的語義應該儘量的簡單和直觀

• The semantics of incompletely or incorrectly synchronized programs should be defined so that potential security hazards are minimized.

• 未完整或正確同步的程序，語義上應該使潛在的安全風險降到最低

• Programmers should be able to reason confidently about how multithreaded programs interact with memory.

• 程序員能自信的分析本身的多線程程序是如何與內存交互的

• It should be possible to design correct, high performance JVM implementations across a wide range of popular hardware architectures.

• 能夠在各類不一樣的硬件架構上，設計出正確且高效的JVM實現

• A new guarantee of initialization safety should be provided. If an object is properly constructed (which means that references to it do not escape during construction), then all threads which see a reference to that object will also see the values for its final fields that were set in the constructor, without the need for synchronization.

• 應該提供一種新的初始化安全保證，即一個對象應該被合適地構造。合適意味着在構造過程當中，它不會被別處引用，這樣的話，等到其餘線程獲得這個對象引用的時候，就已經獲得了對象中final字段的值，將final修飾的字段放在構造方法中設值，就不須要同步了。

• There should be minimal impact on existing code.

• 應該對現有代碼影響最小