将实例字段分配给局部变量 [英] assign instance field to local variable

问题描述

这是来自JDK的HashMap类的keySet（）方法。为什么作者将字段（keySet）分配给局部变量ks？

This is keySet() method on HashMap class from JDK. Why did the author assign the field (keySet) to local variable ks ?

public Set<K> keySet() {
    Set<K> ks;
    return (ks = keySet) == null ? (keySet = new KeySet()) : ks;
}

上面和下面有什么区别？这与线程安全有关吗？

What is the difference between the above and the below ? Does this have something to do with thread-safety ?

public Set<K> keySet() {
    return (keySet == null ? (keySet = new KeySet()) : keySet;
}

推荐答案

为了稍微扩展迈克尔的答案，我希望它能确保 keySet（）方法永远不会返回 null ，可能除了提供所提到的性能优势外。

To expand slightly on Michael's answer, I expect it is there to ensure that the keySet() method never returns null, possibly in addition to providing performance benefits noted.

鉴于此代码：

public Set<K> keySet() {
    return (keySet == null ? (keySet = new KeySet()) : keySet;
}

在多线程代码中至少理论上是可行的 keySet 字段可以在第一次读取之间设置为 null （ keySet == null ）和第二次读取，返回它。我没有看过其余的代码，但我认为还有其他地方 keySet 可能被分配 null 。这是否是因为在wil中看到的问题d，或防御措施对作者来说是一个问题。

It would be at least theoretically possible in multi-threaded code that the keySet field could be set to null between the first read (keySet == null) and the second read, where it is returned. I haven't looked at the rest of the code, but I assume there are other places where keySet is potentially assigned null. Whether this is as a result of an issue seen in the wild, or a defensive measure would be a question for the authors.

实际代码：

public Set<K> keySet() {
    Set<K> ks;
    return (ks = keySet) == null ? (keySet = new KeySet()) : ks;
}

...没有此问题，因为该字段仅读取一次。

...doesn't have this problem as the field is only read once.

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