java - HashMap中的Holder类有什么用？

Question

我发现在HashMap中我们有一个内部类Holder，其描述如下:

/**
 * holds values which can't be initialized until after VM is booted.
 */

我们必须如何以及何时使用该类？它有什么用处。请解释。

Answer 1

这与 Java 7u6 的改进有关，在 Java 8 中已被删除。

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相关文档:

Collections Framework Enhancements in Java SE 7

The alternative hash function improves the performance of these map implementations when a large number of key hash collisions are encountered.

For Java SE 7u6, this alternative hash function is implemented as follows:

The alternative hash function is only applied to maps with a capacity larger than a specified threshold size. By default, the threshold is -1. This value disables the alternative hash function. To enable the alternative hash function, set the jdk.map.althashing.threshold system property to a different value. The recommended value is 512. Setting this system property to 512 causes all maps with a capacity larger than 512 entries to use the alternative hash function. You can set this system property to 0, which causes all maps to use the alternative hash function.

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和

Collections Framework Enhancements in Java SE 8

The alternative String hash function added in 7u6 has been removed from JDK 8, along with the jdk.map.althashing.threshold system property. Instead, hash bins containing a large number of colliding keys improve performance by storing their entries in a balanced tree instead of a linked list.

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现在，回答你的问题:

How and when we have to use that class?

是什么让您认为应该使用它？是私有(private)类without any public docs ，所以你不必关心它。它是 Oracle 的 HashMap 的一个实现细节，您不能直接使用它。您可以间接使用它的唯一方法是通过 jdk.map.althashing.threshold 系统属性。

为什么 Oracle 的工程师要使用这样的支架？由于类加载顺序。当它们彼此有很多依赖关系时，VM 很难加载所有类，它可能会卡住。因此，所有内部类的开发人员都非常确保他们没有使用可能尚未加载/可能导致加载顺序问题的类的方法/属性。

这就是一个这样的助手，它仅在加载所有其他类并且 VM 完全启动后才初始化一个值。 holder 中的值只有在第一次访问类时才会被初始化，并且此访问被 sun.misc.VM.isBooted() 调用屏蔽。