为什么JVM允许设置“高”？ IntegerCache的值，但不是“低”？ [英] Why does the JVM allow to set the "high" value for the IntegerCache, but not the "low"?

问题描述

我们都知道Java的整数（以及其他一些类型）的缓存数量范围 [ - 128,127] 被认为是常用。

We all know that Java has a cache for Integer (and some other types) for number in the range [-128, 127] which are considered to be "commonly used".

缓存的设计如下：

private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];

    static {
        // high value may be configured by property
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            try {
                int i = parseInt(integerCacheHighPropValue);
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
            } catch( NumberFormatException nfe) {
                // If the property cannot be parsed into an int, ignore it.
            }
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);

        // range [-128, 127] must be interned (JLS7 5.1.7)
        assert IntegerCache.high >= 127;
    }

    private IntegerCache() {}
}

我知道我可以通过向JVM提供参数来扩展 high 值：

I know that I can extend the high value by giving a parameter to the JVM :

java -Djava.lang.Integer.IntegerCache.high=xxxx Aclass.class

我不明白为什么我们不允许覆盖 low 值？

What I don't understand is why aren't we allowed to override the low value ?

注意我并没有试图找到一种解决方法，而是理解为什么不允许出于某些不明原因。

Note that I was not trying to find a workaround, but instead understand why it is not allowed for some obscure reasons.

推荐答案

发现已经有一个未解决的 RFP 就此而言。

Found out that there is already an unresolved RFP on this.

Joe Darcy就此问题发表评论：

Joe Darcy commented on the issue :

可以想象，缓存更大范围的
负数也会有所帮助，但是然而，没有迫切需要
这样做。

It is conceivable that it would be helpful to cache a greater range of negative number as well, but as of yet there has been no pressing need to do so.

请注意，RFP提供了一个糟糕的解决方法，如果有人有兴趣在处理之前使用它：

Note that the RFP offer a poor workaround if ever someone was interested in using this before it is handled :

除了实现你的实现你的缓存大小之外，没有办法增加
在转到java.lang.Integer类之前，首先拥有自己的缓存和
。
这显然不太好，因为它需要调用其他类
，而不仅仅是默认使用Integer.valueOf（int i）调用。

这篇关于为什么JVM允许设置“高”？ IntegerCache的值，但不是“低”？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！