JAVA非并发容器--HashMap-hash规则

概述

我相信只要写过JAVA的程序要拿99%的都用过HashMap, 其是我们最常用，也是最基础的一个Ｍap.本篇文章将从存储结构、hash规则、扩容策略、迭代器四方面来分析其源代码。

hash规则

当调用get,put后，首先需要计算key的hash值--hash(key)，然后计算在数组中的index--indexFor(hash, table.length).
hash(key):

  final int hash(Object k) {
        int h = hashSeed;
        if (0 != h && k instanceof String) {
            return sun.misc.Hashing.stringHash32((String) k);
        }
        h ^= k.hashCode();

        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

hash(key)为什么这么写？遵循一个原则:减少hash冲突,说实话，我也没有深入研究why.
indexFor(hash, table.length):

static int indexFor(int h, int length) {
        // assert Integer.bitCount(length) == 1 : "length must be a non-    zero power of 2";
        return h & (length-1);
    }

这句代码很有意思，它隐含了一个条件: table.length一个的2的n次方,
从而用&代替%,提高效率。
hashSeed, 在计算hash的时候，用到了一个hash种子:hashSeed,其是怎么得来的呢?

final boolean initHashSeedAsNeeded(int capacity) {
        boolean currentAltHashing = hashSeed != 0;
        boolean useAltHashing = sun.misc.VM.isBooted() &&
                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
        boolean switching = currentAltHashing ^ useAltHashing;
        if (switching) {
            hashSeed = useAltHashing
                ? sun.misc.Hashing.randomHashSeed(this)
                : 0;
        }
        return switching;
    }

Holder:

    private static class Holder {

        /**
         * Table capacity above which to switch to use alternative hashing.
         */
        static final int ALTERNATIVE_HASHING_THRESHOLD;

        static {
            String altThreshold = java.security.AccessController.doPrivileged(
                new sun.security.action.GetPropertyAction(
                    "jdk.map.althashing.threshold"));

            int threshold;
            try {
                threshold = (null != altThreshold)
                        ? Integer.parseInt(altThreshold)
                        : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;

                // disable alternative hashing if -1
                if (threshold == -1) {
                    threshold = Integer.MAX_VALUE;
                }

                if (threshold < 0) {
                    throw new IllegalArgumentException("value must be positive integer.");
                }
            } catch(IllegalArgumentException failed) {
                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
            }
            ALTERNATIVE_HASHING_THRESHOLD = threshold;
        }
    }

这个地发，没理解透.