ConcurrentHashMap之spread、tabAt、casTabAt、setTabAt、resizeStamp、tableSizeFor

5.1——spread(hash) 源码分析

    / 
	* 1100 0011 1010 0101 0001 1100 0001 1110
     * 0000 0000 0000 0000 1100 0011 1010 0101
     * 1100 0011 1010 0101 1101 1111 1011 1011
     * ---------------------------------------
     * 1100 0011 1010 0101 1101 1111 1011 1011
     * 0111 1111 1111 1111 1111 1111 1111 1111  ==HASH_BITS
     * 0100 0011 1010 0101 1101 1111 1011 1011
     *
     * 当前的hash右移16位 亦或 原来的hash  & HASH_BITS 得到正数的hash值
     * 让高16位 也参与到运算中来
     */
    static final int spread(int h) {
        return (h ^ (h >>> 16)) & HASH_BITS;
    }

5.2——tabAt(tab, index) 源码分析

    //获取数组指定下标位置的元素
    @SuppressWarnings("unchecked")
    static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
        return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
    }

5.3——casTabAt(tab, index, c, v )源码分析

    //用cas的方式去table的指定位置设置值,设置成功返回true
    static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
                                        Node<K,V> c, Node<K,V> v) {
        return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
    }

5.4——setTabAt(tab, index, v) 源码分析

    //指定位置设置值
    static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
        U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
    }

5.5——resizeStamp(int n) 源码分析

    /**
     * Returns the stamp bits for resizing a table of size n.
     * Must be negative when shifted left by RESIZE_STAMP_SHIFT.
     * 扩容标识戳 一致才能参与扩容
     * 16 -> 32
     * numberOfLeadingZeros(16) => 1 0000 =>27 =>0000 0000 0001 1011
     * |		16-1						
     * (1 << (RESIZE_STAMP_BITS - 1)) => 1000 0000 0000 0000 => 32768
     * ---------------------------------------------------------------
     * 0000 0000 0001 1011
     * 1000 0000 0000 0000
     * 1000 0000 0001 1011
     */
    static final int resizeStamp(int n) {
        return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
    }

5.6——tableSizeFor(int c) 源码分析

    /**
     * Returns a power of two table size for the given desired capacity.
     * See Hackers Delight, sec 3.2
     * 返回>=c的最小的2的次方数
     * c=28
     * n=27 => 0b 11011
     * 11011 | 01101 => 11111
     * 11111 | 00111 => 11111
     * ....
     * => 11111 + 1 =100000 = 32
     */
    private static final int tableSizeFor(int c) {
        int n = c - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }