Java中Arrays.sort排序源码分析

本篇以JDK1.7版本来说明：

首先说明：Arrays.sort()方法是根据传出参数的长度的大小来判断用哪种排序方法。

上一波源码：

public static void sort(int[] a) {
        DualPivotQuicksort.sort(a, 0, a.length - 1, null, 0, 0);
    }

final class DualPivotQuicksort {

    /**
     * Prevents instantiation.
     */
    private DualPivotQuicksort() {}

    /*
     * Tuning parameters.
     */

    /**
     * The maximum number of runs in merge sort.
     */
    private static final int MAX_RUN_COUNT = 67;

    /**
     * The maximum length of run in merge sort.
     */
    private static final int MAX_RUN_LENGTH = 33;

    /**
     * If the length of an array to be sorted is less than this
     * constant, Quicksort is used in preference to merge sort.
     */
    private static final int QUICKSORT_THRESHOLD = 286;

    /**
     * If the length of an array to be sorted is less than this
     * constant, insertion sort is used in preference to Quicksort.
     */
    private static final int INSERTION_SORT_THRESHOLD = 47;

    /**
     * If the length of a byte array to be sorted is greater than this
     * constant, counting sort is used in preference to insertion sort.
     */
    private static final int COUNTING_SORT_THRESHOLD_FOR_BYTE = 29;

    /**
     * If the length of a short or char array to be sorted is greater
     * than this constant, counting sort is used in preference to Quicksort.
     */
    private static final int COUNTING_SORT_THRESHOLD_FOR_SHORT_OR_CHAR = 3200;

    /*
     * Sorting methods for seven primitive types.
     */

    /**
     * Sorts the specified range of the array using the given
     * workspace array slice if possible for merging
     *
     * @param a the array to be sorted
     * @param left the index of the first element, inclusive, to be sorted
     * @param right the index of the last element, inclusive, to be sorted
     * @param work a workspace array (slice)
     * @param workBase origin of usable space in work array
     * @param workLen usable size of work array
     */
    static void sort(int[] a, int left, int right,
                     int[] work, int workBase, int workLen) {
        // Use Quicksort on small arrays
        if (right - left < QUICKSORT_THRESHOLD) {
            sort(a, left, right, true);
            return;
        }

        /*
         * Index run[i] is the start of i-th run
         * (ascending or descending sequence).
         */
        int[] run = new int[MAX_RUN_COUNT + 1];
        int count = 0; run[0] = left;

        // Check if the array is nearly sorted
        for (int k = left; k < right; run[count] = k) {
            if (a[k] < a[k + 1]) { // ascending
                while (++k <= right && a[k - 1] <= a[k]);
            } else if (a[k] > a[k + 1]) { // descending
                while (++k <= right && a[k - 1] >= a[k]);
                for (int lo = run[count] - 1, hi = k; ++lo < --hi; ) {
                    int t = a[lo]; a[lo] = a[hi]; a[hi] = t;
                }
            } else { // equal
                for (int m = MAX_RUN_LENGTH; ++k <= right && a[k - 1] == a[k]; ) {
                    if (--m == 0) {
                        sort(a, left, right, true);
                        return;
                    }
                }
            }

            /*
             * The array is not highly structured,
             * use Quicksort instead of merge sort.
             */
            if (++count == MAX_RUN_COUNT) {
                sort(a, left, right, true);
                return;
            }
        }

        // Check special cases
        // Implementation note: variable "right" is increased by 1.
        if (run[count] == right++) { // The last run contains one element
            run[++count] = right;
        } else if (count == 1) { // The array is already sorted
            return;
        }

        // Determine alternation base for merge
        byte odd = 0;
        for (int n = 1; (n <<= 1) < count; odd ^= 1);

        // Use or create temporary array b for merging
        int[] b;                 // temp array; alternates with a
        int ao, bo;              // array offsets from 'left'
        int blen = right - left; // space needed for b
        if (work == null || workLen < blen || workBase + blen > work.length) {
            work = new int[blen];
            workBase = 0;
        }
        if (odd == 0) {
            System.arraycopy(a, left, work, workBase, blen);
            b = a;
            bo = 0;
            a = work;
            ao = workBase - left;
        } else {
            b = work;
            ao = 0;
            bo = workBase - left;
        }

        // Merging
        for (int last; count > 1; count = last) {
            for (int k = (last = 0) + 2; k <= count; k += 2) {
                int hi = run[k], mi = run[k - 1];
                for (int i = run[k - 2], p = i, q = mi; i < hi; ++i) {
                    if (q >= hi || p < mi && a[p + ao] <= a[q + ao]) {
                        b[i + bo] = a[p++ + ao];
                    } else {
                        b[i + bo] = a[q++ + ao];
                    }
                }
                run[++last] = hi;
            }
            if ((count & 1) != 0) {
                for (int i = right, lo = run[count - 1]; --i >= lo;
                    b[i + bo] = a[i + ao]
                );
                run[++last] = right;
            }
            int[] t = a; a = b; b = t;
            int o = ao; ao = bo; bo = o;
        }
    }
}

其实我们没必要一步步看源码，只要理解以下几个阈值的含义：

 /**
     * The maximum number of runs in merge sort.
     */
    private static final int MAX_RUN_COUNT = 67;

    /**
     * The maximum length of run in merge sort.
     * 归并排序中的最大值，归并排序是稳定的，时间复   杂度为O（nlogn）
     */
    private static final int MAX_RUN_LENGTH = 33;

    /**
     * If the length of an array to be sorted is less than this
     * constant, Quicksort is used in preference to merge sort.
     * 快速排序中的最大值，快速排序是不稳定的，时间复杂度为O（nlogn），最坏情况下是O(n^2)
     */
    private static final int QUICKSORT_THRESHOLD = 286;

    /**
     * If the length of an array to be sorted is less than this
     * constant, insertion sort is used in preference to Quicksort.
     */
    private static final int INSERTION_SORT_THRESHOLD = 47;

    /**
     * If the length of a byte array to be sorted is greater than this
     * constant, counting sort is used in       preference to insertion sort.
     */
    private static final int COUNTING_SORT_THRESHOLD_FOR_BYTE = 29;

    /**
     * If the length of a short or char array to be sorted is greater
     * than this constant, counting sort is used in preference to Quicksort.
     */
    private static final int COUNTING_SORT_THRESHOLD_FOR_SHORT_OR_CHAR = 3200;

对于传入的数组长度在不同的阈值内，分别对应这个不同的排序方法，主要是有归并排序、快速排序、插入排序、计数排序。特别注意的是计数排序，主要分为两种情况：
(1)byte array 字节数组长度大于29，则计数排序优先于插排；
(2)short or char array 短整型或者字符数组长度大于3200，则计数排序优先于快速排序。