在进行数据交换时，如果主键不是整型，需要对字符串，或联合主键拼接为字符串，进行hash，再进行取模分片，使用的是String自带的hashCode()方法，本来是件很方便的事，但是有些字符串取hashCode竟然是负数，使得分片为负数，找不到对应的分片，我们先看一下String 生成hashCode的代码：
/** * Returns a hash code for this string. The hash code for a * {@code String} object is computed as * <blockquote><pre> * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] * </pre></blockquote> * using {@code int} arithmetic, where {@code s[i]} is the * <i>i</i>th character of the string, {@code n} is the length of * the string, and {@code ^} indicates exponentiation. * (The hash value of the empty string is zero.) * * @return a hash code value for this object. */ public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { char val[] = value; for (int i = 0; i < value.length; i++) { h = 31 * h + val[i]; } hash = h; } return h; }