- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.4
* Operating System version: Darwin Kernel Version 18.7.0
* Java version: 1.8.0_181-b13)

### Steps to reproduce this issue

1. invoke `StringUtils.split("d,1,2,4", 'a')`
2. invoke `"d,1,2,4".split("a")`

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

`StringUtils.split("d,1,2,4", 'a')` should return `["d,1,2,4"]` just like `"d,1,2,4".split("a")`.

### Actual Result

empty array.

*/
    public static String toString(Throwable e) {
        UnsafeStringWriter w = new UnsafeStringWriter();
        PrintWriter p = new PrintWriter(w);
        p.print(e.getClass().getName());
        if (e.getMessage() != null) {
            p.print(": " + e.getMessage());
        }
        p.println();
        try {
            e.printStackTrace(p);
            return w.toString();
        } finally {
            p.close();
        }
    }
    /**
     * @param msg
     * @param e
     * @return string
     */
    public static String toString(String msg, Throwable e) {
        UnsafeStringWriter w = new UnsafeStringWriter();
        w.write(msg + "\n");
        PrintWriter p = new PrintWriter(w);
        try {
            e.printStackTrace(p);
            return w.toString();
        } finally {
            p.close();