- [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.

assertThat(StringUtils.isJavaIdentifier("1"), is(false));
        assertThat(StringUtils.isJavaIdentifier("abc123"), is(true));
        assertThat(StringUtils.isJavaIdentifier("abc(23)"), is(false));
    }
    @Test
    public void testExceptionToString() throws Exception {
        assertThat(StringUtils.toString(new RuntimeException("abc")), containsString("java.lang.RuntimeException: abc"));
    }
    @Test
    public void testExceptionToStringWithMessage() throws Exception {
        String s = StringUtils.toString("greeting", new RuntimeException("abc"));
        assertThat(s, containsString("greeting"));
        assertThat(s, containsString("java.lang.RuntimeException: abc"));
    }
    @Test
    public void testParseQueryString() throws Exception {
        assertThat(StringUtils.getQueryStringValue("key1=value1&key2=value2", "key1"), equalTo("value1"));
        assertThat(StringUtils.getQueryStringValue("key1=value1&key2=value2", "key2"), equalTo("value2"));
        assertThat(StringUtils.getQueryStringValue("", "key1"), isEmptyOrNullString());
    }
    @Test
    public void testGetServiceKey() throws Exception {
        Map<String, String> map = new HashMap<String, String>();
        map.put(GROUP_KEY, "dubbo");
        map.put(INTERFACE_KEY, "a.b.c.Foo");
        map.put(VERSION_KEY, "1.0.0");
        assertThat(StringUtils.getServiceKey(map), equalTo("dubbo/a.b.c.Foo:1.0.0"));
    }
    @Test
    public void testToQueryString() throws Exception {