jetbrick-commons/src/main/java/jetbrick/io/PathUtils.java
public static String concat(final String parent, final String child) {
        if (parent == null) {
            return normalize(child);
        }
        if (child == null) {
            return normalize(parent);
        }
        return normalize(parent + '/' + child);
    }
public static String concat(final String parent, final String child) {
        if (parent == null) {
            return normalize(child);
        }
        if (child == null) {
            return normalize(parent);
        }
        return normalize(parent + '/' + child);
    }
组合路径.