src/org/nutz/lang/Stopwatch.java
public static Stopwatch beginNano() {
        Stopwatch sw = new Stopwatch();
        sw.nano = true;
        sw.start();
        return sw;
    }
public static Stopwatch beginNano() {
        Stopwatch sw = new Stopwatch();
        sw.nano = true;
        sw.start();
        return sw;
    }
秒表开始计时，计时时间的最小单位是毫微秒

@return 开始计时的秒表对象