tomcat的中文乱码问题

Question

<form action="register1" method="get">
        用户名：<input type="text" name="username" value='aaa'><br>
    </form>

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        String str=req.getParameter("username");
        PrintWriter pw=resp.getWriter();
        pw.println(str);
        pw.println("刷刷刷");
    }

为什么第一个不是乱码，第二个是乱码