index.jsp代码：

    <form action="servlet/Validate" method="post">

    用户名：<input type="text" name="username"><br>

    密&nbsp;&nbsp;&nbsp;&nbsp;码：<input type="password" name="password"><br>

    <input type="submit" value="登陆">

    </form>

Validate代码：

request.setCharacterEncoding("utf-8");

String username=request.getParameter("username");

String password=request.getParameter("password");

if("张三".equals(username)&&"123".equals(password)){

request.getRequestDispatcher("/success.jsp").forward(request, response);

}else{

response.sendRedirect("../error.jsp");

}

success.jsp代码：

 <h1>用户登陆成功！</h1>

    <hr>

    <%request.setCharacterEncoding("utf-8"); %>

    用户名：<%=request.getParameter("username") %>&nbsp;&nbsp;密码：<%=request.getParameter("password") %>

error.jsp代码：

 <h1>用户名或密码错误</h1>

    <hr>

    <a href="index.jsp">点击返回登陆界面</a>