JSP 表单处理
GET 与 POST 表单、参数读取、复选框与下拉框、中文编码。
Key Points表单字段名就是参数名；复选框多值用 paramValues，单选下拉只传一个值。
GET 表单
JSP
<form action="search.jsp" method="get">
关键词：<input name="q">
<button>搜索</button>
</form>GET appends parameters to the URL: search.jsp?q=java. Bookmarkable, shareable, crawlable, but length-limited and visible — never for passwords.
POST 表单
JSP
<form action="save.jsp" method="post">
姓名：<input name="name"><br>
性别：<input type="radio" name="sex" value="m" checked>男
<input type="radio" name="sex" value="f">女<br>
爱好：<input type="checkbox" name="hobby" value="read">阅读
<input type="checkbox" name="hobby" value="run">跑步<br>
城市：<select name="city">
<option value="bj">北京</option>
<option value="sh">上海</option>
</select><br>
<button>提交</button>
</form>接收参数
JSP
<%@ page contentType="text/html;charset=UTF-8" %>
<%
request.setCharacterEncoding("UTF-8");
String name = request.getParameter("name");
String sex = request.getParameter("sex");
String city = request.getParameter("city");
String[] hobbies = request.getParameterValues("hobby");
%>
<p>姓名：<%= name %></p>
<p>性别：<%= "m".equals(sex) ? "男" : "女" %></p>
<p>城市：<%= city %></p>
<p>爱好：<%= hobbies == null ? "未选择" : String.join("、", hobbies) %></p>Output
姓名：张三
性别：男
城市：北京
爱好：阅读、跑步EL 写法
JSP
<p>姓名：${param.name}</p>
<p>城市：${param.city}</p>
<p>爱好：${paramValues.hobby[0]}、${paramValues.hobby[1]}</p>EL prints an empty string instead of null, so no ugly "null" on the page. Array index access throws when out of range, though — iterate with <c:forEach> or check the length first.
回填表单
JSP
<input name="name" value="<c:out value='${param.name}'/>">
<select name="city">
<option value="bj" ${param.city == 'bj' ? 'selected' : ''}>北京</option>
<option value="sh" ${param.city == 'sh' ? 'selected' : ''}>上海</option>
</select>
<input type="checkbox" name="hobby" value="read"
${fn:contains(fn:join(paramValues.hobby, ','), 'read') ? 'checked' : ''}>阅读After a validation failure, re-populate what the user typed or they must redo the whole form. This is the most overlooked detail in form UX and one users feel immediately.
request.setCharacterEncoding("UTF-8") must come before the first parameter read. Once parameters are read the encoding is fixed and later calls do nothing — which looks exactly like "all Chinese input is garbled".GET 表单
JSP
<form action="search.jsp" method="get">
关键词：<input name="q">
<button>搜索</button>
</form>GET 把参数拼在 URL 上：search.jsp?q=java。可以收藏、可以分享、可以被搜索引擎抓取，但长度受限且内容可见——不适合密码。
POST 表单
JSP
<form action="save.jsp" method="post">
姓名：<input name="name"><br>
性别：<input type="radio" name="sex" value="m" checked>男
<input type="radio" name="sex" value="f">女<br>
爱好：<input type="checkbox" name="hobby" value="read">阅读
<input type="checkbox" name="hobby" value="run">跑步<br>
城市：<select name="city">
<option value="bj">北京</option>
<option value="sh">上海</option>
</select><br>
<button>提交</button>
</form>接收参数
JSP
<%@ page contentType="text/html;charset=UTF-8" %>
<%
request.setCharacterEncoding("UTF-8");
String name = request.getParameter("name");
String sex = request.getParameter("sex");
String city = request.getParameter("city");
String[] hobbies = request.getParameterValues("hobby");
%>
<p>姓名：<%= name %></p>
<p>性别：<%= "m".equals(sex) ? "男" : "女" %></p>
<p>城市：<%= city %></p>
<p>爱好：<%= hobbies == null ? "未选择" : String.join("、", hobbies) %></p>Output
姓名：张三
性别：男
城市：北京
爱好：阅读、跑步EL 写法
JSP
<p>姓名：${param.name}</p>
<p>城市：${param.city}</p>
<p>爱好：${paramValues.hobby[0]}、${paramValues.hobby[1]}</p>EL 读不到值时输出空串而不是 null，页面上不会出现难看的「null」。但数组越界会抛异常，多值参数要先用 <c:forEach> 遍历或判断长度。
回填表单
JSP
<input name="name" value="<c:out value='${param.name}'/>">
<select name="city">
<option value="bj" ${param.city == 'bj' ? 'selected' : ''}>北京</option>
<option value="sh" ${param.city == 'sh' ? 'selected' : ''}>上海</option>
</select>
<input type="checkbox" name="hobby" value="read"
${fn:contains(fn:join(paramValues.hobby, ','), 'read') ? 'checked' : ''}>阅读校验失败后要把用户填过的值填回去，否则用户得重填整个表单。这是表单体验里最容易被忽略、也最影响感受的一点。
request.setCharacterEncoding("UTF-8") 必须放在第一次读参数之前。一旦读过参数，编码就定死了，后面再设无效——表现就是「表单里的中文全是乱码」。