Servlet（Server Applet），全称Java Servlet，是用Java编写的服务器端程序。其主要功能在于交互式地浏览和修改数据，生成动态Web内容。本系列将一步步地写出一个Servlet程序。
这篇博文将演示如何分别处理GET和POST请求，以及处理请求中的参数。
编写doGet()和doPost()方法
首先把要实现的功能写好，后面才好调用不是。
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| public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doGet()</code></b>");
writer.print("</body>");
writer.print("</html>");
}
}
|
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| public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doPost()</code></b>");
writer.print("</body>");
writer.print("</html>");
}
}
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区分HTTP方法
因为servlet是调用service()方法来处理请求的，所以对请求做区分也需要在service()方法中进行。
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| @Override
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
HttpServletRequest httpServletRequest = (HttpServletRequest) req;
HttpServletResponse httpServletResponse = (HttpServletResponse) res;
if ("GET".equalsIgnoreCase(httpServletRequest.getMethod())) {
doGet(httpServletRequest, httpServletResponse);
} else if ("POST".equalsIgnoreCase(httpServletRequest.getMethod())) {
doPost(httpServletRequest, httpServletResponse);
} else {
httpServletResponse.sendError(501);
}
}
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运行起来看看效果
首先发个GET请求
再发个POST请求
为什么不用HttpServlet类呢
没错，上面做的，就是自己实现了一个简陋的HttpServlet类，因为是循序渐进嘛，没头没脑的直接砸上来一个，算什么循序渐进。
那么现在就让DemoServlet继承HttpServlet。同时，因为HttpServlet已经在service()方法中实现了判断请求类型，所以DemoServlet中不要覆盖service()方法，只覆盖doGet()和doPost()方法。
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| public class DemoServlet extends HttpServlet {
@Override
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doGet()</code></b>");
writer.print("</body>");
writer.print("</html>");
}
}
@Override
public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doPost()</code></b>");
writer.print("</body>");
writer.print("</html>");
}
}
}
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处理请求中的参数
HTTP请求是可以带参数的，有了参数，那就得处理。
处理GET请求的参数
GET请求里带的参数，名字叫查询字符串(query string)，是一组或多组key=value格式的键值对。
Query string写在URL后面，以一个问号起头，用&分隔各个键值对，即类似http://localhost:8080/appname/servlet?arg1=value1&arg2=value2&...&argN=valueN。
在代码里使用HttpServletRequest#getQueryString()方法，就可以获取到问号后面的query string，分别用&和=分割字符串，就可以取到每个参数的key和value。
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| @Override
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
Optional<String> optionalQueryString = Optional.ofNullable(request.getQueryString());
String[] queryStrings = optionalQueryString.isPresent() ? optionalQueryString.get().split("&") : new String[]{};
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doGet()</code></b>");
writer.print("<br>");
for (String query : queryStrings) {
String[] q = query.split("=");
writer.print(q[0] + " = " + q[1] + "<br>");
}
writer.print("<br>");
writer.print("</body>");
writer.print("</html>");
}
}
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运行一下，结果是这样子的：
处理POST请求的参数
POST请求的参数就叫参数(parameter)，位于请求体(body)里，格式由Content-Type请求头决定。详细介绍可以参考这篇MDN文档。
HttpServletRequest#getParameterMap()方法可以取出请求中的所有参数，并放到一个Map中。
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| @Override
public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
Map parameterMap = request.getParameterMap();
try (PrintWriter writer = response.getWriter()) {
writer.print("<html lang=\"en\">");
writer.print("<body>");
writer.print("<b>Response from DemoServlet</b>");
writer.print("<b>Handled by <code>doPost()</code></b>");
writer.print("<br>");
parameterMap.forEach((k, v) -> writer.print(k + " = " + ((String[]) v)[0] + "<br>"));
writer.print("</body>");
writer.print("</html>");
}
}
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本文中将使用application/x-www-form-urlencoded格式做示例。
系列博文