Servlet 请求和响应

 Servlet的存在就是要为客户服务。servlet的任务是得到一个客户的请求，再发回一个响应。

1 servlet的生命周期

表 servlet的生命周期

1.1 init() 方法

构造函数只是使Servlet类成为一个对象，而执行完init()是成为一个Servlet。

在执行init()方法期间，将会访问到ServletConfig及ServletContext对象。

表 ServletConfig和ServletContext对象

2 HTTP方法

表 http常见方法

2.1 幂等操作

幂等操作的特点是其任意多次执行所产生的影响均与一次执行的影响相同。

GET、HEAD和PUT方法是幂等的。

3 请求 HttpServletRequest

getParameteValue() 是用来从请求中得到一个参数。返回值总是一个String。

如果对应一个给定的参数名有多个参数值，要使用getParameteValues()方法来返回一个String数组。

getReader()和getInputStream()方法：只包含请求体和不包含请求头。

getHeader()和getIntHeader()方法：获取请求头信息。

表 getServletPort()、getLocalPort()和getRemotePort()方法对比

4 实战

编写一个Servlet，要求打印出其生命周期中各个节点，同时编写Servlet对应的多个HTTP方法，获取相关请求参数及做出响应。

public class LifeServlet extends HttpServlet {
 
    public LifeServlet() {
        System.out.println("LifeServlet 无参数构造函数");
    }
 
    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("LifeServlet init()");
    }
 
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.service(req, resp);
        System.out.println("LifeServlet service()");
    }
 
    @Override
    public void destroy() {
        super.destroy();
        System.out.println("LifeServlet destroy");
    }
 
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("doGet");
    }
 
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("doPost");
    }
 
//    运行结果：
//    LifeServlet 无参数构造函数  // 在第一次发起该servlet请求时触发
//    LifeServlet init()
//    doGet
//    LifeServlet service()
//    ...  // 停止tomcat
//    LifeServlet destroy
}
 
public class HttpMethodsServlet extends HttpServlet {
 
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
 
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("param1:" + req.getParameter("params1"));
        String[] param2s = req.getParameterValues("param2");
        System.out.println("param2:" + Arrays.asList(param2s));
        System.out.println("headNames:" + req.getHeaderNames());
        BufferedReader reader = req.getReader();
        System.out.println("reader:" + reader.readLine());
 
        PrintWriter writer = resp.getWriter();
        writer.println("hello response");
        reader.close();
        writer.close();
    }
 
    @Override
    protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
 
    @Override
    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
 
    @Override
    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
 
    @Override
    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
 
    @Override
    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 
    }
}

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
         version="2.4">
    <servlet>
        <servlet-name>lifeServlet</servlet-name>
        <servlet-class>controller.LifeServlet</servlet-class>
    </servlet>
 
    <servlet-mapping>
        <servlet-name>lifeServlet</servlet-name>
        <url-pattern>/lifeServlet</url-pattern>
    </servlet-mapping>
 
    <servlet>
        <servlet-name>httpMethodsServlet</servlet-name>
        <servlet-class>controller.HttpMethodsServlet</servlet-class>
    </servlet>
 
    <servlet-mapping>
        <servlet-name>httpMethodsServlet</servlet-name>
        <url-pattern>/httpMethods</url-pattern>
    </servlet-mapping>
 
</web-app>

web.xml