[Servlet]深刻研究HttpServlet

時間 2019-12-14

標籤 servlet 深刻研究 httpservlet 欄目 Tomcat 简体版

原文原文鏈接

HttpServlet概述

在大多數的Web應用程序中，客戶端都是經過Http協議去訪問服務器端的資源，而咱們編寫的Servlet主要是用於Http協議的請求和響應處理。爲了快速開發應用於Http協議的Servlet類，Sun公司在javax.servlet.http包中提供了一個抽象類HttpServlet，它繼承於GenericServlet，用於建立適合基於Http協議的Web Servlet。html

public abstract class HttpServlet extends GenericServlet {}

下列表中羅列了HttpServlet的全部方法：java

Method Summary
protected void	doDelete(HttpServletRequest req, HttpServletResponse resp)
protected void	doGet(HttpServletRequest req, HttpServletResponse resp)
protected void	doHead(HttpServletRequest req, HttpServletResponse resp)
protected void	doOptions(HttpServletRequest req, HttpServletResponse resp)
protected void	doPost(HttpServletRequest req, HttpServletResponse resp)
protected void	doPut(HttpServletRequest req, HttpServletResponse resp)
protected void	doTrace(HttpServletRequest req, HttpServletResponse resp)
protected long	getLastModified(HttpServletRequest req)
protected void	service(HttpServletRequest req, HttpServletResponse resp)
void	service(ServletRequest req, ServletResponse res)

下面咱們就針對HttpServlet抽象類中提供的各個方法進行討論。web

重寫的service()方法

在HttpServlet類中提供了兩種重載的service()方法：瀏覽器

public abstract class HttpServlet extends GenericServlet {
    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {
        HttpServletRequest  request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
}
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doGet(req, resp);
        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
       } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
}

經過查看HttpServlet的源代碼咱們能夠知道，第一個service()方法是GenericServlet類中的Service()方法的實現。在該service()方法中將ServletRequest和ServletResponse對象轉換成HttpServletRequest（繼承於ServletRequest接口）和HttpServletResponse（繼承於ServletResponse接口），而後調用了第二個service()方法，對客戶端的請求進行處理。服務器

在第二個service()方法中，針對Http 1.1協議中定義的7種請求方式Get、Post、Head、Put、Delete、Trace和Options提供了7種處理方法。這7種方法的參數類型及異常拋出類型與HttpServlet類中的第二個service()方法是一致的。當Servlet容器接收到一個針對HttpServlet對象的請求時，調用該對象的方法順序以下：markdown

調用公用service()方法，將參數類型轉換成HttpServletRequest和HttpServletResponse，而後調用受保護的service()方法。
在受保護的service()方法中，獲取Http請求方法的名字，而後根據請求方法的類型，調用響應的doXXX()方法。

所以，咱們在自定義Servlet繼承於HttpServlet的時候，一般不須要重寫service()方法，只需重寫響應的doXXX()方法便可。app

七種請求處理方法

在HttpServlet抽象類中提供了針對Http 1.1協議中定義的7種請求方式Get、Post、Head、Put、Delete、Trace和Options提供了7種處理方法：jsp

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException

這7種處理請求的方法中，HttpServlet類對Trace和Options方法作了適當的實現，所以咱們不須要重寫doTrace()和doOptions()方法。而對於其餘的5種請求處理方法，HttpServlet類提供的實現都是返回Http錯誤。對於Http 1.0的客戶端請求，這些方法返回的狀態碼爲400，表示客戶端發送的請求在語法上是錯誤的。對於Http 1.1的客戶端請求，這些方法返回的狀態碼爲405，表示對於指定資源的請求方法不被容許。ide

而對於Http協議的實際應用來說，咱們最經常使用的是Get和Post，因此在自定義Servlet時，常重寫doGet()和doPost()兩個請求處理方法。其中doGet()方法用來處理Get方式的請求，doPost()方法用來處理Post方式的請求，下面咱們經過一個案例來看一看：post

建立一個HTML頁面用於發送客戶端請求。

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>index.html</title>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  </head>
  <body>
    <h1>GET方式請求</h1>
    <form id="userinfo" method="get" action="threeServlet">
        用戶名:<input type="text" id="username" name="username">
        <input type="submit" id="submit" value="提交">
    </form>
    <h1>POST方式請求</h1>
    <form id="userinfo" method="post" action="threeServlet">
        用戶名:<input type="text" id="username" name="username">
        <input type="submit" id="submit" value="提交">
    </form>
  </body>
</html>

建立一個Servlet用於處理請求。

public class ThreeServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("這是GET方式發送的請求,Servlet使用doGet()方法來處理.");
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("這是POST方式發送的請求,Servlet使用doPost()方法來處理.");
    }
}

配置web.xml文件。

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <servlet>
<servlet-name>ThreeServlet</servlet-name>
    <servlet-class>app.java.servlet.ThreeServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ThreeServlet</servlet-name>
    <url-pattern>/threeServlet</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>