servlet基础

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一、servlet是什么？

servlet是web项目的服务器端接受客户端请求以及给予响应的载体（入口）。同时，jsp也是由servlet演化而来，是servlet与HTML的结合体。
servlet需要重写doGet方法和doPost方法。这两个方法能够分别响应客户端的get请求和post请求。以get请求为例，如下：
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType("text/html");PrintWriter out = response.getWriter();out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");out.println("<HTML>");out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");out.println("  <BODY>");out.print("    This is ");out.print(this.getClass());out.println(", using the GET method");out.println("  </BODY>");out.println("</HTML>");out.flush();out.close();}

二、servlet是如何处理请求响应的？

servlet在处理get请求时，会接收到request、response两个参数。request中可以获取请求参数。response中可以获取一个输出流，该输出流会将字符返回给客户端浏览器。上边代码返回的内容格式是（text/html），这就告诉浏览器以这种类型来处理，而此时输出流中的字符就是我们看到的html页面。

对于jsp文件来讲，发布服务时会将其变成一个java文件，（这个java文件可以再你的tomcat下找到）然后再编译，其实这个java文件就是一个servlet。jsp中的HTML代码也是通过servlet中的这个输出流返回给客户端的。

jsp生成的java文件如下：

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/7.0.56 * Generated at: 2017-10-27 08:53:59 UTC * Note: The last modified time of this file was set to *       the last modified time of the source file after *       generation to assist with modification tracking. */package org.apache.jsp;import javax.servlet.*;import javax.servlet.http.*;import javax.servlet.jsp.*;import java.util.*;public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase    implements org.apache.jasper.runtime.JspSourceDependent {  private static final javax.servlet.jsp.JspFactory _jspxFactory =          javax.servlet.jsp.JspFactory.getDefaultFactory();  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;  private javax.el.ExpressionFactory _el_expressionfactory;  private org.apache.tomcat.InstanceManager _jsp_instancemanager;  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {    return _jspx_dependants;  }  public void _jspInit() {    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());  }  public void _jspDestroy() {  }  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)        throws java.io.IOException, javax.servlet.ServletException {    final javax.servlet.jsp.PageContext pageContext;    javax.servlet.http.HttpSession session = null;    final javax.servlet.ServletContext application;    final javax.servlet.ServletConfig config;    javax.servlet.jsp.JspWriter out = null;    final java.lang.Object page = this;    javax.servlet.jsp.JspWriter _jspx_out = null;    javax.servlet.jsp.PageContext _jspx_page_context = null;    try {      response.setContentType("text/html;charset=ISO-8859-1");      pageContext = _jspxFactory.getPageContext(this, request, response,      null, true, 8192, true);      _jspx_page_context = pageContext;      application = pageContext.getServletContext();      config = pageContext.getServletConfig();      session = pageContext.getSession();      out = pageContext.getOut();      _jspx_out = out;      out.write('\r');      out.write('\n');String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";      out.write("\r\n");      out.write("\r\n");      out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");      out.write("<html>\r\n");      out.write("  <head>\r\n");      out.write("    <base href=\"");      out.print(basePath);      out.write("\">\r\n");      out.write("    \r\n");      out.write("    <title>My JSP 'index.jsp' starting page</title>\r\n");      out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n");      out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n");      out.write("\t<meta http-equiv=\"expires\" content=\"0\">    \r\n");      out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n");      out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n");      out.write("\t<!--\r\n");      out.write("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n");      out.write("\t-->\r\n");      out.write("  <script>\"undefined\"==typeof CODE_LIVE&&(!function(e){var t={nonSecure:\"8123\",secure:\"8124\"},c={nonSecure:\"http://\",secure:\"https://\"},r={nonSecure:\"127.0.0.1\",secure:\"gapdebug.local.genuitec.com\"},n=\"https:\"===window.location.protocol?\"secure\":\"nonSecure\";script=e.createElement(\"script\"),script.type=\"text/javascript\",script.async=!0,script.src=c[n]+r[n]+\":\"+t[n]+\"/codelive-assets/bundle.js\",e.getElementsByTagName(\"head\")[0].appendChild(script)}(document),CODE_LIVE=!0);</script></head>\r\n");      out.write("  \r\n");      out.write("  <body data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-0\" data-genuitec-path=\"/simpleTestWeb/WebRoot/index.jsp\">\r\n");      out.write("    This is my JSP page. <br data-genuitec-lp-enabled=\"false\" data-genuitec-file-id=\"wc1-0\" data-genuitec-path=\"/simpleTestWeb/WebRoot/index.jsp\">\r\n");      out.write("  </body>\r\n");      out.write("</html>\r\n");    } catch (java.lang.Throwable t) {      if (!(t instanceof javax.servlet.jsp.SkipPageException)){        out = _jspx_out;        if (out != null && out.getBufferSize() != 0)          try {            if (response.isCommitted()) {              out.flush();            } else {              out.clearBuffer();            }          } catch (java.io.IOException e) {}        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);        else throw new ServletException(t);      }    } finally {      _jspxFactory.releasePageContext(_jspx_page_context);    }  }}

目录如下：

三、servlet的继承关系如图：

四、如何实现一个Servlet？

写一个Servlet类时，一般需要三步走：第一步，继承HTTPServlet；第二步，重写doGet（）、doPost（）方法；第三步，在web.xml中定义servlet类，一般需要<servlet><servlet-mapping>两个标签。

五、思考Spring MVC中的DispatcherServlet？

此处可同时思考Spring MVC中web.xml中配置的DispatcherServlet。如下图：
<servlet>    <description>This is the description of my J2EE component</description>    <display-name>This is the display name of my J2EE component</display-name>    <servlet-name>MyServlet</servlet-name>    <servlet-class>test.MyServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>MyServlet</servlet-name>    <url-pattern>/servlet/MyServlet</url-pattern>  </servlet-mapping>
<servlet><servlet-name>springMVC</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>/WEB-INF/conf/springMVC-servlet.xml</param-value></init-param><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>springMVC</servlet-name><url-pattern>/</url-pattern></servlet-mapping>
对于DIspatcherServlet的思考：servlet是我们的web项目前端请求请求后端的入口。请求哪个servlet是配置在web.xml中的。spring MVC中的DispatcherServlet就是一个继承了HTTPServlet的普通的servlet。将其配置在web.xml中后，对后端的请求被DispatcherServlet截获然后分发，自然而然的我们的MVC中的C就由Spring MVC来控制了。

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