第九章 Servlet API

Servlet API 定义了Servlet和服务器之间的一个标准接口，这使得Servlet具有跨应用服务器的特性，通过使用Servlet API，开发人员不必关心服务器的内部运作方式。

Servlet API由两个软件包组成：Javax.servlet包和Javax.servlet.http包

一个存放与HTTP协议无关的一般性Servlet类，另一个存放与HTTP协议相关的功能的类。这两个软件包位于Tomcat的servlet-api.jar中，在myeclipse中创建项目的时候引入Java EE 6也是可以的。

1. servletConfig接口

在Servlet的配置文件中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，程序员通过ServletConfig对象就可以得到当前servlet的初始化参数信息

从一个servlet实例化后，对任何客户端任何时候访问都有效，只对本servlet有效，一个servlet的ServletConfig不能被其他sevlet访问

案例：

配置：通过web.xml文件中的<init-param>标签进行设置，

<servlet-name>ServletConfigDemo1</servlet-name>

<servlet-class>com.silvan.servlet.ServletConfigDemo1</servlet-class>

<init-param>

<param-name>name</param-name>

<param-value>ross</param-value>

</init-param>

<load-on-startup>0</load-on-startup>

</servlet>

<servlet-mapping>

<servlet-name>ServletConfigDemo1</servlet-name>

<url-pattern>/ServletConfigDemo1</url-pattern>

</servlet-mapping>

获取：在servlet实现类里面通过getServletConfig().getInitParameter("name")获取；

package com.silvan.servlet;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class ServletConfigDemo1 extends HttpServlet {

/**

* Constructor of the object.

public ServletDemo1() {

super();

}

/**

* Destruction of the servlet.

public void destroy() {

super.destroy(); // Just puts "destroy" string in log

// Put your code here

}

/**

* The doGet method of the servlet.

* This method is called when a form has its tag value method equals to get.

* @param request the request send by the client to the server

* @param response the response send by the server to the client

* @throws ServletException if an error occurred

* @throws IOException if an error occurred

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

String name = getServletConfig().getInitParameter("name");

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(name);

out.println(" </BODY>");

out.println("</HTML>");

out.flush();

out.close();

}

/**

* The doPost method of the servlet.

* This method is called when a form has its tag value method equals to post.

* @param request the request send by the client to the server

* @param response the response send by the server to the client

* @throws ServletException if an error occurred

* @throws IOException if an error occurred

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

doGet(request, response);

}

/**

* Initialization of the servlet.

* @throws ServletException if an error occurs

public void init() throws ServletException {

String name = getServletConfig().getInitParameter("name");

System.out.println("this is "+name);

}

2. ServletContext接口

WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。

ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。

由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

ServletContext对任何servlet，任何人在任何时间都有效，是真正的全局对象

案例：

配置：通过web.xml文件中的<context-param>标签设置

<context-param>

<param-name>name</param-name>

<param-value> ross</param-value>

</context-param>

<servlet-name>ServletContextDemo1</servlet-name>

<servlet-class>com.silvan.servlet.ServletContextDemo1</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>ServletContextDemo1</servlet-name>

<url-pattern>/ServletContextDemo1</url-pattern>

</servlet-mapping>

获取： getServletContext（）.getInitParameter(“name”);

getServletConfig().getServletContext（）.getInitParameter(“name”);

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

String name1 = getServletContext().getInitParameter("name");

String name2 = getServletConfig().getServletContext().getInitParameter("name");

response.setContentType("text/html;charset=UTF-8");

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("直接getServletContext()取得name为：" +name1);

out.println(" ");

out.print("getServletConfig()对象中取得name为："+name2);

out.println(" </BODY>");

out.println("</HTML>");

out.flush();

out.close();

}

显示

案例：

统计站点访问次数

配置web.xml

<servlet-name>ServletContextDemo21</servlet-name>

<servlet-class>com.silvan.servlet.ServletContextDemo21</servlet-class>

</servlet>

<servlet-name>ServletContextDemo22</servlet-name>

<servlet-class>com.silvan.servlet.ServletContextDemo22</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>ServletContextDemo21</servlet-name>

<url-pattern>/ServletContextDemo21</url-pattern>

</servlet-mapping>

<servlet-mapping>

<servlet-name>ServletContextDemo22</servlet-name>

<url-pattern>/ServletContextDemo22</url-pattern>

</servlet-mapping>

统计访问次数

ServletContextDemo21.java

package com.silvan.servlet;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class ServletContextDemo21 extends HttpServlet {

/**

* Constructor of the object.

public ServletContextDemo21() {

super();

}

/**

* Destruction of the servlet.

public void destroy() {

super.destroy(); // Just puts "destroy" string in log

// Put your code here

}

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

// 每次访问获得原来访问次数+1

ServletContext context = getServletContext();

// 获得访问次数

int count = (Integer) context.getAttribute("count");

// 次数+1保存

context.setAttribute("count", ++count);

System.out.println("context对象地址:"+context);

System.out.println("网站被访问次数为:" + count);

// 浏览器显示中文

response.setContentType("text/html;charset=utf-8");

response.getWriter().println("context对象地址:" + context+" ");

response.getWriter().println("Demo1网站被访问次数为:" + count);

}

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

doGet( request, response);

}

public void init() throws ServletException {

// 在ServletContext对象中保存一个访问次数

ServletContext context = getServletContext();

if(context.getAttribute("count")==null){

// 保存一个数据

context.setAttribute("count", 0);

}

ServletContextDemo22.java

package com.silvan.servlet;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class ServletContextDemo22 extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

// 每次访问获得原来访问次数+1

ServletContext context = getServletContext();

// 获得访问次数

int count = (Integer) context.getAttribute("count");

// 次数+1保存

context.setAttribute("count", ++count);

System.out.println("context对象地址:"+context);

System.out.println("网站被访问次数为:" + count);

// 浏览器显示中文

response.setContentType("text/html;charset=utf-8");

response.getWriter().println("context对象地址:" + context+" ");

response.getWriter().println("Demo1网站被访问次数为:" + count);

}

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

doGet( request, response);

}

public void init() throws ServletException {

// 在ServletContext对象中保存一个访问次数

ServletContext context = getServletContext();

if(context.getAttribute("count")==null){

// 保存一个数据

context.setAttribute("count", 0);

}

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