一、ServletConfig讲解
1.1、配置Servlet初始化参数
在Servlet的配置文件web.xml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。
例如:
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<servlet>
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<servlet-name>ServletConfigDemo1</servlet-name>
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<servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class>
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<!--配置ServletConfigDemo1的初始化参数 -->
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<init-param>
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<param-name>name</param-name>
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<param-value>gacl</param-value>
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</init-param>
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<init-param>
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<param-name>password</param-name>
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<param-value>123</param-value>
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</init-param>
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<init-param>
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<param-name>charset</param-name>
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<param-value>UTF-8</param-value>
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</init-param>
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</servlet>
1.2、通过ServletConfig获取Servlet的初始化参数
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,我们通过ServletConfig对象就可以得到当前servlet的初始化参数信息。
例如:
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package gacl.servlet.study;
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import java.io.IOException;
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import java.util.Enumeration;
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import javax.servlet.ServletConfig;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletConfigDemo1 extends HttpServlet {
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/**
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* 定义ServletConfig对象来接收配置的初始化参数
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*/
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private ServletConfig config;
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/**
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* 当servlet配置了初始化参数后,web容器在创建servlet实例对象时,
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* 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,
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* 将ServletConfig对象传递给servlet。进而,程序员通过ServletConfig对象就可以
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* 得到当前servlet的初始化参数信息。
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*/
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@Override
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public void init(ServletConfig config) throws ServletException {
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this.config = config;
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}
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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//获取在web.xml中配置的初始化参数
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String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数
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response.getWriter().print(paramVal);
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response.getWriter().print("<hr/>");
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//获取所有的初始化参数
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Enumeration<String> e = config.getInitParameterNames();
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while(e.hasMoreElements()){
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String name = e.nextElement();
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String value = config.getInitParameter(name);
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response.getWriter().print(name + "=" + value + "<br/>");
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}
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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this.doGet(request, response);
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}
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}
运行结果如下:
二、ServletContext对象
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。
三、ServletContext的应用
3.1、多个Servlet通过ServletContext对象实现数据共享
范例:ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享
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package gacl.servlet.study;
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import java.io.IOException;
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import javax.servlet.ServletContext;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletContextDemo1 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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String data = "xdp_gacl";
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/**
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* ServletConfig对象中维护了ServletContext对象的引用,开发人员在编写servlet时,
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* 可以通过ServletConfig.getServletContext方法获得ServletContext对象。
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*/
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ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象
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context.setAttribute("data", data); //将data存储到ServletContext对象中
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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doGet(request, response);
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}
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}
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package gacl.servlet.study;
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import java.io.IOException;
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import javax.servlet.ServletContext;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletContextDemo2 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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ServletContext context = this.getServletContext();
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String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
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response.getWriter().print("data="+data);
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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doGet(request, response);
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}
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}
先运行ServletContextDemo1,将数据data存储到ServletContext对象中,然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了,这样就实现了数据共享,如下图所示:
3.2、获取WEB应用的初始化参数
在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数,如下所示:
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<?xml version="1.0" encoding="UTF-8"?>
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<web-app version="3.0"
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xmlns="http://java.sun.com/xml/ns/javaee"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
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http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
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<display-name></display-name>
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<!-- 配置WEB应用的初始化参数 -->
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<context-param>
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<param-name>url</param-name>
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<param-value>jdbc:mysql://localhost:3306/test</param-value>
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</context-param>
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<welcome-file-list>
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<welcome-file>index.jsp</welcome-file>
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</welcome-file-list>
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</web-app>
获取Web应用的初始化参数,代码如下:
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package gacl.servlet.study;
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import java.io.IOException;
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import javax.servlet.ServletContext;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletContextDemo3 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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ServletContext context = this.getServletContext();
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//获取整个web站点的初始化参数
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String contextInitParam = context.getInitParameter("url");
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response.getWriter().print(contextInitParam);
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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doGet(request, response);
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}
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}
运行结果:
3.3、用servletContext实现请求转发
ServletContextDemo4
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package gacl.servlet.study;
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import java.io.IOException;
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import java.io.PrintWriter;
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import javax.servlet.RequestDispatcher;
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import javax.servlet.ServletContext;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletContextDemo4 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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String data = "<h1><font color='red'>abcdefghjkl</font></h1>";
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response.getOutputStream().write(data.getBytes());
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ServletContext context = this.getServletContext();//获取ServletContext对象
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RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)
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rd.forward(request, response);//调用forward方法实现请求转发
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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}
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}
ServletContextDemo5
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package gacl.servlet.study;
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import java.io.IOException;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ServletContextDemo5 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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response.getOutputStream().write("servletDemo5".getBytes());
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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this.doGet(request, response);
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}
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}
运行结果:
访问的是ServletContextDemo4,浏览器显示的却是ServletContextDemo5的内容,这就是使用ServletContext实现了请求转发
3.4、利用ServletContext对象读取资源文件
项目目录结构如下:
代码范例:使用servletContext读取资源文件
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package gacl.servlet.study;
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import java.io.FileInputStream;
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import java.io.FileNotFoundException;
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import java.io.IOException;
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import java.io.InputStream;
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import java.text.MessageFormat;
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import java.util.Properties;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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/**
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* 使用servletContext读取资源文件
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*
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* @author gacl
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*
-
*/
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public class ServletContextDemo6 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
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/**
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* response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码;
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* 这样就不会出现中文乱码了
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*/
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response.setHeader("content-type","text/html;charset=UTF-8");
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readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件
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response.getWriter().println("<hr/>");
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readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
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response.getWriter().println("<hr/>");
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readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
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response.getWriter().println("<hr/>");
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readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件
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}
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/**
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* 读取src目录下的gacl.servlet.study包中的db4.properties配置文件
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* @param response
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* @throws IOException
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*/
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private void readPropCfgFile2(HttpServletResponse response)
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throws IOException {
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InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");
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Properties prop = new Properties();
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prop.load(in);
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String driver = prop.getProperty("driver");
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String url = prop.getProperty("url");
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String username = prop.getProperty("username");
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String password = prop.getProperty("password");
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response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件:");
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response.getWriter().println(
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MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
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driver,url, username, password));
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}
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/**
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* 读取src目录下的db.config包中的db3.properties配置文件
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* @param response
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* @throws FileNotFoundException
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* @throws IOException
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*/
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private void readPropCfgFile(HttpServletResponse response)
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throws FileNotFoundException, IOException {
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//通过ServletContext获取web资源的绝对路径
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String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");
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InputStream in = new FileInputStream(path);
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Properties prop = new Properties();
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prop.load(in);
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String driver = prop.getProperty("driver");
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String url = prop.getProperty("url");
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String username = prop.getProperty("username");
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String password = prop.getProperty("password");
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response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件:");
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response.getWriter().println(
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MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
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driver,url, username, password));
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}
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/**
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* 通过ServletContext对象读取WebRoot目录下的properties配置文件
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* @param response
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* @throws IOException
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*/
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private void readWebRootDirPropCfgFile(HttpServletResponse response)
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throws IOException {
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/**
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* 通过ServletContext对象读取WebRoot目录下的properties配置文件
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* “/”代表的是项目根目录
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*/
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InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");
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Properties prop = new Properties();
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prop.load(in);
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String driver = prop.getProperty("driver");
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String url = prop.getProperty("url");
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String username = prop.getProperty("username");
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String password = prop.getProperty("password");
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response.getWriter().println("读取WebRoot目录下的db2.properties配置文件:");
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response.getWriter().print(
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MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
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driver,url, username, password));
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}
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/**
-
* 通过ServletContext对象读取src目录下的properties配置文件
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* @param response
-
* @throws IOException
-
*/
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private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {
-
/**
-
* 通过ServletContext对象读取src目录下的db1.properties配置文件
-
*/
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InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");
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Properties prop = new Properties();
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prop.load(in);
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String driver = prop.getProperty("driver");
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String url = prop.getProperty("url");
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String username = prop.getProperty("username");
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String password = prop.getProperty("password");
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response.getWriter().println("读取src目录下的db1.properties配置文件:");
-
response.getWriter().println(
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MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
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driver,url, username, password));
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}
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public void doPost(HttpServletRequest request, HttpServletResponse response)
-
throws ServletException, IOException {
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this.doGet(request, response);
-
}
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}
运行结果如下:
代码范例:使用类装载器读取资源文件
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package gacl.servlet.study;
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import java.io.FileOutputStream;
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import java.io.IOException;
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import java.io.InputStream;
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import java.io.OutputStream;
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import java.text.MessageFormat;
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import java.util.Properties;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
-
/**
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* 用类装载器读取资源文件
-
* 通过类装载器读取资源文件的注意事项:不适合装载大文件,否则会导致jvm内存溢出
-
* @author gacl
-
*
-
*/
-
public class ServletContextDemo7 extends HttpServlet {
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public void doGet(HttpServletRequest request, HttpServletResponse response)
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throws ServletException, IOException {
-
/**
-
* response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码;
-
* 这样就不会出现中文乱码了
-
*/
-
response.setHeader("content-type","text/html;charset=UTF-8");
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test1(response);
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response.getWriter().println("<hr/>");
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test2(response);
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response.getWriter().println("<hr/>");
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//test3();
-
test4();
-
}
-
/**
-
* 读取类路径下的资源文件
-
* @param response
-
* @throws IOException
-
*/
-
private void test1(HttpServletResponse response) throws IOException {
-
//获取到装载当前类的类装载器
-
ClassLoader loader = ServletContextDemo7.class.getClassLoader();
-
//用类装载器读取src目录下的db1.properties配置文件
-
InputStream in = loader.getResourceAsStream("db1.properties");
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Properties prop = new Properties();
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prop.load(in);
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String driver = prop.getProperty("driver");
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String url = prop.getProperty("url");
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String username = prop.getProperty("username");
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String password = prop.getProperty("password");
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response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件:");
-
response.getWriter().println(
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MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
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driver,url, username, password));
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}
-
/**
-
* 读取类路径下面、包下面的资源文件
-
* @param response
-
* @throws IOException
-
*/
-
private void test2(HttpServletResponse response) throws IOException {
-
//获取到装载当前类的类装载器
-
ClassLoader loader = ServletContextDemo7.class.getClassLoader();
-
//用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件
-
InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties");
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Properties prop = new Properties();
-
prop.load(in);
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String driver = prop.getProperty("driver");
-
String url = prop.getProperty("url");
-
String username = prop.getProperty("username");
-
String password = prop.getProperty("password");
-
response.getWriter().println("用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件:");
-
response.getWriter().println(
-
MessageFormat.format(
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"driver={0},url={1},username={2},password={3}",
-
driver,url, username, password));
-
}
-
/**
-
* 通过类装载器读取资源文件的注意事项:不适合装载大文件,否则会导致jvm内存溢出
-
*/
-
public void test3() {
-
/**
-
* 01.avi是一个150多M的文件,使用类加载器去读取这个大文件时会导致内存溢出:
-
* java.lang.OutOfMemoryError: Java heap space
-
*/
-
InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
-
System.out.println(in);
-
}
-
/**
-
* 读取01.avi,并拷贝到e:\根目录下
-
* 01.avi文件太大,只能用servletContext去读取
-
* @throws IOException
-
*/
-
public void test4() throws IOException {
-
// path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi
-
// path=01.avi
-
String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");
-
/**
-
* path.lastIndexOf("\\") + 1是一个非常绝妙的写法
-
*/
-
String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名
-
InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");
-
byte buffer[] = new byte[1024];
-
int len = 0;
-
OutputStream out = new FileOutputStream("e:\\" + filename);
-
while ((len = in.read(buffer)) > 0) {
-
out.write(buffer, 0, len);
-
}
-
out.close();
-
in.close();
-
}
-
public void doPost(HttpServletRequest request, HttpServletResponse response)
-
throws ServletException, IOException {
-
this.doGet(request, response);
-
}
-
}
运行结果如下:
四、在客户端缓存Servlet的输出
对于不经常变化的数据,在servlet中可以为其设置合理的缓存时间值,以避免浏览器频繁向服务器发送请求,提升服务器的性能。例如:
-
package gacl.servlet.study;
-
import java.io.IOException;
-
import javax.servlet.ServletException;
-
import javax.servlet.http.HttpServlet;
-
import javax.servlet.http.HttpServletRequest;
-
import javax.servlet.http.HttpServletResponse;
-
public class ServletDemo5 extends HttpServlet {
-
public void doGet(HttpServletRequest request, HttpServletResponse response)
-
throws ServletException, IOException {
-
String data = "abcddfwerwesfasfsadf";
-
/**
-
* 设置数据合理的缓存时间值,以避免浏览器频繁向服务器发送请求,提升服务器的性能
-
* 这里是将数据的缓存时间设置为1天
-
*/
-
response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000);
-
response.getOutputStream().write(data.getBytes());
-
}
-
public void doPost(HttpServletRequest request, HttpServletResponse response)
-
throws ServletException, IOException {
-
this.doGet(request, response);
-
}
-
}