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servlet(二)
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:
1 <servlet> 2 <servlet-name>ServletConfigDemo1</servlet-name> 3 <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class> 4 <!--配置ServletConfigDemo1的初始化参数 --> 5 <init-param> 6 <param-name>name</param-name> 7 <param-value>gacl</param-value> 8 </init-param> 9 <init-param>10 <param-name>password</param-name>11 <param-value>123</param-value>12 </init-param>13 <init-param>14 <param-name>charset</param-name>15 <param-value>UTF-8</param-value>16 </init-param>17 </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:
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import java.util.Enumeration; 5 import javax.servlet.ServletConfig; 6 import javax.servlet.ServletException; 7 import javax.servlet.http.HttpServlet; 8 import javax.servlet.http.HttpServletRequest; 9 import javax.servlet.http.HttpServletResponse;10 11 public class ServletConfigDemo1 extends HttpServlet {12 13 /**14 * 定义ServletConfig对象来接收配置的初始化参数15 */16 private ServletConfig config;17 18 /**19 * 当servlet配置了初始化参数后,web容器在创建servlet实例对象时,20 * 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,21 * 将ServletConfig对象传递给servlet。进而,程序员通过ServletConfig对象就可以22 * 得到当前servlet的初始化参数信息。23 */24 @Override25 public void init(ServletConfig config) throws ServletException {26 this.config = config;27 }28 29 public void doGet(HttpServletRequest request, HttpServletResponse response)30 throws ServletException, IOException {31 //获取在web.xml中配置的初始化参数32 String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数33 response.getWriter().print(paramVal);34 35 response.getWriter().print("<hr/>");36 //获取所有的初始化参数37 Enumeration<String> e = config.getInitParameterNames();38 while(e.hasMoreElements()){39 String name = e.nextElement();40 String value =http://www.mamicode.com/ config.getInitParameter(name);41 response.getWriter().print(name + "=" + value + "<br/>");42 }43 }44 45 public void doPost(HttpServletRequest request, HttpServletResponse response)46 throws ServletException, IOException {47 this.doGet(request, response);48 }49 50 }
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
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 10 public class ServletContextDemo1 extends HttpServlet {11 12 public void doGet(HttpServletRequest request, HttpServletResponse response)13 throws ServletException, IOException {14 String data = "http://www.mamicode.com/xdp_gacl";15 /**16 * ServletConfig对象中维护了ServletContext对象的引用,开发人员在编写servlet时,17 * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。18 */19 ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象20 context.setAttribute("data", data); //将data存储到ServletContext对象中21 }22 23 public void doPost(HttpServletRequest request, HttpServletResponse response)24 throws ServletException, IOException {25 doGet(request, response);26 }27 }
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 10 public class ServletContextDemo2 extends HttpServlet {11 12 public void doGet(HttpServletRequest request, HttpServletResponse response)13 throws ServletException, IOException {14 ServletContext context = this.getServletContext();15 String data = http://www.mamicode.com/(String) context.getAttribute("data");//从ServletContext对象中取出数据16 response.getWriter().print("data="http://www.mamicode.com/+data);17 }18 19 public void doPost(HttpServletRequest request, HttpServletResponse response)20 throws ServletException, IOException {21 doGet(request, response);22 }23 }
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:
1 <?xml version="1.0" encoding="UTF-8"?> 2 <web-app version="3.0" 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 3 http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"> 4 <display-name></display-name> 5 <!-- 配置WEB应用的初始化参数 --> 6 <context-param> 7 <param-name>url</param-name> 8 <param-value>jdbc:mysql://localhost:3306/test</param-value> 9 </context-param>10 11 <welcome-file-list>12 <welcome-file>index.jsp</welcome-file>13 </welcome-file-list>14 </web-app>
获取Web应用的初始化参数,代码如下:
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import javax.servlet.ServletContext; 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 10 11 public class ServletContextDemo3 extends HttpServlet {12 13 public void doGet(HttpServletRequest request, HttpServletResponse response)14 throws ServletException, IOException {15 16 ServletContext context = this.getServletContext();17 //获取整个web站点的初始化参数18 String contextInitParam = context.getInitParameter("url");19 response.getWriter().print(contextInitParam);20 }21 22 public void doPost(HttpServletRequest request, HttpServletResponse response)23 throws ServletException, IOException {24 doGet(request, response);25 }26 27 }
运行结果:
3.3、用servletContext实现请求转发
ServletContextDemo4
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import java.io.PrintWriter; 5 import javax.servlet.RequestDispatcher; 6 import javax.servlet.ServletContext; 7 import javax.servlet.ServletException; 8 import javax.servlet.http.HttpServlet; 9 import javax.servlet.http.HttpServletRequest;10 import javax.servlet.http.HttpServletResponse;11 12 public class ServletContextDemo4 extends HttpServlet {13 14 public void doGet(HttpServletRequest request, HttpServletResponse response)15 throws ServletException, IOException {16 String data = "http://www.mamicode.com/abcdefghjkl
";17 response.getOutputStream().write(data.getBytes());18 ServletContext context = this.getServletContext();//获取ServletContext对象19 RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)20 rd.forward(request, response);//调用forward方法实现请求转发21 }22 23 public void doPost(HttpServletRequest request, HttpServletResponse response)24 throws ServletException, IOException {25 }26 }
ServletContextDemo5
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 import javax.servlet.ServletException; 5 import javax.servlet.http.HttpServlet; 6 import javax.servlet.http.HttpServletRequest; 7 import javax.servlet.http.HttpServletResponse; 8 9 public class ServletContextDemo5 extends HttpServlet {10 11 public void doGet(HttpServletRequest request, HttpServletResponse response)12 throws ServletException, IOException {13 response.getOutputStream().write("servletDemo5".getBytes());14 }15 16 public void doPost(HttpServletRequest request, HttpServletResponse response)17 throws ServletException, IOException {18 this.doGet(request, response);19 }20 21 }
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
1 package gacl.servlet.study; 2 3 import java.io.FileInputStream; 4 import java.io.FileNotFoundException; 5 import java.io.IOException; 6 import java.io.InputStream; 7 import java.text.MessageFormat; 8 import java.util.Properties; 9 import javax.servlet.ServletException; 10 import javax.servlet.http.HttpServlet; 11 import javax.servlet.http.HttpServletRequest; 12 import javax.servlet.http.HttpServletResponse; 13 14 /** 15 * 使用servletContext读取资源文件 16 * 17 * @author gacl 18 * 19 */ 20 public class ServletContextDemo6 extends HttpServlet { 21 22 public void doGet(HttpServletRequest request, HttpServletResponse response) 23 throws ServletException, IOException { 24 /** 25 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码; 26 * 这样就不会出现中文乱码了 27 */ 28 response.setHeader("content-type","text/html;charset=UTF-8"); 29 readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件 30 response.getWriter().println("<hr/>"); 31 readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件 32 response.getWriter().println("<hr/>"); 33 readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件 34 response.getWriter().println("<hr/>"); 35 readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件 36 37 } 38 39 /** 40 * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件 41 * @param response 42 * @throws IOException 43 */ 44 private void readPropCfgFile2(HttpServletResponse response) 45 throws IOException { 46 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties"); 47 Properties prop = new Properties(); 48 prop.load(in); 49 String driver = prop.getProperty("driver"); 50 String url = prop.getProperty("url"); 51 String username = prop.getProperty("username"); 52 String password = prop.getProperty("password"); 53 response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件:"); 54 response.getWriter().println( 55 MessageFormat.format( 56 "driver={0},url={1},username={2},password={3}", 57 driver,url, username, password)); 58 } 59 60 /** 61 * 读取src目录下的db.config包中的db3.properties配置文件 62 * @param response 63 * @throws FileNotFoundException 64 * @throws IOException 65 */ 66 private void readPropCfgFile(HttpServletResponse response) 67 throws FileNotFoundException, IOException { 68 //通过ServletContext获取web资源的绝对路径 69 String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties"); 70 InputStream in = new FileInputStream(path); 71 Properties prop = new Properties(); 72 prop.load(in); 73 String driver = prop.getProperty("driver"); 74 String url = prop.getProperty("url"); 75 String username = prop.getProperty("username"); 76 String password = prop.getProperty("password"); 77 response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件:"); 78 response.getWriter().println( 79 MessageFormat.format( 80 "driver={0},url={1},username={2},password={3}", 81 driver,url, username, password)); 82 } 83 84 /** 85 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 86 * @param response 87 * @throws IOException 88 */ 89 private void readWebRootDirPropCfgFile(HttpServletResponse response) 90 throws IOException { 91 /** 92 * 通过ServletContext对象读取WebRoot目录下的properties配置文件 93 * “/”代表的是项目根目录 94 */ 95 InputStream in = this.getServletContext().getResourceAsStream("/db2.properties"); 96 Properties prop = new Properties(); 97 prop.load(in); 98 String driver = prop.getProperty("driver"); 99 String url = prop.getProperty("url");100 String username = prop.getProperty("username");101 String password = prop.getProperty("password");102 response.getWriter().println("读取WebRoot目录下的db2.properties配置文件:");103 response.getWriter().print(104 MessageFormat.format(105 "driver={0},url={1},username={2},password={3}", 106 driver,url, username, password));107 }108 109 /**110 * 通过ServletContext对象读取src目录下的properties配置文件111 * @param response112 * @throws IOException113 */114 private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {115 /**116 * 通过ServletContext对象读取src目录下的db1.properties配置文件117 */118 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");119 Properties prop = new Properties();120 prop.load(in);121 String driver = prop.getProperty("driver");122 String url = prop.getProperty("url");123 String username = prop.getProperty("username");124 String password = prop.getProperty("password");125 response.getWriter().println("读取src目录下的db1.properties配置文件:");126 response.getWriter().println(127 MessageFormat.format(128 "driver={0},url={1},username={2},password={3}", 129 driver,url, username, password));130 }131 132 public void doPost(HttpServletRequest request, HttpServletResponse response)133 throws ServletException, IOException {134 this.doGet(request, response);135 }136 137 }
运行结果如下:
代码范例:使用类装载器读取资源文件
1 package gacl.servlet.study; 2 3 import java.io.FileOutputStream; 4 import java.io.IOException; 5 import java.io.InputStream; 6 import java.io.OutputStream; 7 import java.text.MessageFormat; 8 import java.util.Properties; 9 10 import javax.servlet.ServletException; 11 import javax.servlet.http.HttpServlet; 12 import javax.servlet.http.HttpServletRequest; 13 import javax.servlet.http.HttpServletResponse; 14 15 /** 16 * 用类装载器读取资源文件 17 * 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 18 * @author gacl 19 * 20 */ 21 public class ServletContextDemo7 extends HttpServlet { 22 23 public void doGet(HttpServletRequest request, HttpServletResponse response) 24 throws ServletException, IOException { 25 /** 26 * response.setContentType("text/html;charset=UTF-8");目的是控制浏览器用UTF-8进行解码; 27 * 这样就不会出现中文乱码了 28 */ 29 response.setHeader("content-type","text/html;charset=UTF-8"); 30 test1(response); 31 response.getWriter().println("<hr/>"); 32 test2(response); 33 response.getWriter().println("<hr/>"); 34 //test3(); 35 test4(); 36 37 } 38 39 /** 40 * 读取类路径下的资源文件 41 * @param response 42 * @throws IOException 43 */ 44 private void test1(HttpServletResponse response) throws IOException { 45 //获取到装载当前类的类装载器 46 ClassLoader loader = ServletContextDemo7.class.getClassLoader(); 47 //用类装载器读取src目录下的db1.properties配置文件 48 InputStream in = loader.getResourceAsStream("db1.properties"); 49 Properties prop = new Properties(); 50 prop.load(in); 51 String driver = prop.getProperty("driver"); 52 String url = prop.getProperty("url"); 53 String username = prop.getProperty("username"); 54 String password = prop.getProperty("password"); 55 response.getWriter().println("用类装载器读取src目录下的db1.properties配置文件:"); 56 response.getWriter().println( 57 MessageFormat.format( 58 "driver={0},url={1},username={2},password={3}", 59 driver,url, username, password)); 60 } 61 62 /** 63 * 读取类路径下面、包下面的资源文件 64 * @param response 65 * @throws IOException 66 */ 67 private void test2(HttpServletResponse response) throws IOException { 68 //获取到装载当前类的类装载器 69 ClassLoader loader = ServletContextDemo7.class.getClassLoader(); 70 //用类装载器读取src目录下的gacl.servlet.study包中的db4.properties配置文件 71 InputStream in = loader.getResourceAsStream("gacl/servlet/study/db4.properties"); 72 Properties prop = new Properties(); 73 prop.load(in); 74 String driver = prop.getProperty("driver"); 75 String url = prop.getProperty("url"); 76 String username = prop.getProperty("username"); 77 String password = prop.getProperty("password"); 78 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:"); 79 response.getWriter().println( 80 MessageFormat.format( 81 "driver={0},url={1},username={2},password={3}", 82 driver,url, username, password)); 83 } 84 85 /** 86 * 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 87 */ 88 public void test3() { 89 /** 90 * 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: 91 * java.lang.OutOfMemoryError: Java heap space 92 */ 93 InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi"); 94 System.out.println(in); 95 } 96 97 /** 98 * 读取01.avi,并拷贝到e:\根目录下 99 * 01.avi文件太大,只能用servletContext去读取100 * @throws IOException101 */102 public void test4() throws IOException {103 // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi104 // path=01.avi105 String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");106 /**107 * path.lastIndexOf("\\") + 1是一个非常绝妙的写法108 */109 String filename = path.substring(path.lastIndexOf("\\") + 1);//获取文件名110 InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/01.avi");111 byte buffer[] = new byte[1024];112 int len = 0;113 OutputStream out = new FileOutputStream("e:\\" + filename);114 while ((len = in.read(buffer)) > 0) {115 out.write(buffer, 0, len);116 }117 out.close();118 in.close();119 }120 121 public void doPost(HttpServletRequest request, HttpServletResponse response)122 throws ServletException, IOException {123 124 this.doGet(request, response);125 }126 127 }
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:
1 package gacl.servlet.study; 2 3 import java.io.IOException; 4 5 import javax.servlet.ServletException; 6 import javax.servlet.http.HttpServlet; 7 import javax.servlet.http.HttpServletRequest; 8 import javax.servlet.http.HttpServletResponse; 9 10 public class ServletDemo5 extends HttpServlet {11 12 public void doGet(HttpServletRequest request, HttpServletResponse response)13 throws ServletException, IOException {14 String data = "http://www.mamicode.com/abcddfwerwesfasfsadf";15 /**16 * 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17 * 这里是将数据的缓存时间设置为1天18 */19 response.setDateHeader("expires",System.currentTimeMillis() + 24 * 3600 * 1000);20 response.getOutputStream().write(data.getBytes());21 }22 23 public void doPost(HttpServletRequest request, HttpServletResponse response)24 throws ServletException, IOException {25 26 this.doGet(request, response);27 }28 29 }
servlet(二)