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 @Override 25 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 = 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 = "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 = (String) context.getAttribute("data");//从ServletContext对象中取出数据 16 response.getWriter().print("data="+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 = "<h1><font color=‘red‘>abcdefghjkl</font></h1>"; 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 response 112 * @throws IOException 113 */ 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 IOException 101 */ 102 public void test4() throws IOException { 103 // path=G:\Java学习视频\JavaWeb学习视频\JavaWeb\day05视频\01.avi 104 // path=01.avi 105 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 = "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 }
