javaweb Servlet开发总结（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：

<servlet> <servlet-name>ServletConfigDemo1</servlet-name> <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class> <!--配置ServletConfigDemo1的初始化参数 --> <init-param>  <param-name>name</param-name>  <param-value>gacl</param-value> </init-param>  <init-param>  <param-name>password</param-name>  <param-value>123</param-value> </init-param> <init-param>  <param-name>charset</param-name>  <param-value>UTF-8</param-value> </init-param></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：

package gacl.servlet.study;import java.io.IOException;import java.util.Enumeration;import javax.servlet.ServletConfig;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ServletConfigDemo1 extends HttpServlet { /**  * 定义ServletConfig对象来接收配置的初始化参数  */ private ServletConfig config;  /**  * 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。  */ @Override public void init(ServletConfig config) throws ServletException {  this.config = config; } public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  //获取在web.xml中配置的初始化参数  String paramVal = this.config.getInitParameter("name");//获取指定的初始化参数  response.getWriter().print(paramVal);    response.getWriter().print("<hr/>");  //获取所有的初始化参数  Enumeration<String> e = config.getInitParameterNames();  while(e.hasMoreElements()){   String name = e.nextElement();   String value = config.getInitParameter(name);   response.getWriter().print(name + "=" + value + "<br/>");  } } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  this.doGet(request, response); }}

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

package gacl.servlet.study;import java.io.IOException;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 ServletContextDemo1 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  String data = "xdp_gacl";  /**   * 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。   */  ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象  context.setAttribute("data", data); //将data存储到ServletContext对象中 } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  doGet(request, response); }}

package gacl.servlet.study;import java.io.IOException;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 ServletContextDemo2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  ServletContext context = this.getServletContext();  String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据  response.getWriter().print("data="+data); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  doGet(request, response); }}

　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：

<?xml version="1.0" encoding="UTF-8"?><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  http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"> <display-name></display-name> <!-- 配置WEB应用的初始化参数 --> <context-param>  <param-name>url</param-name>  <param-value>jdbc:mysql://localhost:3306/test</param-value> </context-param> <welcome-file-list>  <welcome-file>index.jsp</welcome-file> </welcome-file-list></web-app>

获取Web应用的初始化参数，代码如下：

package gacl.servlet.study;import java.io.IOException;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 ServletContextDemo3 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  ServletContext context = this.getServletContext();  //获取整个web站点的初始化参数  String contextInitParam = context.getInitParameter("url");  response.getWriter().print(contextInitParam); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  doGet(request, response); }}

运行结果：

3.3、用servletContext实现请求转发

ServletContextDemo4

package gacl.servlet.study;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.RequestDispatcher;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 ServletContextDemo4 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  String data = "<h1><font color='red'>abcdefghjkl</font></h1>";  response.getOutputStream().write(data.getBytes());  ServletContext context = this.getServletContext();//获取ServletContext对象  RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//获取请求转发对象(RequestDispatcher)  rd.forward(request, response);//调用forward方法实现请求转发 } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException { }}

ServletContextDemo5

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 ServletContextDemo5 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  response.getOutputStream().write("servletDemo5".getBytes()); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  this.doGet(request, response); }}

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

package gacl.servlet.study;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.IOException;import java.io.InputStream;import java.text.MessageFormat;import java.util.Properties;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * 使用servletContext读取资源文件 *  * @author gacl *  */public class ServletContextDemo6 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   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");  readSrcDirPropCfgFile(response);//读取src目录下的properties配置文件  response.getWriter().println("<hr/>");  readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件  response.getWriter().println("<hr/>");  readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件  response.getWriter().println("<hr/>");  readPropCfgFile2(response);//读取src目录下的gacl.servlet.study包中的db4.properties配置文件   } /**  * 读取src目录下的gacl.servlet.study包中的db4.properties配置文件  * @param response  * @throws IOException  */ private void readPropCfgFile2(HttpServletResponse response)   throws IOException {  InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/gacl/servlet/study/db4.properties");  Properties prop = new Properties();  prop.load(in);  String driver = prop.getProperty("driver");  String url = prop.getProperty("url");  String username = prop.getProperty("username");  String password = prop.getProperty("password");  response.getWriter().println("读取src目录下的gacl.servlet.study包中的db4.properties配置文件：");  response.getWriter().println(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 读取src目录下的db.config包中的db3.properties配置文件  * @param response  * @throws FileNotFoundException  * @throws IOException  */ private void readPropCfgFile(HttpServletResponse response)   throws FileNotFoundException, IOException {  //通过ServletContext获取web资源的绝对路径  String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db3.properties");  InputStream in = new FileInputStream(path);  Properties prop = new Properties();  prop.load(in);  String driver = prop.getProperty("driver");  String url = prop.getProperty("url");  String username = prop.getProperty("username");  String password = prop.getProperty("password");  response.getWriter().println("读取src目录下的db.config包中的db3.properties配置文件：");  response.getWriter().println(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 通过ServletContext对象读取WebRoot目录下的properties配置文件  * @param response  * @throws IOException  */ private void readWebRootDirPropCfgFile(HttpServletResponse response)   throws IOException {  /**   * 通过ServletContext对象读取WebRoot目录下的properties配置文件   * “/”代表的是项目根目录   */  InputStream in = this.getServletContext().getResourceAsStream("/db2.properties");  Properties prop = new Properties();  prop.load(in);  String driver = prop.getProperty("driver");  String url = prop.getProperty("url");  String username = prop.getProperty("username");  String password = prop.getProperty("password");  response.getWriter().println("读取WebRoot目录下的db2.properties配置文件：");  response.getWriter().print(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 通过ServletContext对象读取src目录下的properties配置文件  * @param response  * @throws IOException  */ 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   */  InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db1.properties");  Properties prop = new Properties();  prop.load(in);  String driver = prop.getProperty("driver");  String url = prop.getProperty("url");  String username = prop.getProperty("username");  String password = prop.getProperty("password");  response.getWriter().println("读取src目录下的db1.properties配置文件：");  response.getWriter().println(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  this.doGet(request, response); }}

运行结果如下：

代码范例：使用类装载器读取资源文件

package gacl.servlet.study;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.text.MessageFormat;import java.util.Properties;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * 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 { public void doGet(HttpServletRequest request, HttpServletResponse response)   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");  test1(response);  response.getWriter().println("<hr/>");  test2(response);  response.getWriter().println("<hr/>");  //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");  Properties prop = new Properties();  prop.load(in);  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db1.properties配置文件：");  response.getWriter().println(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 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");  Properties prop = new Properties();  prop.load(in);  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(      "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); }}

以上就是本文的全部内容，希望对大家掌握javaweb Servlet开发技术有所帮助。