javaweb Servlet開發總結（二）

2019-11-26 14:22:53

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一、ServletConfig講解

1.1、配置Servlet初始化參數

　　在Servlet的配置文件web.xml中，可以使用一個或多個<init-param>標簽為servlet配置一些初始化參數。

例如：

<servlet> <servlet-name>ServletConfigDemo1</servlet-name> <servlet-class>gacl.servlet.study.ServletConfigDemo1</servlet-class> <!--配置ServletConfigDemo1的初始化參數 --> <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、通過ServletConfig獲取Servlet的初始化參數

　　當servlet配置了初始化參數后，web容器在創建servlet實例對象時，會自動將這些初始化參數封裝到ServletConfig對象中，并在調用servlet的init方法時，將ServletConfig對象傳遞給servlet。進而，我們通過ServletConfig對象就可以得到當前servlet的初始化參數信息。

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 { /**  * 定義ServletConfig對象來接收配置的初始化參數  */ private ServletConfig config;  /**  * 當servlet配置了初始化參數后，web容器在創建servlet實例對象時，  * 會自動將這些初始化參數封裝到ServletConfig對象中，并在調用servlet的init方法時，  * 將ServletConfig對象傳遞給servlet。進而，程序員通過ServletConfig對象就可以  * 得到當前servlet的初始化參數信息。  */ @Override public void init(ServletConfig config) throws ServletException {  this.config = config; } public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  //獲取在web.xml中配置的初始化參數  String paramVal = this.config.getInitParameter("name");//獲取指定的初始化參數  response.getWriter().print(paramVal);    response.getWriter().print("<hr/>");  //獲取所有的初始化參數  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); }}

運行結果如下：

二、ServletContext對象

　　WEB容器在啟動時，它會為每個WEB應用程序都創建一個對應的ServletContext對象，它代表當前web應用。
　　ServletConfig對象中維護了ServletContext對象的引用，開發人員在編寫servlet時，可以通過ServletConfig.getServletContext方法獲得ServletContext對象。
　　由于一個WEB應用中的所有Servlet共享同一個ServletContext對象，因此Servlet對象之間可以通過ServletContext對象來實現通訊。ServletContext對象通常也被稱之為context域對象。

三、ServletContext的應用

3.1、多個Servlet通過ServletContext對象實現數據共享

范例：ServletContextDemo1和ServletContextDemo2通過ServletContext對象實現數據共享

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對象中維護了ServletContext對象的引用，開發人員在編寫servlet時，   * 可以通過ServletConfig.getServletContext方法獲得ServletContext對象。   */  ServletContext context = this.getServletConfig().getServletContext();//獲得ServletContext對象  context.setAttribute("data", data); //將data存儲到ServletContext對象中 } 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");//從ServletContext對象中取出數據  response.getWriter().print("data="+data); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  doGet(request, response); }}

　　先運行ServletContextDemo1，將數據data存儲到ServletContext對象中，然后運行ServletContextDemo2就可以從ServletContext對象中取出數據了，這樣就實現了數據共享，如下圖所示：

3.2、獲取WEB應用的初始化參數

在web.xml文件中使用<context-param>標簽配置WEB應用的初始化參數，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應用的初始化參數 --> <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>

獲取Web應用的初始化參數，代碼如下：

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();  //獲取整個web站點的初始化參數  String contextInitParam = context.getInitParameter("url");  response.getWriter().print(contextInitParam); } public void doPost(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  doGet(request, response); }}

運行結果：

3.3、用servletContext實現請求轉發

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();//獲取ServletContext對象  RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//獲取請求轉發對象(RequestDispatcher)  rd.forward(request, response);//調用forward方法實現請求轉發 } 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); }}

運行結果：

訪問的是ServletContextDemo4，瀏覽器顯示的卻是ServletContextDemo5的內容，這就是使用ServletContext實現了請求轉發

3.4、利用ServletContext對象讀取資源文件

項目目錄結構如下：

代碼范例：使用servletContext讀取資源文件

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讀取資源文件 *  * @author gacl *  */public class ServletContextDemo6 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {   /**   * response.setContentType("text/html;charset=UTF-8");目的是控制瀏覽器用UTF-8進行解碼；   * 這樣就不會出現中文亂碼了   */  response.setHeader("content-type","text/html;charset=UTF-8");  readSrcDirPropCfgFile(response);//讀取src目錄下的properties配置文件  response.getWriter().println("<hr/>");  readWebRootDirPropCfgFile(response);//讀取WebRoot目錄下的properties配置文件  response.getWriter().println("<hr/>");  readPropCfgFile(response);//讀取src目錄下的db.config包中的db3.properties配置文件  response.getWriter().println("<hr/>");  readPropCfgFile2(response);//讀取src目錄下的gacl.servlet.study包中的db4.properties配置文件   } /**  * 讀取src目錄下的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("讀取src目錄下的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)); } /**  * 讀取src目錄下的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 {  //通過ServletContext獲取web資源的絕對路徑  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("讀取src目錄下的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)); } /**  * 通過ServletContext對象讀取WebRoot目錄下的properties配置文件  * @param response  * @throws IOException  */ private void readWebRootDirPropCfgFile(HttpServletResponse response)   throws IOException {  /**   * 通過ServletContext對象讀取WebRoot目錄下的properties配置文件   * “/”代表的是項目根目錄   */  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("讀取WebRoot目錄下的db2.properties配置文件：");  response.getWriter().print(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 通過ServletContext對象讀取src目錄下的properties配置文件  * @param response  * @throws IOException  */ private void readSrcDirPropCfgFile(HttpServletResponse response) throws IOException {  /**   * 通過ServletContext對象讀取src目錄下的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("讀取src目錄下的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); }}

運行結果如下：

代碼范例：使用類裝載器讀取資源文件

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;/** * 用類裝載器讀取資源文件 * 通過類裝載器讀取資源文件的注意事項:不適合裝載大文件，否則會導致jvm內存溢出 * @author gacl * */public class ServletContextDemo7 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response)   throws ServletException, IOException {  /**   * response.setContentType("text/html;charset=UTF-8");目的是控制瀏覽器用UTF-8進行解碼；   * 這樣就不會出現中文亂碼了   */  response.setHeader("content-type","text/html;charset=UTF-8");  test1(response);  response.getWriter().println("<hr/>");  test2(response);  response.getWriter().println("<hr/>");  //test3();  test4();   }  /**  * 讀取類路徑下的資源文件  * @param response  * @throws IOException  */ private void test1(HttpServletResponse response) throws IOException {  //獲取到裝載當前類的類裝載器  ClassLoader loader = ServletContextDemo7.class.getClassLoader();  //用類裝載器讀取src目錄下的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("用類裝載器讀取src目錄下的db1.properties配置文件：");  response.getWriter().println(    MessageFormat.format(      "driver={0},url={1},username={2},password={3}",       driver,url, username, password)); } /**  * 讀取類路徑下面、包下面的資源文件  * @param response  * @throws IOException  */ private void test2(HttpServletResponse response) throws IOException {  //獲取到裝載當前類的類裝載器  ClassLoader loader = ServletContextDemo7.class.getClassLoader();  //用類裝載器讀取src目錄下的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("用類裝載器讀取src目錄下的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)); }  /**  * 通過類裝載器讀取資源文件的注意事項:不適合裝載大文件，否則會導致jvm內存溢出  */ public void test3() {  /**   * 01.avi是一個150多M的文件，使用類加載器去讀取這個大文件時會導致內存溢出：   * java.lang.OutOfMemoryError: Java heap space   */  InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");  System.out.println(in); }  /**  * 讀取01.avi,并拷貝到e:/根目錄下  * 01.avi文件太大，只能用servletContext去讀取  * @throws IOException  */ public void test4() throws IOException {  // path=G:/Java學習視頻/JavaWeb學習視頻/JavaWeb/day05視頻/01.avi  // path=01.avi  String path = this.getServletContext().getRealPath("/WEB-INF/classes/01.avi");  /**   * path.lastIndexOf("http://") + 1是一個非常絕妙的寫法   */  String filename = path.substring(path.lastIndexOf("http://") + 1);//獲取文件名  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); }}

運行結果如下：

四、在客戶端緩存Servlet的輸出

對于不經常變化的數據，在servlet中可以為其設置合理的緩存時間值，以避免瀏覽器頻繁向服務器發送請求，提升服務器的性能。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";  /**   * 設置數據合理的緩存時間值，以避免瀏覽器頻繁向服務器發送請求，提升服務器的性能   * 這里是將數據的緩存時間設置為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); }}