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：
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<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：
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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
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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); }} |
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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：
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<?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：
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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
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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
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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
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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
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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("\\") + 1是一個非常絕妙的寫法 */ String filename = path.substring(path.lastIndexOf("\\") + 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：
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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); }} |
以上就是本文的全部內容，希望對大家掌握javaweb Servlet開發技術有所幫助。
