javaweb Servlet開發(fā)總結(jié)（二）

更新時(shí)間：2016年05月05日 09:24:34 作者：孤傲蒼狼

這篇文章主要為大家詳細(xì)介紹了javaweb Servlet開發(fā)總結(jié)的第二篇，感興趣的小伙伴們可以參考一下

一、ServletConfig講解

1.1、配置Servlet初始化參數(shù)

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

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(shù) -->
 <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(shù)

　　當(dāng)servlet配置了初始化參數(shù)后，web容器在創(chuàng)建servlet實(shí)例對(duì)象時(shí)，會(huì)自動(dòng)將這些初始化參數(shù)封裝到ServletConfig對(duì)象中，并在調(diào)用servlet的init方法時(shí)，將ServletConfig對(duì)象傳遞給servlet。進(jìn)而，我們通過ServletConfig對(duì)象就可以得到當(dāng)前servlet的初始化參數(shù)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(duì)象來接收配置的初始化參數(shù)
  */
 private ServletConfig config;
 
 /**
  * 當(dāng)servlet配置了初始化參數(shù)后，web容器在創(chuàng)建servlet實(shí)例對(duì)象時(shí)，
  * 會(huì)自動(dòng)將這些初始化參數(shù)封裝到ServletConfig對(duì)象中，并在調(diào)用servlet的init方法時(shí)，
  * 將ServletConfig對(duì)象傳遞給servlet。進(jìn)而，程序員通過ServletConfig對(duì)象就可以
  * 得到當(dāng)前servlet的初始化參數(shù)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(shù)
  String paramVal = this.config.getInitParameter("name");//獲取指定的初始化參數(shù)
  response.getWriter().print(paramVal);
  
  response.getWriter().print("<hr/>");
  //獲取所有的初始化參數(shù)
  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(yùn)行結(jié)果如下：

二、ServletContext對(duì)象

　　WEB容器在啟動(dòng)時(shí)，它會(huì)為每個(gè)WEB應(yīng)用程序都創(chuàng)建一個(gè)對(duì)應(yīng)的ServletContext對(duì)象，它代表當(dāng)前web應(yīng)用。
　　ServletConfig對(duì)象中維護(hù)了ServletContext對(duì)象的引用，開發(fā)人員在編寫servlet時(shí)，可以通過ServletConfig.getServletContext方法獲得ServletContext對(duì)象。
　　由于一個(gè)WEB應(yīng)用中的所有Servlet共享同一個(gè)ServletContext對(duì)象，因此Servlet對(duì)象之間可以通過ServletContext對(duì)象來實(shí)現(xiàn)通訊。ServletContext對(duì)象通常也被稱之為context域?qū)ο蟆?/p>

三、ServletContext的應(yīng)用

3.1、多個(gè)Servlet通過ServletContext對(duì)象實(shí)現(xiàn)數(shù)據(jù)共享

范例：ServletContextDemo1和ServletContextDemo2通過ServletContext對(duì)象實(shí)現(xiàn)數(shù)據(jù)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(duì)象中維護(hù)了ServletContext對(duì)象的引用，開發(fā)人員在編寫servlet時(shí)，
   * 可以通過ServletConfig.getServletContext方法獲得ServletContext對(duì)象。
   */
  ServletContext context = this.getServletConfig().getServletContext();//獲得ServletContext對(duì)象
  context.setAttribute("data", data); //將data存儲(chǔ)到ServletContext對(duì)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(duì)象中取出數(shù)據(jù)
  response.getWriter().print("data="+data);
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }
}

　　先運(yùn)行ServletContextDemo1，將數(shù)據(jù)data存儲(chǔ)到ServletContext對(duì)象中，然后運(yùn)行ServletContextDemo2就可以從ServletContext對(duì)象中取出數(shù)據(jù)了，這樣就實(shí)現(xiàn)了數(shù)據(jù)共享，如下圖所示：

3.2、獲取WEB應(yīng)用的初始化參數(shù)

在web.xml文件中使用<context-param>標(biāo)簽配置WEB應(yīng)用的初始化參數(shù)，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(yīng)用的初始化參數(shù) -->
 <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(yīng)用的初始化參數(shù)，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(gè)web站點(diǎn)的初始化參數(shù)
  String contextInitParam = context.getInitParameter("url");
  response.getWriter().print(contextInitParam);
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  doGet(request, response);
 }

}

運(yùn)行結(jié)果：

3.3、用servletContext實(shí)現(xiàn)請(qǐng)求轉(zhuǎn)發(fā)

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(duì)象
  RequestDispatcher rd = context.getRequestDispatcher("/servlet/ServletContextDemo5");//獲取請(qǐng)求轉(zhuǎn)發(fā)對(duì)象(RequestDispatcher)
  rd.forward(request, response);//調(diào)用forward方法實(shí)現(xiàn)請(qǐng)求轉(zhuǎn)發(fā)
 }

 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(yùn)行結(jié)果：

訪問的是ServletContextDemo4，瀏覽器顯示的卻是ServletContextDemo5的內(nèi)容，這就是使用ServletContext實(shí)現(xiàn)了請(qǐng)求轉(zhuǎn)發(fā)

3.4、利用ServletContext對(duì)象讀取資源文件

項(xiàng)目目錄結(jié)構(gòu)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(jìn)行解碼；
   * 這樣就不會(huì)出現(xiàn)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(duì)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(duì)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(duì)象讀取WebRoot目錄下的properties配置文件
   * “/”代表的是項(xiàng)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(duì)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(duì)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(yùn)行結(jié)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(xiàng):不適合裝載大文件，否則會(huì)導(dǎo)致jvm內(nèi)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(jìn)行解碼；
   * 這樣就不會(huì)出現(xiàn)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(dāng)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(dāng)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(xiàng):不適合裝載大文件，否則會(huì)導(dǎo)致jvm內(nèi)存溢出
  */
 public void test3() {
  /**
   * 01.avi是一個(gè)150多M的文件，使用類加載器去讀取這個(gè)大文件時(shí)會(huì)導(dǎo)致內(nèi)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(xué)習(xí)視頻\JavaWeb學(xué)習(xí)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(gè)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(yùn)行結(jié)果如下：

四、在客戶端緩存Servlet的輸出

對(duì)于不經(jīng)常變化的數(shù)據(jù)，在servlet中可以為其設(shè)置合理的緩存時(shí)間值，以避免瀏覽器頻繁向服務(wù)器發(fā)送請(qǐng)求，提升服務(wù)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(shè)置數(shù)據(jù)合理的緩存時(shí)間值，以避免瀏覽器頻繁向服務(wù)器發(fā)送請(qǐng)求，提升服務(wù)器的性能
   * 這里是將數(shù)據(jù)的緩存時(shí)間設(shè)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);
 }

}