深刻分析JavaWeb Item6 -- servletConfig 與servletContext詳解
1、ServletConfig講解
首先看ServletConfig API文檔html
1.一、配置Servlet初始化參數
在Servlet的配置文件web.xml中，可使用一個或多個<init-param>標籤爲servlet配置一些初始化參數。java
例如：mysql
<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>
1.二、經過ServletConfig獲取Servlet的初始化參數
當servlet配置了初始化參數後，web容器在建立servlet實例對象時，會自動將這些初始化參數封裝到ServletConfig對象中，並在調用servlet的init方法時，將ServletConfig對象傳遞給servlet。進而，咱們經過ServletConfig對象就能夠獲得當前servlet的初始化參數信息。程序員
例如：web
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配置了初始化參數後，web容器在建立servlet實例對象時，
* 會自動將這些初始化參數封裝到ServletConfig對象中，並在調用servlet的init方法時，
* 將ServletConfig對象傳遞給servlet。進而，程序員經過ServletConfig對象就能夠
* 獲得當前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/>");
//獲取全部的初始化參數
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);
}
}
運行結果以下：sql
2、ServletContext對象
WEB容器在啓動時，它會爲每一個WEB應用程序都建立一個對應的ServletContext對象，它表明當前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，能夠經過ServletConfig.getServletContext方法得到ServletContext對象，可是還有更簡潔的this.getServletContext()方法；
<font color="red">因爲一個WEB應用中的全部Servlet共享同一個ServletContext對象，所以Servlet對象之間能夠經過ServletContext對象來實現通信。ServletContext對象一般也被稱之爲context域對象：1，是一個容器 2。做用範圍是應用程序範圍。數據庫
3、ServletContext的應用
3.一、多個Servlet經過ServletContext對象實現數據共享
範例：ServletContextDemo1和ServletContextDemo2經過ServletContext對象實現數據共享apache
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，
* 能夠經過ServletConfig.getServletContext方法得到ServletContext對象。
*/
ServletContext context = this.getServletConfig().getServletContext();//得到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，而後運行ServletContextDemo2就能夠從ServletContext對象中取出數據了，這樣就實現了數據共享，以下圖所示：瀏覽器
3.二、獲取WEB應用的初始化參數
若是想在全部的Servlet應用中都要配置並讀取初始化參數，則能夠在web.xml文件的<web-app>中使用<context-param>標籤配置WEB應用的初始化參數，以下所示：緩存
<?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，代碼以下：
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：
3.三、用servletContext實現請求轉發
實現Servlet的轉發。
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，瀏覽器顯示的倒是ServletContextDemo5的內容，這就是使用ServletContext實現了請求轉發
3.四、利用ServletContext對象讀取資源文件
利用ServletContext對象讀取資源文件,由於文件的位置不一樣，全部讀取的方式也不一樣，通常來講分爲兩種狀況：
在Servlet的context域中讀取文件，工程目錄下的src目錄發佈到服務器中，會映射到「/WEB-INF/classes」文件夾下。因此要一一對應。並且這個是相對目錄，相對於web服務器的目錄。若是要用傳統的文件讀取文件，則要使用絕對路勁
PrintWriter out = response.getWriter();
ServletContext context = this.getServletContext();
String path = context.getRealPath("/WEB-INF/classes/itcast.properties");
InputStream in = new FileInputStream(path);
Properties pro = new Properties();
pro.load(in);
若是是非servlet中讀取配置文件，則要使用類加載器去讀取。稍後講到
項目目錄結構以下：
代碼範例：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 {
//經過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));
}
/**
* 經過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 {
/**
* 經過ServletContext對象讀取WebRoot目錄下的properties配置文件
* 「/」表明的是項目根目錄
*/
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));
}
/**
* 經過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 {
/**
* 經過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);
}
}
運行結果以下：
使用類裝載器讀取資源文件
咱們在非servlet中讀取資源文件時（好比在數據庫的dao層讀取配置文件），採用類裝載器 classLoader,你能夠先採用servlet服務先讀取，而後在把servlet傳遞給dao，這樣雖然能夠實現，可是，這樣損壞了咱們編代碼的設計原則，就是層之間不能有交織在一塊兒的東西。
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
* 經過類裝載器讀取資源文件的注意事項:不適合裝載大文件，不然會致使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));
}
/**
* 經過類裝載器讀取資源文件的注意事項:不適合裝載大文件，不然會致使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致使內存溢出：
* java.lang.OutOfMemoryError: Java heap space
*/
InputStream in = ServletContextDemo7.class.getClassLoader().getResourceAsStream("01.avi");
System.out.println(in);
}
/**
* 讀取01.avi,並拷貝到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是一個很是絕妙的寫法
*/
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);
}
}
運行結果以下：
使用類裝載器讀取資源文件，存在的問題是；類裝載器，每次只會裝載一次。
//若是讀取資源文件的程序不是servlet的話，
//就只能經過類轉載器去讀了，文件不能太大
//用傳遞參數方法很差，耦合性高
public class UserDao {
private static Properties dbconfig=new Properties();
static {
InputStream in=UserDao.class.getClassLoader().getResourceAsStream("db.properties");
try {
dbconfig.load(in);
} catch (IOException e) {
throw new ExceptionInInitializerError(e);
}
//上面代碼類裝載器只裝載一次,下面代碼用類裝載方式獲得文件位置
URL url=UserDao.class.getClassLoader().getResource("db.properties");
String str=url.getPath();
//file:/C:/apache-tomcat-7.0.22/webapps/day05/WEB-INF/classes/db.properties
try {
InputStream in2=new FileInputStream(str);
try {
dbconfig.load(in2);
} catch (IOException e) {
throw new ExceptionInInitializerError(e);
}
} catch (FileNotFoundException e1) {
throw new ExceptionInInitializerError(e1);
}
}
public void update() {
System.out.println(dbconfig.get("url"));
}
}
4、在客戶端緩存Servlet的輸出
對於不常常變化的數據，在servlet中能夠爲其設置合理的緩存時間值，以免瀏覽器頻繁向服務器發送請求，提高服務器的性能。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，以免瀏覽器頻繁向服務器發送請求，提高服務器的性能
* 這裏是將數據的緩存時間設置爲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);
}
}
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