深入分析JavaWeb Item6 -- servletConfig 与servletContext详解
一、ServletConfig讲解
首先看ServletConfig API文档
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
例如：
<servlet>
<servlet-name>ServletConfigDemo1servlet-name>
<servlet-class>gacl.servlet.study.ServletConfigDemo1servlet-class>
<init-param>
<param-name>nameparam-name>
<param-value>gaclparam-value>
init-param>
<init-param>
<param-name>passwordparam-name>
<param-value>123param-value>
init-param>
<init-param>
<param-name>charsetparam-name>
<param-value>UTF-8param-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("
");
//获取所有的初始化参数
Enumeration e = config.getInitParameterNames();
while(e.hasMoreElements()){
String name = e.nextElement();
String value = config.getInitParameter(name);
response.getWriter().print(name + "=" + value + "
");
}
}
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，但是还有更简洁的this.getServletContext()方法；
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：1，是一个容器 2。作用范围是应用程序范围。
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
如果想在所有的Servlet应用中都要配置并读取初始化参数，则可以在web.xml文件的
<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>
<context-param>
<param-name>urlparam-name>
<param-value>jdbc:mysql://localhost:3306/testparam-value>
context-param>
<welcome-file-list>
<welcome-file>index.jspwelcome-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
实现Servlet的转发。
- 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 = "abcdefghjkl
";
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
利用ServletContext对象读取资源文件,因为文件的位置不同，所有读取的方式也不同，一般来说分为两种情况：
- 在Servlet的context域中读取文件，工程目录下的src目录发布到服务器中，会映射到“/WEB-INF/classes”文件夹下。所以要一一对应。而且这个是相对目录，相对于web服务器的目录。如果要用传统的文件读取文件，则要使用绝对路劲
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);如果是非servlet中读取配置文件，则要使用类加载器去读取。稍后讲到
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("
");
readWebRootDirPropCfgFile(response);//读取WebRoot目录下的properties配置文件
response.getWriter().println("
");
readPropCfgFile(response);//读取src目录下的db.config包中的db3.properties配置文件
response.getWriter().println("
");
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：
使用类装载器读取资源文件
我们在非servlet中读取资源文件时（比如在数据库的dao层读取配置文件），采用类装载器 classLoader,你可以先采用servlet服务先读取，然后在把servlet传递给dao，这样虽然可以实现，但是，这样损坏了我们编代码的设计原则，就是层之间不能有交织在一起的东西。
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("
");
test2(response);
response.getWriter().println("
");
//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：
使用类装载器读取资源文件，存在的问题是；类装载器，每次只会装载一次。
//如果读取资源文件的程序不是servlet的话，
//就只能通过类转载器去读了，文件不能太大
//用传递参数方法不好，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);
}
//上面代码类装载器只装载一次,下面代码用类装载方式得到文件位置
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"));
}
}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);
}
}