java_web学习第二天(ServletConfig和ServletContext对象的作用和使用)

ServletConfig

官方定义：public abstract interface ServletConfigA servlet configuration object used by a servlet container used to pass information to a servlet during initialization.一个servlet的配置对象，用来给servlet容器传递信息到servlet，在Servlet初始化的时候。ServletConfig的几个方法：1.getInitParameter(java.lang.String name) ------>根据name，获取初始化信息 Returns a String containing the value of the named initialization parameter, or null if the parameter does not exist.2.getInitParameterNames() ------>获取所有初始化信息的名称          Returns the names of the servlet's initialization parameters as an Enumeration of String objects, or an empty Enumeration if the servlet has no initialization parameters.3.getServletContext() ------>获取ServletContext对象           Returns a reference to the ServletContext in which the caller is executing.4.getServletName() ------>获取这个servlet实例的名称          Returns the name of this servlet instance.
例子：
在web.xml中配置servlet并且设置servlet的初始化信息；

<servlet><servlet-name>Servlet_03</servlet-name><servlet-class>com.enterise.always.servlet.Servlet_03</servlet-class><init-param><param-name>name_01</param-name><param-value>value_01</param-value></init-param><init-param><param-name>name_02</param-name><param-value>value_02</param-value></init-param></servlet><servlet-mapping><servlet-name>Servlet_03</servlet-name><url-pattern>/servlet/Servlet_03</url-pattern></servlet-mapping>

java：

package com.enterise.always.servlet;import java.io.IOException;import java.util.Enumeration;import javax.servlet.ServletConfig;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 Servlet_03 extends HttpServlet {private static final long serialVersionUID = 1L;protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {doPost(req, resp);}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {//1.获取ServletConfig对象ServletConfig config = getServletConfig();//1.getInitParameterString para_01 = config.getInitParameter("name_01");String para_02 = config.getInitParameter("name_02");System.out.println("para_01----------->"+para_01);System.out.println("para_02----------->"+para_02);//2.getInitParameterNamesEnumeration enumeration = config.getInitParameterNames();while(enumeration.hasMoreElements()){String name = (String) enumeration.nextElement();String value = config.getInitParameter(name);System.out.println("name----------->"+name);System.out.println("value----------->"+value);}//3.getServletContextServletContext servletContext = config.getServletContext();System.out.println("servletContext----------->"+servletContext);//4.getServletNameString servletName = config.getServletName();System.out.println("servletName----------->"+servletName);}}

后台打印数据：

para_01----------->value_01para_02----------->value_02name----------->name_02value----------->value_02name----------->name_01value----------->value_01servletContext----------->org.apache.catalina.core.ApplicationContextFacade@b398da----------->Servlet_03

例子：

在servlet创建的时候，获取数据库的链接信息：

1.在web.xml中配置数据库的连接信息

<servlet><servlet-name>Servlet_04</servlet-name><servlet-class>com.enterise.always.servlet.Servlet_04</servlet-class><init-param><param-name>driver</param-name><param-value>com.mysql.jdbc.driver</param-value></init-param><init-param><param-name>url</param-name><param-value>jdbc:mysql://localhost:3306/data</param-value></init-param><init-param><param-name>user</param-name><param-value>root</param-value></init-param><init-param><param-name>password</param-name><param-value>root</param-value></init-param><load-on-startup>2</load-on-startup></servlet>

2.在servlet中获取数据:

public void init(ServletConfig config) throws ServletException {System.out.println("Servlet_04.init()--2");Enumeration enumeration = config.getInitParameterNames();while(enumeration.hasMoreElements()){String name = (String) enumeration.nextElement();String value = config.getInitParameter(name);System.out.println("name----------->"+name);System.out.println("value-----------     >"+value);}}

后台打印数据：

2013-8-31 15:59:36 org.apache.catalina.core.ApplicationContext log信息: SessionListener: contextInitialized()Servlet_04.init()--2name----------->drivervalue-----------     >com.mysql.jdbc.drivername----------->passwordvalue-----------     >rootname----------->uservalue-----------     >rootname----------->urlvalue-----------     >jdbc:mysql://localhost:3306/data2013-8-31 15:59:36 org.apache.coyote.http11.Http11Protocol start信息: Starting Coyote HTTP/1.1 on http-80802013-8-31 15:59:36 org.apache.jk.common.ChannelSocket init信息: JK: ajp13 listening on /0.0.0.0:80092013-8-31 15:59:36 org.apache.jk.server.JkMain start信息: Jk running ID=0 time=0/22  config=null2013-8-31 15:59:36 org.apache.catalina.startup.Catalina start信息: Server startup in 738 ms

从数据打印信息中来看，在servlet创建的时候，就会调用初始化的方法。

原因是在web.xml中配置了：

<load-on-startup>2</load-on-startup>

用途：

如果在<servlet>元素中配置了一个<load-on-startup>元素，那么WEB应用程序在启动时，就会装载并创建Servlet的实例对象、以及调用Servlet实例对象的init()方法。

为web应用写一个InitServlet，这个servlet配置为启动时装载，为整个web应用创建必要的数据库表和数据。

如果没有在servlet配置这个参数的话，在装载并创建servlet对象的时候是不会调用init方法的。

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ServletContext：

官方文档定义：
public abstract interface ServletContext
Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file. 


There is one context per "web application" per Java Virtual Machine. (A "web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace such as /catalog and possibly installed via a .war file.) 


In the case of a web application marked "distributed" in its deployment descriptor, there will be one context instance for each virtual machine. In this situation, the context cannot be used as a location to share global information (because the information won't be truly global). Use an external resource like a database instead. 


The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized. 


安装在一个服务器中的一个特定URL名字空间（比如，/myapplication）下的所有Servlet，JSP，JavaBean等Web部件的集合构成了一个Web的应用，每一个Web应用（同一JVM），容器都会有一个背景对象，而javax.servlet.ServletContext接口就提供了访问这个背景对象的途径。


也就是说在整个web应用中，只有一个ServletContext，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。


1.获取ServletContext实例：

//1.第一种方式
ServletContext servletContext_01 = this.getServletContext();
//2.第二种方式
ServletContext servletContext_02 = this.getServletConfig().getServletContext();
2.利用ServletContext实现数据共享：
在Servlet_03中：
//往servletContext对象中设置数据
ServletContext servletContext = config.getServletContext();
servletContext.setAttribute("name", "共享数据---》");
在Servlet_04中：
//从servletContext对象中获取数据
ServletContext servletContext = getServletConfig().getServletContext();
System.out.println("value------------>"+servletContext.getAttribute("name"));

3.在web.xml中配置初始化信息。<!-- context的配置信息 --><context-param><param-name>name_param1</param-name><param-value>value_param1</param-value></context-param><context-param><param-name>name_param2</param-name><param-value>value_param2</param-value></context-param>获取初始化参数ServletContext context = this.getServletContext();Enumeration enumeration = context.getInitParameterNames();while(enumeration.hasMoreElements()){String name = (String) enumeration.nextElement();String value = (String) context.getInitParameter(name);System.out.println("name-------------->"+name);System.out.println("value----------------->"+value.toString());}控制台打印数据：name-------------->name_param2value----------------->value_param2name-------------->name_param1value----------------->value_param14.实现servlet的转发//4.实现servlet的转发。ServletContext context = this.getServletContext();//在request设置参数，然后在jsp界面中获取。req.setAttribute("name", "value");RequestDispatcher requestDispatcher = context.getRequestDispatcher("/index.jsp");requestDispatcher.forward(req,resp);在jsp界面获取的数据： <%    String value = (String)request.getAttribute("name");    System.out.println("value-------------->"+value);     %>

5.读取资源的配置文件。ServletContext context = this.getServletContext();InputStream inputStream = context.getResourceAsStream("/db.properties");ByteArrayOutputStream outStream = new ByteArrayOutputStream();byte[] by = new byte[1024];int len = 0;while((len = inputStream.read(by))!= -1){outStream.write(by, 0, len);}String content = new String(outStream.toByteArray());System.out.println("content------------->"+content);outStream.close();inputStream.close();