Servlet的自动加载、ServletConfig对象、ServletContext对象

一、 Servlet的自动加载

默认情况下，第一次访问servlet的时候，创建servlet对象。如果servlet构造函数里面的代码或者init方法里面的代码比较多，就会导致用户第一次访问servlet的时候比较慢。这个时候，我们可以改变servlet对象的创建时机：提前到加载web应用的时候。在servlet的配置信息中，加上一个<load-on-startup>标签即可。

<servlet><servlet-name>loadOnStartup</servlet-name><servlet-class>com.by.servlet.LoadOnStartupServlet</servlet-class><!--容器是否在启动时加载该servlet,数字越小优先级越高越高--><load-on-startup>1</load-on-startup>
</servlet>

servlet实例:

public class LoadOnStartupServlet implements HttpServlet {public LoadOnStartupServlet(){System.out.println("LoadOnStartupServlet constructor method has run....");}
}

这样配置之后，servlet的构造函数和init方法就会在web应用加载的时候就会执行。

二、ServletConfig对象

是什么

ServletConfig是javax.servlet.包下的一个接口，ServletConfig它是Servlet的一个配置对象；

ServletConfig是由tomcat容器创建，通过init方法传入给Servlet；
ServletConfig对象如何获取?

在GenericServlet里面定义了：

public ServletConfig getServletConfig() {return this.config;
}

常用方法

getInitParameter(String parameterName); //根据参数名称获取指定的参数值
getInitParameterNames(); //获取所有的参数名称

需求：

获取servlet里面定义的参数

<servlet><servlet-name>demo2</servlet-name><servlet-class>com.by.servlet.ServletConfigServlet</servlet-class><!--Servlet的初始化参数--><init-param><param-name>username</param-name><param-value>root</param-value></init-param><init-param><param-name>password</param-name><param-value>root123</param-value></init-param></servlet><servlet-mapping><servlet-name>demo2</servlet-name><url-pattern>/demo2</url-pattern></servlet-mapping>

ServletConfigServlet

package com.by.servlet;import javax.servlet.*;
import javax.servlet.http.HttpServlet;public class ServletConfigServlet extends HttpServlet {@Overridepublic void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {//1.获取ServletConfig对象ServletConfig servletConfig = getServletConfig();//2.获取Servlet中的初始化参数String username = servletConfig.getInitParameter("username");System.out.println(username);String password = servletConfig.getInitParameter("password");System.out.println(password);//3.获取ServletContext对象(域对象)ServletContext servletContext = servletConfig.getServletContext();}
}

思考：param参数可不可以在另外的Servlet中获取? 不能

public class ServletConfigServlet2 extends HttpServlet {@Overridepublic void goGet(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {//1.获取ServletConfig对象ServletConfig servletConfig = getServletConfig();//2.获取Servlet中的初始化参数String username = servletConfig.getInitParameter("username");//不能获取System.out.println(username);String password = servletConfig.getInitParameter("password");//不能获取System.out.println(password);//3.获取ServletContext对象(域对象)ServletContext servletContext = servletConfig.getServletContext();}
}

三、ServletContext对象

是什么

ServletContext是javax.servlet包下的一个接口，又称上下文对象，是配置对象也是一个域对象；

当服务器启动时，会为服务器中的每一个web应用程序创建一个ServletContext对象；

在web应用中的servlet要想实现资源的共享，可以通过ServletContext来完成；
如何获取这个对象:

public ServletContext getServletContext() {//获取ServletContext对象return this.getServletConfig().getServletContext();
}

这个对象里面也有很多方法：

getInitParameter() //获取指定参数名称的全局参数值
getRealPath(String path) //获得当前项目的服务器磁盘路径
getContextPath() //获取项目的根路径
getAttribute(String parameterName) //获取ServletContext域中指定名称的参数值；
setAttribute(String paramterName,Object parameterValue) //存储参数到ServletContext域中；
removeAttribute(String parameterNam) //将ServletContext域中指定名称的参数移除；

需求

获取servlet里面定义的参数

 <context-param><param-name>username</param-name><param-value>root</param-value></context-param><context-param><param-name>password</param-name><param-value>root456</param-value></context-param>

package com.by.servlet;import javax.servlet.*;
import javax.servlet.http.HttpServlet;public class ServletContextServlet extends HttpServlet {@Overridepublic void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {//获取ServletContext对象ServletContext servletContext = getServletContext();//1.获取全局初始化参数String username = servletContext.getInitParameter("username");System.out.println(username);String password = servletContext.getInitParameter("password");System.out.println(password);//2.获取服务器真实路径String upload = servletContext.getRealPath("");System.out.println(upload);//3.获取项目的根路径String contextPath = servletContext.getContextPath();System.out.println(contextPath);//4.往ServletContext域中，存储一个名称为msg的属性，值为"hello"String str = "hello";servletContext.setAttribute("msg",str);}
}

思考：param参数可不可以在另外的Servlet中获取? 不能

package com.by.servlet;import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;public class ServletContextServlet2 extends HttpServlet {@Overridepublic void doGet(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {//从ServletContext域中取出msg的值ServletContext servletContext = getServletContext();//1.获取全局初始化参数String username = servletContext.getInitParameter("username");System.out.println(username);String password = servletContext.getInitParameter("password");System.out.println(password);//2.获得msg属性Object msg = servletContext.getAttribute("msg");//能获取System.out.println(msg);}
}

package com.by.servlet;import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;public class ServletContextServlet3 extends HttpServlet {@Overridepublic void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {//1.获取ServletContext对象ServletContext servletContext = getServletContext();//将ServletContext域中的msg参数移除servletContext.removeAttribute("msg");}
}

  <servlet><servlet-name>context3=</servlet-name><servlet-class>com.by.servlet.ServletContextServlet</servlet-class></servlet><servlet-mapping><servlet-name>context3=</servlet-name><url-pattern>/context=</url-pattern></servlet-mapping><servlet><servlet-name>context2</servlet-name><servlet-class>com.by.servlet.ServletContextServlet2</servlet-class></servlet><servlet-mapping><servlet-name>context2</servlet-name><url-pattern>/context2</url-pattern></servlet-mapping><servlet><servlet-name>context3</servlet-name><servlet-class>com.by.servlet.ServletContextServlet3</servlet-class></servlet><servlet-mapping><servlet-name>context3</servlet-name><url-pattern>/context3</url-pattern></servlet-mapping>