JAVAWEB学习总结 SERVLET开发（二）

一、ServletConfig对象

1.1、配置servlet初始化参数

  在servlet的配置文件中web.xml中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。

  例如：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <servlet>
    <servlet-name>ServletDemo6</servlet-name>
    <servlet-class>my.servlet.demo.ServletDemo6</servlet-class>
    <!--配置ServletConfigDemo1的初始化参数 -->
    <init-param>
        <param-name>name</param-name>
        <param-value>my</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>
 <servlet-mapping>
    <servlet-name>ServletDemo6</servlet-name>
    <url-pattern>/ServletDemo6</url-pattern>
    </servlet-mapping>
</web-app>

1.2、通过ServletConfig获取servlet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 my.servlet.demo;
//导入必需的 java 库
import java.io.*;
import java.util.Enumeration;

import javax.servlet.*;
import javax.servlet.http.*;

// 扩展 HttpServlet 类
public class ServletDemo1 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。
*/

  public void init(ServletConfig config) throws ServletException
  {
      // 执行必需的初始化
      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("<hr/>");
//获取所有的初始化参数
   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 destroy()
  {
      // 什么也不做
  }
}

　　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ServletContxt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。

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对象通常也被称为context域对象。

2.1、多个servlet通过ServletContext对象实现数据共享

例子：ServletDemo1和ServletDemo2通过ServletContext对象实现数据共享

package my.servlet.demo;
//导入必需的 java 库
import java.io.*;
import java.util.Enumeration;

import javax.servlet.*;
import javax.servlet.http.*;

// 扩展 HttpServlet 类
public class ServletDemo1 extends HttpServlet {
 
 private ServletConfig config;

  public void init(ServletConfig config) throws ServletException
  {
      // 执行必需的初始化
      this.config=config;
  }
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
	 String data="itnnn";
	 ServletContext context=config.getServletContext();
	 context.setAttribute("data", data);
  }
  
  public void destroy()
  {
      // 什么也不做
  }
}

　　

package my.servlet.demo;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo2
 */
@WebServlet("/ServletDemo2")
public class ServletDemo2 extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		ServletContext context =this.getServletContext();
		String data=(String) context.getAttribute("data");
		response.getWriter().print("data="+data);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

}

　　先运行ServletDemo1，将数据data存储到ServletContext对象中，然后运行ServletDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，运行结果如下图所示：

2.2、获取WEB应用的初始化参数

在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，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应用的初始化参数 -->
    <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>

package my.servlet.demo;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


@WebServlet("/ServletDemo3")
public class ServletDemo3 extends HttpServlet {


	protected 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 );
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

}

　　运行结果：

2.3、用ServletContext实现请求转发

ServletDemo4.java

package my.servlet.demo;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletDemo4 extends HttpServlet {
 
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String data="<h1><font color='red'>abc</font></h1>";
		response.getOutputStream().write(data.getBytes());
		ServletContext context=this.getServletContext();
		RequestDispatcher rd =context.getRequestDispatcher("/ServletDemo5");
		rd.forward(request, response);
	}


	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

}

　　ServletDemo5.java

package my.servlet.demo;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletDemo5 extends HttpServlet {



	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		response.getOutputStream().write("servletDemo5".getBytes());
	}


	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

}

　　web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <servlet>
    <description>This is the description of my J2EEcomponent</description>
    <display-name>This is the display of my J2EEcomponent</display-name>
    <servlet-name>ServletDemo4</servlet-name>
    <servlet-class>my.servlet.demo.ServletDemo4</servlet-class>
  </servlet>
  <servlet>
    <description>This is the description of my J2EEcomponent</description>
    <display-name>This is the display of my J2EEcomponent</display-name>
    <servlet-name>ServletDemo5</servlet-name>
    <servlet-class>my.servlet.demo.ServletDemo5</servlet-class>
  </servlet>
    <servlet-mapping>
    <servlet-name>ServletDemo4</servlet-name>
    <url-pattern>/ServletDemo4</url-pattern>
   </servlet-mapping>
  <servlet-mapping>
    <servlet-name>ServletDemo5</servlet-name>
    <url-pattern>/ServletDemo5</url-pattern>
    </servlet-mapping>
</web-app>

　　运行结果：

访问的是ServletDemo4，浏览器显示的却是ServletDemo5的内容，这就是使用ServletContext实现了请求转发。

2.4、利用ServletContext对象读取资源文件

 2.5、在客户端缓存servlet输出