JavaWeb-Servlet开发2

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ServletConfig

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<init-param>标签为servlet配置一些初始化参数。

<servlet>
    <servlet-name>servletDemo</servlet-name>
    <servlet-class>com.fpc.servletDemo</servlet-class>
    <!-- 配置servletDemo的初始化参数 -->
    <init-param>
        <param-name>name</param-name>
        <param-value>fpc</param-value>
    </init-param>
    
    <init-param>
    <param-name>password</param-name>
    <param-value>123456</param-value>
    </init-param>
    
    <init-param>
    <param-name>charset</param-name>
    <param-value>UTF-8</param-value>
    </init-param>
    
    </servlet>

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。

package com.fpc;

import java.io.IOException;
import java.io.PrintWriter;
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 servletDemo extends HttpServlet {
    int i = 1;
    private ServletConfig config;
    
    
    @Override
    public void init(ServletConfig config) throws ServletException {
        // TODO Auto-generated method stub
        this.config = config;
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        //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().write(paramVal);
        
        response.getWriter().write("<br/>");
        //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().write(name + " = " + value + "<br/>");
        }
        
    }

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

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">");
        out.println("<HTML>");
        out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
        out.println("  <BODY>");
        out.print("    This is ");
        out.print(this.getClass());
        out.println(", using the POST method");
        out.println("  </BODY>");
        out.println("</HTML>");
        out.flush();
        out.close();
    }
    
}

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。

运行结果：

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。

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对象通常也称之为context域对象。

多个Servlet通过ServletContext对象实现数据共享

例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享ServletContextDemo1

package com.fpc;

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 {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String data = "fpc_20171206";
        
        /*
         * ServletConfig对象维护了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
         * 
         * */
        
        //获取ServletContext对象
        ServletContext  servletContext = this.getServletConfig().getServletContext();
        //将数据存储到ServletContext对象中
        servletContext.setAttribute("name_date", data);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

ServletContextDemo2

package com.fpc;

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 {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        ServletContext servletContext = this.getServletConfig().getServletContext();
        String data = (String) servletContext.getAttribute("name_date");
        resp.getWriter().write("context : " + data );
        
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后再运行ServletContextDemo2即㐓从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示：

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：

<!-- 配置web应用的初始化参数 -->
      <context-param>
          <param-name>url</param-name>
          <param-value>jdbc:mysql://10.0.20.252:306/test</param-value>
      </context-param>

获取web应用的初始化参数，代码如下：

String contextInitParam = servletContext.getInitParameter("url");
        resp.getWriter().write("url : " + contextInitParam);

运行结果：

用ServletContext实现请求转发

ServletContextDemod1

package com.fpc;

import java.io.IOException;

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 ServletContextDemo1 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String data = "<h1><font color='blue'>fpc_20171206</font></h1>";
        resp.getOutputStream().write(data.getBytes());
        ServletContext context = this.getServletContext();//获取ServletContext对象
        RequestDispatcher rd = context.getRequestDispatcher("/servletContext/servletContextDemo2");//获取请求转发对象
        rd.forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

ServletContextDemo2

package com.fpc;

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 {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.getOutputStream().write("ServletContextDemo2".getBytes());
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

运行结果：

访问的是ServletContextDemo1，浏览器显示的却是ServletContextDemo2的内容，这就是使用了ServletContext实现了请求转发。

利用ServletContext对象读取资源文件

目录结构

package com.fpc;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

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 {
    private void readFile(HttpServletResponse response) throws IOException {
        //通过ServletContext获取文件的绝对路径
        String path = this.getServletContext().getRealPath("/WEB-INF/classes/db/config/db.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("name");
        String password = prop.getProperty("password");
        response.getWriter().write("dirver : " + driver + " url : " + url + " username : " + username + " password : " + password);
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        resp.getOutputStream().write("ServletContextDemo2".getBytes());
        //目的是控制浏览器用UTF-8进行编码；这样不会出现中文乱码
        resp.setContentType("text/html;charset=UTF-8");
        resp.setHeader("content-type", "text/html;charset=UTF-8");
        readFile(resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

运行结果：

访问路径为：/WEB-INF/classes/db/config/db.properties

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，提升服务器性能。

ServletContextDemo1

package com.fpc;

import java.io.IOException;

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 ServletContextDemo1 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
//        String data = "<h1><font color='blue'>fpc_20171206</font></h1>";
//        resp.getOutputStream().write(data.getBytes());
//        ServletContext context = this.getServletContext();//获取ServletContext对象
//        RequestDispatcher rd = context.getRequestDispatcher("/servletContext/servletContextDemo2");//获取请求转发对象
//        rd.forward(req, resp);
        String data = "fpc_2017/12/06 14:07";
        /*
         * 设置合理的缓存时间值，以避免浏览器向服务器发送请求，提升服务器的性能
         * 这里是将数据的缓存时间设置为1天
         * */
        resp.setDateHeader("expires", System.currentTimeMillis() + 24 *3600*1000);
        resp.getOutputStream().write(data.getBytes());
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.doPost(req, resp);
    }
    
}

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原文地址：https://www.cnblogs.com/fangpengchengbupter/p/7992488.html