java web学习笔记 servlet

关于java web

web.xml中一般配置的都是与servlet先关的可以配置servlet filter listener context-param用来配置web应用的启动参数，可用通过ServletContext对象获取参数，然后作用与整个web应用的生命周期

Filter 接口的使用

• 创建Filter 类
• 在web.xml中配置Filter 或者直接使用注解
• 创建Filter必须实现javax.servlet.Filter接口，该接口中定义了三个方法
• void init(FilterConfig config)
• void destroy();
• void doFilter(ServletRequest req,ServletResponse rep,FilterChain chain)

下面是一个日志Filter,这个Filter负责拦截所有的用户请求，并将请求的信息记录在日志中

package com.ws.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
@WebFilter(filterName="log",urlPatterns="/*")
public class LogFilter implements Filter {
    private FilterConfig config;
    public void init(FilterConfig config){
        this.config=config;
    }

    public void destroy(){
        this.config=null;
    }

    public void doFilter(ServletRequest req,ServletResponse rep,FilterChain chain)
    throws IOException,ServletException
    {
        //下面代码用于对用户请求执行预处理
        ServletContext context=this.config.getServletContext();
        long before=System.currentTimeMillis();
        System.out.println("开始过滤...");
        HttpServletRequest hrequest=(HttpServletRequest)req;
        System.out.println("Filter已经拦截到的用户请求的地址："+hrequest.getServletPath());
        //Filter 只是链式处理，请求依然放行到目的地址
        chain.doFilter(req, rep);
        //下面的代码用于对服务器响应后的处理
        long after=System.currentTimeMillis();
        System.out.println("过滤结束...");
        System.out.println("请求被定为到"+hrequest.getRequestURI()+" 所花的时间为:"+(after-before)+"s");

    }
}

Listener 接口

当web应用在web容器中运行时，会再web容器内发生各种事件，web应用启动，session开始，用户请求到达，响应用户请求，session结束，web应用关闭

• ServletContextListener:用于监听web应用的启动和关闭

• ServletContextAttributeListener:用于监听ServletContext范围（application）内属性的改变

• ServletRequestListener:用于监听用户请求

• ServletRequestAttributeListener:用于监听request内范围属性的变化

• HttpSessionListener: 用于监听用户的session的开始和结束

• HttpSessionAttributeListener:用于监听session范围内属性的变化

下面将创建一个获取数据库链接的Listener ,该Listener会再应用启动时获取数据库链接，并将获取的链接设为application内的属性

• 实现Listerner类

package com.ws.listener;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class GetConnListener implements ServletContextListener {
    //应用启动时触发
    public void contextInitialized(ServletContextEvent sce){
        try{
            ServletContext application=sce.getServletContext();
            String driver=application.getInitParameter("driver");
            String url=application.getInitParameter("url");
            String user=application.getInitParameter("user");
            String pass=application.getInitParameter("pass");
            Class.forName(driver);
            Connection conn=DriverManager.getConnection(url,user,pass);
            application.setAttribute("conn", conn);
        }
        catch(Exception ex){
            System.out.println("Listener 中获取数据库连接出现异常"+ex.getMessage());
        }
    }

    //应用关闭时触发
    public void contextDestroyed(ServletContextEvent sce){
        ServletContext application=sce.getServletContext();
        Connection conn=(Connection)application.getAttribute("conn");
        if(conn!=null){
            try{
                conn.close();
            }
            catch(SQLException ex){
                ex.printStackTrace();
            }
        }
    }

}

• 配置Listener
  
  ** 使用@WebListener
  
  ** 在web.xml配置

<listener>
    <listener-class>com.ws.listener.GetConnListener</listener-class>
</listener>

还需要添加context-param参数设置

<context-param>
    <param-name>driver</param-name>
    <param-value>com.mysql.jdbc.Driver</param-value>
  </context-param>

  <context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/javaee</param-value>
  </context-param>

  <context-param>
    <param-name>user</param-name>
    <param-value>root</param-value>
  </context-param>

  <context-param>
    <param-name>pass</param-name>
    <param-value>123456</param-value>
  </context-param>

• 使用ServletContextAttrbuteListener

  需要实现如下三个方法
  
  void attributeAdded(ServletContextAttributeEvent event);
  
  void attributeRemoved(ServletContextAttributeEvent event);
  
  void attributeReplaced(ServletContextAttributeEvent event);

ServletRequestListener接口与ServletContextListener接口类似 HttpSessionListern接口 initialize换成create