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本人博客：http://wxmimperio.coding.io/
本人邮箱：wxmimperio@163.com
本文源码——GitHub：
https://github.com/imperio-wxm/wordpressCode/tree/master/Filter
【没有过滤器】：
用户直接访问WEB资源。
【有过滤器】：
WEB容器启动时进行过滤器的加载，用户发出请求到过滤器，过滤器判断请求是否符合规则，符合规则的请求通过过滤器发送给WEB资源，WEB资源响应的信息返回给过滤器，过滤器再将WEB资源的响应返回给用户。
1.实例化——通过web.xml进行配置加载，在容器启动时只会实例化一次
2.初始化——调用init()方法，加载信息只会执行一次
3.过滤——使用doFilter()方法进行多次过滤
4.销毁——WEB容器关闭时调用destroy()方法进行销毁
所有的Servlet过滤器都必须实现javax.servlet.Filter接口，并实现该接口中的三个方法。
【生命周期代码演示】
package com.filter;
import javax.servlet.*;
import java.io.IOException;
/**
* Created by wxm-Imperio
*/
public class FirstFilter implements Filter {
@Override
public void destroy() {
System.out.println("destroy，firstFilter");
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
throws IOException, ServletException {
System.out.println("start，doFilter，firstFilter");
filterChain.doFilter(servletRequest, servletResponse);
System.out.println("End，doFilter，firstFilter");
}
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("init。firstFilter");
}
}
【web.xml配置】
<filter>
<filter-name>Filter的名字</filter-name>
<filter-class>Filter类的名字</filter-class>
<init-param>
<description>描述信息可以省略或者放在此处</description>
<param-name>参数名称</param-name>
<param-value>参数的值</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>Filter的名字</filter-name>
<url-pattern>URL</url-pattern>
<dispatcher></dispatcher>//过滤器类型
</filter-mapping>
注意：一般Filter配置在所有的Servlet之前。
【过滤器链的web.xml配置】
<!--过滤器1的配置信息--!>
<filter>
<filter-name>Filter1的名字</filter-name>
<filter-class>Filter1类的名字</filter-class>
<init-param>
<description>描述信息可以省略或者放在此处</description>
<param-name>参数名称</param-name>
<param-value>参数的值</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>Filter1的名字</filter-name>
<url-pattern>URL</url-pattern>
<dispatcher></dispatcher>//过滤器类型
</filter-mapping>
<!--过滤器2的配置信息--!>
<filter>
<filter-name>Filter2的名字</filter-name>
<filter-class>Filter2类的名字</filter-class>
<init-param>
<description>描述信息可以省略或者放在此处</description>
<param-name>参数名称</param-name>
<param-value>参数的值</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>Filter2的名字</filter-name>
<url-pattern>URL</url-pattern>
<dispatcher></dispatcher>//过滤器类型
</filter-mapping>
注意：过滤器的执行顺序就是在web.xml里配置的顺序。
Servlet2.5：
@WebFilter(servletNames = {"SimpleServlet"} filterName = "SimpleFilter")
public class LessThanSixFilter implements Filter {
//类中内容
}
【ERROR的配置信息】
<error-page> <error-code>404</error-code> <location>/error.jsp</location> </error-page> <filter> <filter-name>errorFilter</filter-name> <filter-class>com.filter.ErrorFilter</filter-class> </filter> <filter-mapping> <filter-name>errorFilter</filter-name> <url-pattern>/error.jsp</url-pattern> </filter-mapping>
【ErrorFilter过滤器代码】
public class ErrorFilter implements Filter {
@Override
public void destroy() {
}
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
throws IOException, ServletException {
System.out.println("检测到有错误信息");
}
}
Servlet3.0：
【异步操作处理代码：过滤器不等待线程，直接执行后面内容，实现异步处理】
AsynServlet
//设置Servlet支持异步
@WebServlet(name = "AsynServlet", asyncSupported = true)
public class AsynServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("Servlet执行开始时间" + new Date());
//实现异步操作
AsyncContext context = request.startAsync();
//开启异步线程
new Thread(new Executor(context)).start();
System.out.println("Servlet执行结束时间" + new Date());
}
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}
//内部类模拟线程
public class Executor implements Runnable {
private AsyncContext context;
//实现构造方法
public Executor(AsyncContext context) {
this.context = context;
}
@Override
public void run() {
//执行相关的复杂业务
try {
Thread.sleep(1000 * 10);
//context.getRequest();
//context.getResponse();
System.out.println("业务执行完成时间" + new Date());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
AsvnFilter过滤器
//注解 @WebFilter(filterName =
"AsynFilter", value = {"/AsynServlet"}, asyncSupported = true, dispatcherTypes = {DispatcherType.REQUEST,DispatcherType.ASYNC}) public class AsynFilter implements Filter { @Override public void destroy() { } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
throws IOException, ServletException { System.out.println("Start..........AsynFilter"); filterChain.doFilter(servletRequest, servletResponse); System.out.println("End..........AsynFilter"); } }
web.xml配置
<servlet> <servlet-name>AsynServlet</servlet-name> <servlet-class>com.servlet.AsynServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>AsynServlet</servlet-name> <url-pattern>/AsynServlet</url-pattern> </servlet-mapping>
Filter的init方法中提供了一个FilterConfig对象，提供相关的操作：
如获取Filter中配置的初始化参数web.xml配置：
<filter>
<filter-name>LoginFilter</filter-name>
<filter-class>com.itzhai.login.LoginFilter</filter-class>
<init-param>
<param-name>username</param-name>
<param-value>arthinking</param-value>
</init-param>
</filter>
在init()方法中获取：
@Override
public void init(FilterConfig filterConfig) throws ServletException {
//获取Filter初始化参数
String username = filterConfig.getInitParameter("username");
}
在Filter中访问application：
ServletContext context = filterConfig.getServletContext();
也可以在doFilter方法中根据转换好的request获取：
HttpServletRequest req = (HttpServletRequest)request; ServletContext context = req.getSession().getServletContext();
慕课网中用户登录的加强版Demo，利用过滤器对用于请求和服务器回应进行过滤。
GitHub源码：https://github.com/imperio-wxm/projectDemo
参考：JSP应用开发详解（第三版）、慕课网
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原文地址：http://www.cnblogs.com/wnlja/p/4346659.html
