本文主要是介绍Filter过滤器周期、Filter拦截过滤、Filter执行链，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
Servlet过滤器的概念：
Servlet过滤器本身并不生成请求和响应对象，它只提供过滤作用。
Servlet过滤器能够在Servlet被调用之前检查Request对象，修改Request Header和Request内容。
在Servlet被调用之后检查Response对象，修改Response Header和Response内容。
Servlet过滤器负责过滤的Web组件可以是Servlet、JSP或HTML文件。
来看一个简单小例子
Filter也是需要在web.xml进行配置：
<?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"xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"id="WebApp_ID" version="2.5"><filter><filter-name>helloFilter</filter-name><filter-class>com.safly.HelloFilter</filter-class><init-param><param-name>name</param-name><param-value>root</param-value></init-param></filter><filter-mapping><filter-name>helloFilter</filter-name><url-pattern>/test.jsp</url-pattern></filter-mapping>
</web-app>HelloFilter
package com.safly;import java.io.IOException;import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;public class HelloFilter implements Filter{public void destroy() {System.out.println("destroy...");}public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {System.out.println("doFilter");chain.doFilter(request, response);}//加载web应用时，即被创建public void init(FilterConfig filterConfig) throws ServletException {System.out.println("init...");}}
test.jsp页面只有一行字输出
浏览器输出：
http://localhost:8080/day01/test.jsp
Web服务器启动，根据web.xml配置，创建注册的Filter实例对象，创建完毕后，调用init方法，该法只执行一次，下次请求时，会调用doFilter，关闭服务器调用destroy
来看另一个简单小例子
我们来看下login.jsp页面吧–里面有用户名、密码
<body>
<font color="red">${requestScope.message }</font><form action="hello.jsp" method="post">username:<input type="text" name="username" value="${param.username }"/>password:<input type="password" name="password"/><input type="submit" value="Submit"/></form>
</body>提交后跳转到hello.jsp页面，如果验证成功，就提示hello界面
<body>Hello:${param.username }
</body>条件是在跳转到hello.jsp页面之前，需要进行拦截验证，UsernameFilter和PasswordFilter进行拦截hello.jsp页面
PasswordFilter
<context-param><param-name>password</param-name><param-value>1234</param-value></context-param><filter><filter-name>UsernameFilter</filter-name><filter-class>com.safly.UsernameFilter</filter-class><init-param><param-name>username</param-name><param-value>Tom</param-value></init-param></filter><filter-mapping><filter-name>UsernameFilter</filter-name><url-pattern>/hello.jsp</url-pattern></filter-mapping><!-- --><filter><filter-name>PasswordFilter</filter-name><filter-class>com.safly.PasswordFilter</filter-class></filter><filter-mapping><filter-name>PasswordFilter</filter-name><url-pattern>/hello.jsp</url-pattern></filter-mapping>UsernameFilter在web.xml中username：Tom，如果在login.jsp页面输入的用户名为Tom，那么就会去继续验证密码是否正确，如果输入的用户名字不是Tom,就转发到login.jsp页面
package com.safly;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;public class UsernameFilter implements Filter {public void destroy() {}public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {String initUser = filterConfig.getInitParameter("username");String username = request.getParameter("username");if (!initUser.equals(username)) {request.setAttribute("message", "用户名不正确");request.getRequestDispatcher("/login.jsp").forward(request, response);return;}chain.doFilter(request, response);}private FilterConfig filterConfig;public void init(FilterConfig filterConfig) throws ServletException {this.filterConfig = filterConfig;}
}
PasswordFilter如果web.xml中的password：1234，如果在login.jsp输入的密码不是1234，就转发到login.jsp,如果密码也输入正确，就进入到hello.jsp页面进行欢迎提示
package com.safly;
import java.io.IOException;import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;public class PasswordFilter implements Filter {public void destroy() {}public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {String initPassword = filterConfig.getServletContext().getInitParameter("password");String password = request.getParameter("password");if (!initPassword.equals(password)) {request.setAttribute("message", "密码不正确");request.getRequestDispatcher("/login.jsp").forward(request, response);return;}chain.doFilter(request, response);}private FilterConfig filterConfig;public void init(FilterConfig filterConfig) throws ServletException {this.filterConfig = filterConfig;}
}以下是4种情况：
Filter执行链流程
首先定义一个hello.jsp界面，里面只有一个链接跳转到second.jsp界面
<body>
<a href="second.jsp">To Test Page</a>
</body>但是在跳转到second.jsp界面做了2个拦截
<filter><filter-name>HelloFilter</filter-name><filter-class>com.safly.HelloFilter</filter-class></filter><filter-mapping><filter-name>HelloFilter</filter-name><url-pattern>/second.jsp</url-pattern></filter-mapping><filter><filter-name>SecondFilter</filter-name><filter-class>com.safly.SecondFilter</filter-class></filter><filter-mapping><filter-name>SecondFilter</filter-name><url-pattern>/second.jsp</url-pattern></filter-mapping>HelloFilter
package com.safly;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class HelloFilter implements Filter {public void destroy() {}public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {System.out.println("before helloFilter'chain");chain.doFilter(request, response);System.out.println("after hellofilter'chain");}public void init(FilterConfig filterConfig) throws ServletException {}
}SecondFilter
package com.safly;
import java.io.IOException;import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class SecondFilter implements Filter {public void destroy() { }public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {System.out.println("before secondFilter'chain");chain.doFilter(request, response);System.out.println("after secondFilter'chain");}public void init(FilterConfig filterConfig) throws ServletException {}}浏览器输入
http://localhost:8080/day01/second.jsp
log日志输出如下：
before helloFilter'chain
before secondFilter'chain
second.jsp
after secondFilter'chain
after hellofilter'chain通过截图来看下Filter链执行的流程吧
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