Filter过滤器周期、Filter拦截过滤、Filter执行链
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：
<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>helloFilterfilter-name>
<filter-class>com.safly.HelloFilterfilter-class>
<init-param>
<param-name>nameparam-name>
<param-value>rootparam-value>
init-param>
filter>
<filter-mapping>
<filter-name>helloFilterfilter-name>
<url-pattern>/test.jspurl-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
Hello:${param.username }
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>passwordparam-name>
<param-value>1234param-value>
context-param>
<filter>
<filter-name>UsernameFilterfilter-name>
<filter-class>com.safly.UsernameFilterfilter-class>
<init-param>
<param-name>usernameparam-name>
<param-value>Tomparam-value>
init-param>
filter>
<filter-mapping>
<filter-name>UsernameFilterfilter-name>
<url-pattern>/hello.jspurl-pattern>
filter-mapping>
<filter>
<filter-name>PasswordFilterfilter-name>
<filter-class>com.safly.PasswordFilterfilter-class>
filter>
<filter-mapping>
<filter-name>PasswordFilterfilter-name>
<url-pattern>/hello.jspurl-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 Pagea>
body>但是在跳转到second.jsp界面做了2个拦截
<filter>
<filter-name>HelloFilterfilter-name>
<filter-class>com.safly.HelloFilterfilter-class>
filter>
<filter-mapping>
<filter-name>HelloFilterfilter-name>
/second.jsp
filter-mapping>
<filter>
<filter-name>SecondFilterfilter-name>
<filter-class>com.safly.SecondFilterfilter-class>
filter>
<filter-mapping>
<filter-name>SecondFilterfilter-name>
/second.jsp
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
