原出处:https://blog.csdn.net/u010902721/article/details/51673760
Filter(过滤器)的功能
Filter顾名思义就是针对浏览器的请求进行过滤,甚至是拦截。它可以在一个请求到达servlet之前进行请求的预处理,在一个请求离开servlet后,到达浏览器之前进行后处理。多个Filter可以组成Filter Chain(过滤器链)。
Filter可以完全控制一个请求,在一个Filter调用chain.doFilter()之前,此次请求是不会离开本Filter的。
Filter(过滤器)的工作流程是:
1、在Servlet Request到达时拦截,并做处理;
2、调用chain.doFilter()将Servlet Request传递下去;
3、在Servlet Response离开之前拦截,并做后处理。
Filter(过滤器)必须要实现的方法
- public void init(FilterConfig config)。在Filter的生命周期中只执行一次,在服务器(如tomcat)加载war包时就执行。参数FilterConfig是根据web.xml文件中的配置生成的配置信息,可以用来初始化本Filter的私有属性。
- public void destroy()。也是只执行一次,做资源释放。
- public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)。这个函数中调用chain.doFilter(req, resp)之前的代码属于对request的预处理,调用chain.doFilter(req, resp)之后的代码属于对response的后处理。不调用此代码就表示拦截,不向后传递。
多个Filter(过滤器)的执行顺序
当一个工程中有多个Filter(过滤器)时,Filter的执行顺序一定得弄清楚。是根据在web.xml中配置的先后顺序,类似于进出栈的顺序来执行的。
例如,下面demo中在web.xml中配置了两个filter和一个servlet,两个Filter的配置顺序是filter1在前,filter2在后。执行顺序如下图:
浏览器浏览器Filter1Filter1Filter2Filter2ServletServletHttpServletRequestHttpServletRequestHttpServletRequestHttpServletResponseHttpServletResponseHttpServletResponse
demo
web.xml里面配置了两个filter和一个servlet:
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<filter>
<filter-name>filter1</filter-name>
<filter-class>com.zcx.filterTest.filter1</filter-class>
<init-param>
<!-- 为filter1的私有属性配置值-->
<param-name>paraName</param-name>
<param-value>paraValue</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>filter1</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>filter2</filter-name>
<filter-class>com.zcx.filterTest.filter2</filter-class>
</filter>
<filter-mapping>
<filter-name>filter2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<servlet>
<servlet-name>Servlet1</servlet-name>
<servlet-class>com.zcx.filterTest.Servlet1</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Servlet1</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
</web-app>
新建filter1:
package com.zcx.filterTest;
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 filter1 implements Filter{
private String paraName;
public void init(FilterConfig config) throws ServletException{
paraName = config.getInitParameter("paraName");
System.out.println("init filter1, paraName=" + paraName);
}
public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException{
System.out.println("Before filter1, paraName=" + paraName);
chain.doFilter(req, resp);
System.out.println("After filter1, paraName=" + paraName);
}
public void destroy(){
System.out.println("Destory filter1");
paraName = null;
}
}
新建filter2:
package com.zcx.filterTest;
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 filter2 implements Filter{
public void init(FilterConfig config) throws ServletException{
System.out.println("init filter2");
}
public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException{
System.out.println("Before filter2");
chain.doFilter(req, resp);
System.out.println("After filter2");
}
public void destroy(){
System.out.println("Destory filter2");
}
}
新建一个servlet:
package com.zcx.filterTest;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Servlet1 extends HttpServlet{
/**
*
*/
private static final long serialVersionUID = 4657145885789315799L;
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
System.out.println("doget...");
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
System.out.println("dopost...");
}
}
在tomcat部署此工程时进行初始化工作,控制台输出:
init filter1, paraName=paraValue
init filter2
在浏览器中访问此工程时,控制台输出:
Before filter1, paraName=paraValue
Before filter2
doget…
After filter2
After filter1, paraName=paraValue