Java Web之Filter

Filter被称为过滤器或者拦截器，基本功能就是对调用servler过程的拦截，在servlet进行响应和处理前后实现一些特殊功能。其实，Filter过滤器就是一个实现了javax.servlet.Filter接口的类，在javax.servlet.Filter接口中定义了3个方法：

init(FilterConfig filterConfig) : 用来初始化过滤器，可以在init()中完成与构造方法类似的初始化功能，如果初始化代码中要使用FilterConfig对象，那么，这些初始化代码就只能在Filter的init()方法中编写，而不能在构造方法中编写。
doFilter(ServletRequest request, ServletResponse response, FilterChain chain) ： request和response为Web服务器或Filter链中的上一个Filter对象传递过来的请求和响应对象；参数chain代表Filter链的对象，在当前Filter对象中的doFilter()方法内需要调用FilterChain对象的doFilter()方法，才能把请求交付给Filter链的下一个Filter或者目标程序去处理。
public void destroy() : destroy()方法在Web服务器卸载Filter之前被调用，该方法用于释放Filter对象打开的资源，例如关闭数据库和IO流。

1、实现第一个Filter程序

Index.java文件如下：

 1 package com.net;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 
11 public class Index extends HttpServlet {
12     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
13         PrintWriter out = response.getWriter();
14         out.println("Index");
15     }
16 
17     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
18         doGet(request, response);
19     }
20 }

MyFilter.java文件如下：

 1 package com.net;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.Filter;
 7 import javax.servlet.FilterChain;
 8 import javax.servlet.FilterConfig;
 9 import javax.servlet.ServletException;
10 import javax.servlet.ServletRequest;
11 import javax.servlet.ServletResponse;
12 
13 public class MyFilter implements Filter {
14     public void init(FilterConfig fConfig) throws ServletException {
15         System.out.println("MyFilter.init()");
16     }
17 
18     public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
19         PrintWriter out = response.getWriter();
20         out.println("Hello MyFilter");
21         chain.doFilter(request, response);
22     }
23     
24     public void destroy() {
25         System.out.println("MyFilter.destroy()");
26     }
27 }

web.xml配置文件添加如下代码：

 1 <filter>
 2     <filter-name>MyFilter</filter-name>
 3     <filter-class>com.net.MyFilter</filter-class>
 4 </filter>
 5 <filter-mapping>
 6     <filter-name>MyFilter</filter-name>
 7     <url-pattern>/Index</url-pattern>
 8 </filter-mapping>
 9 
10 
11 <servlet>
12     <servlet-name>Index</servlet-name>
13     <servlet-class>com.net.Index</servlet-class>
14 </servlet>
15 <servlet-mapping>
16     <servlet-name>Index</servlet-name>
17     <url-pattern>/Index</url-pattern>
18 </servlet-mapping>

测试结果如下：

2、Filter链

Filter链示例图如下：

测试程序如下，Index.java文件和MyFilter.java文件参考实现第一个Filter程序中代码，MyFilter2.java文件和web.xml配置文件如下：

MyFilter2.java文件如下：

 1 package com.net;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.Filter;
 7 import javax.servlet.FilterChain;
 8 import javax.servlet.FilterConfig;
 9 import javax.servlet.ServletException;
10 import javax.servlet.ServletRequest;
11 import javax.servlet.ServletResponse;
12 
13 public class MyFilter2 implements Filter {
14     public void init(FilterConfig fConfig) throws ServletException {
15     }
16 
17     public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
18         PrintWriter out = response.getWriter();
19         out.println("Hello MyFilter2 Start");
20         chain.doFilter(request, response);
21         out.println("Hello MyFilter2 End");
22     }
23     
24     public void destroy() {
25     }
26 }

web.xml配置文件添加如下代码：

<filter>
    <filter-name>MyFilter</filter-name>
    <filter-class>com.net.MyFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>MyFilter</filter-name>
    <url-pattern>/Index</url-pattern>
</filter-mapping>

<filter>
    <filter-name>MyFilter2</filter-name>
    <filter-class>com.net.MyFilter2</filter-class>
</filter>
<filter-mapping>
    <filter-name>MyFilter2</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

<servlet>
    <servlet-name>Index</servlet-name>
    <servlet-class>com.net.Index</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Index</servlet-name>
    <url-pattern>/Index</url-pattern>
</servlet-mapping>

测试结果如下：

注意：Filter之间的调用顺序什么样的呢？这与他们在web.xml中声明的前后顺序有关，比如按照上面web.xml配置，Filter的调用顺序为：

请求->MyFilter -> MyFilter2 -> Index(servlet)

响应 <- MyFilter <- MyFilter2 <- Index(servlet)

3、Filter的生命周期

Filter的创建和销毁由web服务器负责。 web 应用程序启动时，web 服务器将创建Filter 的实例对象，并调用其init方法，完成对象的初始化功能，从而为后续的用户请求作好拦截的准备工作，filter对象只会创建一次，init方法也只会执行一次。通过init方法的参数，可获得代表当前filter配置信息的FilterConfig对象。FilterConfig接口封装了Filter程序在web.xml中所有注册信息，并提供了一些获取这些配置信息的方法，如下所示：

MyFilter.java文件如下

package com.net;

import java.io.IOException;
import java.io.PrintWriter;

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 MyFilter implements Filter {
    public void init(FilterConfig fConfig) throws ServletException {
        System.out.println("MyFilter.init()");
        
        //得到过滤器的名字
        String filterName = fConfig.getFilterName();
        //得到在web.xml文件中配置的初始化参数
        String initParam = fConfig.getInitParameter("like");
        
        System.out.println(filterName);
        System.out.println(initParam);
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        PrintWriter out = response.getWriter();
        out.println("Hello MyFilter");
        chain.doFilter(request, response);
    }
    
    public void destroy() {
        System.out.println("MyFilter.destroy()");
    }
}

MyFilter.java对应的web.xml配置项如下：

<filter>
    <filter-name>MyFilter</filter-name>
    <filter-class>com.net.MyFilter</filter-class>
    <init-param>
        <param-name>like</param-name>
        <param-value>java</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>MyFilter</filter-name>
    <url-pattern>/Index</url-pattern>
</filter-mapping>