Servlet学习笔记（七）—— 自己定义过滤器的编写改进：自己定义实现FilterChain

笔记六中实现了三种过滤器：字符编码过滤、登录权限过滤、敏感词过滤，可是有个缺陷就是，限定了过滤顺序，而不能实现先进行request过滤。最后response过滤，而且中间几项过滤的顺序不能动态改变。所以这里做个改进。实现一个过滤顺序的FilterChain。

多个Filter的运行顺序在这篇博文中得到非常细致的解说，总结一点。多个过滤器的运行顺序是依据web.xml中不同<filter-mapping>的顺序来先后运行的，比方：

<?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">
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <filter>
        <filter-name>firstFilter</filter-name>
        <filter-class>com.test.filter.FirstFilter</filter-class>
    </filter>
    <filter>
        <filter-name>secondFilter</filter-name>
        <filter-class>com.test.filter.SecondFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>secondFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>firstFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>   

    <servlet>
        <servlet-name>firstServlet</servlet-name>
        <servlet-class>com.alimama.servlet.FirstServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>firstServlet</servlet-name>
        <url-pattern>/firstServlet</url-pattern>
    </servlet-mapping>
</web-app>   

将会先运行secondFilter，再运行firstFilter。

以下是截取的一段实现Filter接口的doFilter代码，用户自己定义的Filter都实现了这个接口：

 public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        System.out.println("before invoke secondFilter's chain.doFilter() ..");
        chain.doFilter(request, response);
        System.out.println("after invoke secondFilter's chain.doFilter() ..");
    }   

能够看到第四行的chain.doFilter(request,response);这句代码是Servlet里的Filter技术的核心，它的主要职能是将请求传递给下一个Filter。

不是一下子所有初始化所有的Filter对象。

根据这个思想，我自定义了一个FilterChain来实现多个过滤器的过滤顺序。

//FilterChain.java
package lewa;

import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class FilterChain {
	//private static final String[] filters={"LoginFilter","WordFilter"};
	private static final Filter[] filters={new LoginFilter(),new WordFilter()};
	private static int count=0;
	public void doFilter(ServletRequest request,ServletResponse response,FilterChain fc) throws InstantiationException, IllegalAccessException, Exception{
		if(count<filters.length){
			//Class<Filter> cls=(Class<Filter>)Class.forName(filters[count]);
			//cls.doFilter(request, response);
			filters[count++].doFilter(request, response, fc);
		}
	}
}

静态成员变量filters里的过滤器实例的顺序能够自行定义，以后想更改过滤顺序仅仅要改这个String数组就能够。

对应的其它Filter也做了更改：

//Filter.java
package lewa;

import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public interface Filter {
	public void init();
	public void doFilter(ServletRequest reuqest,ServletResponse response,FilterChain fc)throws Exception;
	public void destroy();
}

//EncodingFilter.java
package lewa;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class EncodingFilter implements Filter{

	@Override
	public void init() {
		// TODO Auto-generated method stub

	}

	@Override
	public void doFilter(ServletRequest reuqest, ServletResponse response,FilterChain fc)throws InstantiationException, IllegalAccessException, Exception{
		// TODO Auto-generated method stub
		reuqest.setCharacterEncoding("UTF-8");
		fc.doFilter(reuqest, response,fc);
		response.setContentType("text/html;charset=UTF-8");
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
	}

}

//LoginFilter.java
package lewa;

import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginFilter implements Filter{
	public LoginFilter(){}

	@Override
	public void init() {
		// TODO Auto-generated method stub

	}

	@Override
	public void doFilter(ServletRequest reuqest, ServletResponse response,FilterChain fc)
			throws  Exception {
		// TODO Auto-generated method stub
		HttpServletRequest req=(HttpServletRequest)reuqest;
		HttpServletResponse resp=(HttpServletResponse)response;
		HttpSession session=req.getSession();
		if(session.getAttribute("username")==null){
			resp.sendRedirect("login.html");
		}else{
		fc.doFilter(reuqest, response,fc);//这里必须有一个else，不能直接是运行下一个Filter,由于假设未登陆。
那么敏感词过滤就不必再做，并且能够尝试不加else，你会发现。未登陆前留言会跳转到登陆界面。登陆之后再回到留言界面。
这时候count已经不满足count<filters.length,所以此时敏感词过滤将不会运行。
                }
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub

	}

}

//WordFilter.java
package lewa;

import java.io.IOException;
import java.io.InputStream;
import java.util.Enumeration;
import java.util.Properties;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class WordFilter implements Filter{

	private static Pattern pattern=null;
	public static String str=null;

	private void loadKeyWordProperties(){
		StringBuffer patternBuffer = new StringBuffer();
		try{
			InputStream in =WordFilter.class.getClassLoader().getResourceAsStream("words.properties");
			Properties properties = new Properties();
			properties.load(in);
			Enumeration<?> enu=properties.propertyNames();
			while(enu.hasMoreElements()){
				patternBuffer.append((String)enu.nextElement()+"|");
			}
			patternBuffer.deleteCharAt(patternBuffer.length()-1);
			pattern = Pattern.compile(new String(patternBuffer.toString().getBytes("ISO-8859-1"),"UTF-8"));
		}catch(IOException e){
			e.printStackTrace();
		}
	}
	@Override
	public void init() {
		// TODO Auto-generated method stub

	}

	@Override
	public void doFilter(ServletRequest reuqest, ServletResponse response,FilterChain fc)
			throws InstantiationException, IllegalAccessException, Exception {
		// TODO Auto-generated method stub
		loadKeyWordProperties();
		str=reuqest.getParameter("liuyan");
		try{
			Matcher m=pattern.matcher(str);
			str=m.replaceAll("**");
		}catch(Exception e){
			e.printStackTrace();
		}
		fc.doFilter(reuqest, response,fc);
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub

	}
}

最后Servlet里的代码更改：

package lewa;

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

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class Summary
 */
public class Summary extends HttpServlet {
	private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public Summary() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		Filter filter=new EncodingFilter();
		FilterChain  fc=new FilterChain();
		try {
			filter.doFilter(request, response,fc);
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		PrintWriter out=response.getWriter();
		HttpSession session=request.getSession();
		String name=(String) session.getAttribute("username");
		if(name!=null&&!"".equals(name)) {
			name=new String(name.getBytes("ISO-8859-1"),"UTF-8");
		}
		out.println("<html><head><title>留言板内容</title></head><body>");
		out.println("用户名："+name);
		out.println("<br/>"+WordFilter.str+"</body></html>");
	}
}
 

自己定义过滤器链的编写这个是源代码的下载地址。

希望大家多多不吝赐教~~