一、過濾器配置web.xml

 <filter>
        <filter-name>GuestbookFilter</filter-name>
        <filter-class>test.GuestbookFilter</filter-class>
        <init-param>
            <!-- 配置不雅字句內容的替換檔案，內容如：fuck=****，每行一句 -->
            <param-name>word_file</param-name>
            <param-value>/WEB-INF/word.txt</param-value>
        </init-param>
    </filter>
    
    <filter-mapping>
        <!-- 儲存過濾 -->
        <filter-name>GuestbookFilter</filter-name>
        <url-pattern>/process.jsp</url-pattern>
    </filter-mapping>
    
     <filter-mapping>
         <!-- 顯示過濾 -->
        <filter-name>GuestbookFilter</filter-name>
        <url-pattern>/index.jsp</url-pattern>
        <dispatcher>REQUEST</dispatcher>
        <dispatcher>FORWARD</dispatcher>
    </filter-mapping>

二、過濾器編寫GuestbookFilter.java，利用MyRequestWrapper和MyResponseWrapper來替換請求中的特殊字元和響應中的不雅字句

package test;

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class GuestbookFilter implements Filter
{
    private static final String WORD_FILE="word_file";
    HashMap<String,String> hm=new HashMap<String,String>();
    
    /**
     * 在init()方法中，讀取儲存了不雅字句和替換內容的檔案，
     * 並以不雅字句作為key，替換內容作為value，儲存到Hashmap物件中。
     */
    public void init(FilterConfig filterConfig) throws ServletException{   
        String configPath=filterConfig.getInitParameter(WORD_FILE);
        ServletContext sc=filterConfig.getServletContext();
        String filePath=sc.getRealPath(configPath);
        try{
            FileReader fr=new FileReader(filePath);
            BufferedReader br=new BufferedReader(fr);
            String line;
            while(null!=(line=br.readLine())){
                String[] strTemp=line.split("=");
                hm.put(strTemp[0],strTemp[1]);
            }
        }catch(IOException ie){
            throw new ServletException("讀取過濾檔案資訊出錯！");
        }
    }
    public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain)
                  throws IOException, ServletException{
        HttpServletRequest httpReq=(HttpServletRequest)request;
        HttpServletResponse httpResp=(HttpServletResponse)response;
        //得到請求和響應物件的封裝類物件。
        MyRequestWrapper reqWrapper=new MyRequestWrapper(httpReq);
        MyResponseWrapper respWrapper=new MyResponseWrapper(httpResp);
        chain.doFilter(reqWrapper,respWrapper);
        String content = new String(respWrapper.toByteArray());
        String result=replaceText(content);
        httpResp.setContentType("text/html;charset=GB2312");
        PrintWriter out = httpResp.getWriter();
        out.println(result);
        out.close();
    }
    
    /**
     * 對內容中的不雅字句進行過濾。
     */
    public String replaceText(String content) throws IOException{
        StringBuffer sb=new StringBuffer(content);
        Set keys=hm.keySet();
        Iterator it=keys.iterator();
        while(it.hasNext()){
            String key=(String)it.next();
            int index=sb.indexOf(key);
            if(-1!=index){
                sb.replace(index,index+key.length(),(String)hm.get(key));
            }
        }
        return sb.toString();
    }
    public void destroy(){}
}

三、請求引數過濾MyRequestWrapper.java，替換請求中的特殊字元

package test;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

public final class MyRequestWrapper extends HttpServletRequestWrapper{
    public MyRequestWrapper(HttpServletRequest request){
        super(request);
    }
    /**
     * 覆蓋基類的getParameter()方法，對請求引數的值進行過濾。
     */
    public java.lang.String getParameter(java.lang.String name){
        String value=super.getParameter(name);
        if(null!=value)
            return toHtml(value.trim());
        else
            return null;
    }
    /**
     * 將特殊字元轉換為對應的實體引用或字元引用。
     */
    private String toHtml(String str){
        if(str==null)
            return null;
        StringBuffer sb = new StringBuffer();
        int len = str.length();
        for (int i = 0; i < len; i++){
            char c = str.charAt(i);
            switch(c){                  
                case ' ':sb.append("&nbsp;");break;
                case '\n':sb.append("<br>");break;
                case '\r':break;
                case '\'':sb.append("&#39;");break;
                case '<': sb.append("&lt;");break;
                case '>':sb.append("&gt;");break;
                case '&':sb.append("&amp;");break;
                case '"':sb.append("&#34;");break;
                case '\\':sb.append("&#92;");break;
                default:sb.append(c);
            }
        }
        return sb.toString();
    }
}
四、對響應資料進行過濾MyResponseWrapper.java，替換響應中的不雅字句，用ByteArrayServletOutputStream類的物件替換Web容器中的物件

package test;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class MyResponseWrapper extends HttpServletResponseWrapper{
    private ByteArrayOutputStream baos;
    private ByteArrayServletOutputStream basos;
    private PrintWriter pw;
    public MyResponseWrapper(HttpServletResponse response){
        super(response);
        //建立ByteArrayOutputStream物件。
        baos=new ByteArrayOutputStream();
        //用ByteArrayOutputStream物件作為引數，
        //構造ByteArrayServletOutputStream物件。
        basos=new ByteArrayServletOutputStream(baos);
        //用ByteArrayOutputStream物件作為引數，
        //構造PrintWriter物件。
        pw=new PrintWriter(baos);
    }
    public PrintWriter getWriter(){
        return pw;
    }
    public ServletOutputStream getOutputStream(){
        return basos;
    }
    /**
     * 以位元組陣列的形式返回輸出流緩衝區中的內容。
     */
    public byte[] toByteArray(){
        return baos.toByteArray();
    }
}

五、替換Web容器中的ServletOutputStream物件ByteArrayServletOutputStream.java

package test;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import javax.servlet.ServletOutputStream;

//用該類的物件替換HttpServletResponse.getOutputStream()方法返回的ServletOutputStream物件
public class ByteArrayServletOutputStream extends ServletOutputStream{
    ByteArrayOutputStream baos;
    ByteArrayServletOutputStream(ByteArrayOutputStream baos){
        this.baos = baos;
    }
    public void write(int data) throws IOException{
        baos.write(data);
    }
}