Servlet过滤器的核心：
1.在web.xml中配置Filter所对应的java类
2.实现javax.servlet.Filter接口的类
3.实现Filter接口中的方法,即init方法，并进行初始化
4.实现doFilter方法，完成对过滤器的请求和响应
5调用FilterChain接口对象的doFilter方法，像后续过滤器进行传递请求或者跳转到响应的页面
filter过滤器主要使用于前台向后台传递数据是的过滤操作。直接给几个已经写好的代码：
一、使浏览器不缓存页面的过滤器
Java代码
import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class MyFilter implements Filter {
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException
{
((HttpServletResponse) response).setHeader("Pragma","no-cache");
((HttpServletResponse) response).setDateHeader ("Expires", -1);
filterChain.doFilter(request, response);
}
public void destroy()
{
}
public void init(FilterConfig filterConfig) throws ServletException
{
}
}
二、检测用户是否登陆的过滤器
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.util.List;
import java.util.ArrayList;
import java.util.StringTokenizer;
import java.io.IOException;
public class CheckLoginFilter
implements Filter
{
protected FilterConfig filterConfig = null;
private String redirectURL = null;
private List notCheckURLList = new ArrayList();
private String sessionKey = null;
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException
{
HttpServletRequest request = (HttpServletRequest) servletRequest;
HttpServletResponse response = (HttpServletResponse) servletResponse;
HttpSession session = request.getSession();
if(sessionKey == null)
{
filterChain.doFilter(request, response);
return;
}
if((!checkRequestURIIntNotFilterList(request)) && session.getAttribute(sessionKey) == null)
{
response.sendRedirect(request.getContextPath() + redirectURL);
return;
}
filterChain.doFilter(servletRequest, servletResponse);
}
public void destroy()
{
notCheckURLList.clear();
}
private boolean checkRequestURIIntNotFilterList(HttpServletRequest request)
{
String uri = request.getServletPath() + (request.getPathInfo() == null ? "" : request.getPathInfo());
return notCheckURLList.contains(uri);
}
public void init(FilterConfig filterConfig) throws ServletException
{
this.filterConfig = filterConfig;
redirectURL = filterConfig.getInitParameter("redirectURL");
sessionKey = filterConfig.getInitParameter("checkSessionKey");
String notCheckURLListStr = filterConfig.getInitParameter("notCheckURLList");
if(notCheckURLListStr != null)
{
StringTokenizer st = new StringTokenizer(notCheckURLListStr, ";");
notCheckURLList.clear();
while(st.hasMoreTokens())
{
notCheckURLList.add(st.nextToken());
}
}
}
}
三、字符编码的过滤器
import javax.servlet.*;
import java.io.IOException;
public class SetCharacterEncodingFilter implements Filter {
private FilterConfig filterConfig;
public void init(FilterConfig filterConfig) throws ServletException {
this.filterConfig = filterConfig;
}
//Process the request/response pair
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) {
try {
request.setCharacterEncoding("utf-8");
response.setCharacterEncoding("utf-8");
filterChain.doFilter(request, response);
HttpServletRequest ser =(HttpServletRequest) request;
HttpServletResponse seo = (HttpServletResponse) response;
//获得跳转的路径,即可知道跳转的路径
System.out.println(ser.getRequestURL());
} catch (ServletException sx) {
filterConfig.getServletContext().log(sx.getMessage());
} catch (IOException iox) {
filterConfig.getServletContext().log(iox.getMessage());
}
}
//Clean up resources
public void destroy() {
}
}
四、记录用户的访问操作器
package com.mymd.filter;
import java.io.IOException;
import javax.servlet.*;
import java.text.SimpleDateFormat;
import javax.servlet.http.HttpServletRequest;
import com.qwserv.itm.api.pfl.sm.vo.Person;
import java.sql.*;
import com.qwserv.itm.api.ServiceAccess;
import com.qwserv.itm.util.toolkit.DebugUtil;
public class VisitFilter implements Filter {
protected static DebugUtil log = DebugUtil.getInstances("pfl-log", VisitFilter.class);
public void destroy() {
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,
ServletException {
//记录用户的访问操作
HttpServletRequest request1 = (HttpServletRequest)request;
StringBuffer url = request1.getRequestURL();
//对url进行过滤，如果是js/css/image则不进行处理
if (judgeFile(url.toString())){
String operTime = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"). format(new java.util.Date());
String hostIp = request.getRemoteAddr();
String sessionId = request1.getRequestedSessionId();
String userId = "";
Person person = (Person)request1.getSession().getAttribute("userObj");
if (null != person && null != person.getUser()){
userId = person.getUser().getId();
}
String queryString = request1.getQueryString();
if (null != queryString) {
url.append('?');
url.append(queryString);
}
//保存到数据库中
saveToDb(userId,hostIp,sessionId,url.toString(),operTime,"");
}
// Pass control on to the next filter
chain.doFilter(request, response);
}
public void init(FilterConfig filterConfig) throws ServletException {
}
public boolean judgeFile(String url){
if (url.endsWith(".gif") || url.endsWith(".jpg") || url.endsWith(".png")
|| url.endsWith(".bmp") || url.endsWith(".css") || url.endsWith(".js")
|| url.endsWith(".jsx")){
return false;
} else {
return true;
}
}
public int saveToDb(String userId, String hostIp,String sessionId,String url,
String operTime,String desc){
//将报表任务数据保存到数据库中
Connection conn = null;
Statement st = null;
try {
//构造sql表达式，将数据插入数据库
conn = ServiceAccess.getSystemSupportService().getDefaultConnection();
st = conn.createStatement();
String sql = "insert into LOG_Visit_HISTORY(USERID,URL,Detail,SessionID,HostName,StartDate) values('"+
userId + "','" + url + "','" + desc + "','" + sessionId
+ "','" + hostIp + "','" + operTime + "')";
if (ServiceAccess.getSystemSupportService().getConnectionType (conn==ServiceAccess.getSystemSupportService().JCA_TYPE_ORACLE){
sql = "insert into LOG_Visit_HISTORY(Id,USERID,URL,Detail,SessionID,HostName,StartDate) values(LOG_Visit_SEQ.nextval,'"+
userId + "','" + url + "','" + desc + "','" + sessionId
+ "','" + hostIp + "',TO_DATE('" + operTime
+ "','YYYY-MM-DD HH24:MI:SS'))";
}
st.executeUpdate(sql);
} catch (Exception e) {
e.printStackTrace();
log.error("--------------------The url String is:" + url + "-------------------------------");
return 1; //表示操作失败
} finally {
if (null != st)
{
try{
st.close();
}
catch(Exception e)
{
e.printStackTrace();
}
st = null;
}
if (conn != null) {
try {
conn.close();
} catch (Exception e) {
e.printStackTrace();
}
conn = null;
}
}
return 0; //表示操作成功
}
}
<filter>
<filter-name>VisitFilter</filter-name>
<filter-class>com.mymd.filter.VisitFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>VisitFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
五.Filter防止用户访问一些未被授权的资源
package com.mymd.filter;
import java.io.IOException;
import javax.servlet.*;
public class AuthFilter implements Filter {
public void destroy() {
}
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse,
FilterChain filterChain) throws IOException, ServletException {
/**1,doFilter方法的第一个参数为ServletRequest对象。此对象给过滤器提供了对进入的信息（包括表单数据、cookie和HTTP请求头）的完全访问。第二个参数为ServletResponse，通常在简单的过滤器中忽略此参数。最后一个参数为FilterChain，此参数用来调用servlet或JSP页。 */
HttpServletRequest request = (HttpServletRequest)servletRequest;
//如果处理HTTP请求，并且需要访问诸如getHeader或getCookies等在ServletRequest中无法得到的方 法，就要把此request对象构造成HttpServletRequest
HttpServletResponse response = (HttpServletResponse)servletResponse。
String currentURL = request.getRequestURI();
//取得根目录所对应的绝对路径: 就是我们浏览器中所显示的路径
String targetURL = currentURL.substring(currentURL.indexOf("/", 1), currentURL.length());
//截取到当前文件名用于比较
HttpSession session = request.getSession(false);
if (!"/login.jsp".equals(targetURL)) {
//判断当前页是否是重定向以后的登录页面页面，如果是就不做session的判断，防止出现死循环
if (session == null || session.getAttribute("user") == null) {
//*用户登录以后需手动添加session
System.out.println("request.getContextPath()=" + request.getContextPath());
response.sendRedirect(request.getContextPath() + "/login.jsp");//如果session为空表示用户没有登录就重定向到login.jsp页面
return;
}
}
//加入filter链继续向下执行
filterChain.doFilter(request, response);
//.调用FilterChain对象的doFilter方法。Filter接口的doFilter方法取一个FilterChain对象作为它的一个参数。在调用此对象的doFilter方法时，激活下一个相关的过滤器。如果没有另一个过滤器与servlet或JSP页面关联，则servlet或JSP页面被激活。
}
public void init(FilterConfig filterConfig) throws ServletException {
}
}
<filter>
<filter-name>AuthFilter</filter-name>
<filter-class>com.mymd.filter.AuthFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>AuthFilter</filter-name>
<url-pattern>*.jsp</url-pattern>
//表示对所有jsp文件有效
</filter-mapping>