一.过滤器作用
- 用户认证与授权管理，统计web应用访问量(找到就访问，没的话转到其他页面)和访问命中率和形成访问报告。
- 实现web应用的日志处理功能
- 实现数据压缩和加密功能
- 实现xml和xstl的转换。
二.预备知识
-
- init() ：这个方法在容器实例化过滤器时被调用，它主要设计用于使过滤器为处理做准备。该方法接受一个 FilterConfig 类型的对象作为输入。
- doFilter() ：与 servlet 拥有一个 service() 方法（这个方法又调用 doPost() 或者 doGet() ）来处理请求一样，过滤器拥有单个用于处理请求和响应的方法?D?D doFilter() 。这个方法接受三个输入参数：一个 ServletRequest 、 response 和一个 FilterChain (可能多个filter，按顺序执行)对象。 这里的ServletRequest和ServletResponse一般需要转换成具体的Servlet实现对于的对象，如：HttpServletRequest和HttpServletResponse。
doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
- destroy() ：正如您想像的那样，这个方法执行任何清理操作，这些操作可能需要在自动垃圾收集之前进行。当Servlet容器在销毁过滤器实例前，该方法销毁过滤器占用的资源。
三.代码
1.访问时间限制
//访问时的过滤器（在过滤器中使用servlet初始化参数）
//下面利用init设定一个正常访问时间范围，对那些不在此时间段的访问作出记录
import java.io.IOException;
import java.text.DateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
public class SimpleFilter2 implements Filter {
@SuppressWarnings("unused")
private FilterConfig config;
private ServletContext context;
private int startTime, endTime;
private DateFormat formatter;
public void init(FilterConfig config) throws ServletException {
this.config = config;
context = config.getServletContext();
formatter = DateFormat.getDateTimeInstance(DateFormat.MEDIUM,
DateFormat.MEDIUM);
try {
startTime = Integer.parseInt(config.getInitParameter("startTime"));// web.xml
endTime = Integer.parseInt(config.getInitParameter("endTime"));// web.xml
} catch (NumberFormatException nfe) { // Malformed or null
// Default: access at or after 10 p.m. but before 6 a.m. is
// considered unusual.
startTime = 22; // 10:00 p.m.
endTime = 6; // 6:00 a.m.
}
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) {
try {
System.out.println("Within SimpleFilter2:Filtering the Request...");
HttpServletRequest req = (HttpServletRequest) request;
GregorianCalendar calendar = new GregorianCalendar();
int currentTime = calendar.get(Calendar.HOUR_OF_DAY);
if (isUnusualTime(currentTime, startTime, endTime)) {
context.log("WARNING: " + req.getRemoteHost() + " accessed "
+ req.getRequestURL() + " on "
+ formatter.format(calendar.getTime()));
// The log file is under <CATALINA_HOME>/logs.One log per day.
}
chain.doFilter(request, response);
System.out
.println("Within SimpleFilter2:Filtering the Response...");
} catch (IOException ioe) {
ioe.printStackTrace();
} catch (ServletException se) {
se.printStackTrace();
}
}
public void destroy() {}
// Is the current time between the start and end
// times that are marked as abnormal access times?
private boolean isUnusualTime(int currentTime, int startTime, int endTime) {
// If the start time is less than the end time (i.e.,
// they are two times on the same day), then the
// current time is considered unusual if it is
// between the start and end times.
if (startTime < endTime) {
return ((currentTime >= startTime) && (currentTime < endTime));
}
// If the start time is greater than or equal to the
// end time (i.e., the start time is on one day and
// the end time is on the next day), then the current
// time is considered unusual if it is NOT between
// the end and start times.
else {
return (!isUnusualTime(currentTime, endTime, startTime));
}
}
}
参考资料：http://zhangjunhd.blog.51cto.com/113473/20629
2.登陆限制
public class LoginFilter implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("init LoginFilter");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
//把ServletRequest和ServletResponse转换成真正的类型
HttpServletRequest req = (HttpServletRequest)request;
HttpSession session = req.getSession();
//由于web.xml中设置Filter过滤全部请求，可以排除不需要过滤的url
String requestURI = req.getRequestURI();
if(requestURI.endsWith("login.jsp")){
chain.doFilter(request, response);
return;
}
//判断用户是否登录，进行页面的处理
if(null == session.getAttribute("user")){
//未登录用户，重定向到登录页面
((HttpServletResponse)response).sendRedirect("login.jsp");
return;
} else {
//已登录用户，允许访问
chain.doFilter(request, response);
}
}
@Override
public void destroy() {
System.out.println("destroy!!!");
}
}
3.过滤敏感词
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
//转换成实例的请求和响应对象
HttpServletRequest req = (HttpServletRequest)request;
HttpServletResponse resp = (HttpServletResponse)response;
//获取评论并屏蔽关键字
String comment = req.getParameter("comment");
comment = comment.replace("A", "***");
//重新设置参数
req.setAttribute("comment", comment);
//继续执行
chain.doFilter(request, response);
}
