一个完整的java项目是少不了权限控制功能的，不然谁都能进那还了得?实现权限控制功能就需要靠过滤器了，本篇文章就来看看如果实现过滤器吧。
示例1：检测用户是否登陆的过滤器
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());
}
}
}
}示例2：防止用户访问一些未被授权资源的过滤器
package com.drp.util.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
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 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:
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.drp.util.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>以上就是今天的全部内容，相比拦截器来说，过滤器可能较为简单，但还是不能小觑的;更多java基础知识请持续关注本站了解。
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