在关闭 Cookie 的情况下，系统登出时抛空。
04-Jan-2018 15:49:13.902 严重 [http-nio-8080-exec-8] org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [LogOutServlet] in context with path [/GuestOrderApp] threw exception
java.lang.NullPointerException
at controller.LogOutServlet.doGet(LogOutServlet.java:27)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
at filter.EncodeFilter.doFilter(EncodeFilter.java:26)
...定位到 EncodeFilter.java。
package filter;
import org.apache.log4j.Logger;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import java.io.IOException;
/**
* 解决表单中的中文请求后的乱码问题
*/
@WebFilter("/*")
public class EncodeFilter implements Filter {
private final static Logger logger = Logger.getLogger(EncodeFilter.class);
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html; charset=utf-8");
filterChain.doFilter(request, response);
}
@Override
public void destroy() {
}
}抛错的第 26 行是 filterChain.doFilter(request, response); 直观感觉就是 filterChain 抛空。然后想了一会儿不明白为什么他会抛空。。然后觉得其他都正常的，应该是哪个地方 jsessionid 没有加上去。所以再仔细看了一下报错信息，定位到 LogOutServlet.java，第 27 行是 logger.debug("user " + session.getAttribute("user_id") + "已手动退出");。session 为空的话，一定是哪个地方 jsessionid 没加啦。然后看 logout button 的链接，确实是没加。然后把链接 encode 一下就好了。
需要注意的一点其实不是这个bug，是对 Filter 实现的理解吧。之前一直不知道 filterChain.doFilter(request, response); 这句话到底是在干什么，现在知道啦。就是请求来了，进了filter，做到 filterChain.doFilter(request, response); 之前的，然后 filterChain.doFilter(request, response); 实际是去真正调用servlet实现，然后做完了返回该 Filter 了再做 filterChain.doFilter(request, response); 之后的。