servlet中的IllegalStateException

IllegalStateException在java web开发中比较常见，IllegalStateException的根本原因是java servlet在提交响应后，还尝试写内容。

所以避免IllegalStateException的一个好方法就是提交响应，比如forward或者redirect之后，就不要再写内容，一个方法是在redirect之后加上return;

比如这个比较常见的IllegalStateException，原因就是已经sendRedirect，提交响应了，然后还尝试写内容，这样就导致了IllegalStateException

Java.lang.IllegalStateException: Cannot call sendRedirect() after the response has been committed

下面给出会异常的代码：

public class LoginFilter extends OncePerRequestFilter{
   ...
    @Override
    protected void doFilterInternal(final HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
        HttpServletRequest wrappedRequest = new RemoteUserRequestWrapper(request);
        if (StringUtils.isEmpty(wrappedRequest.getRemoteUser()) ) {
            response.sendRedirect("login.do");
            //return;
        }
        filterChain.doFilter(new RemoteUserRequestWrapper(request), response);
    }
}

这段代码response.sendRedirect之后，跳到 login.do，login.do里的代码逻辑是有再次重定向等等逻辑的，然后Filter里代码，没return，执行后是会IllegalStateException的

然后只要在sendRedirect之后加上return就可以：

public class LoginFilter extends OncePerRequestFilter{
   ...
    @Override
    protected void doFilterInternal(final HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
        HttpServletRequest wrappedRequest = new RemoteUserRequestWrapper(request);
        if (StringUtils.isEmpty(wrappedRequest.getRemoteUser()) ) {
            response.sendRedirect("login.do");
            return;
        }
        filterChain.doFilter(new RemoteUserRequestWrapper(request), response);
    }
}