java - 刷新数据时发生异常。这是什么，为什么我会得到这个？

Question

发生了一些有趣的事情。当我调用一个 url abc.com 时，一个 servlet 被寻址但请求通过过滤器(到 servlet)。我尝试从 servlet 将请求分派(dispatch)到 jsp 页面。但是当我尝试分派(dispatch)请求时，我抛出了一个异常:java.lang.IllegalStateException: Exception occurred when flushing数据

过滤代码

public class FirstSiteFilter implements Filter {
private FilterConfig fc;

@Override
public void init(FilterConfig config) throws ServletException {
    this.fc = config;
}

@Override
public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain) 
        throws ServletException,IOException {
    String IP = request.getRemoteAddr();
    request.setAttribute("client IP from the filter", IP);
    chain.doFilter(request, response);
}

@Override
public void destroy() {

}   
}

Servlet 代码(请求从过滤器链接到这个 servlet)

public class FW_FirstSite extends HttpServlet {


@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    String IP = (String)request.getAttribute("client IP from the filter");
    request.setAttribute("Client IP", IP);
    // THE FOLLOWING TWO STATEMENTS CAUSE AN EXCEPTION
    // IF I COMMENT THEM OUT NO EXCEPTION IS THROWN BUT AN EXCEPTION IS THROWN IF I LEAVE THEM AS IT IS
    RequestDispatcher rd = request.getRequestDispatcher("index.jsp");
    rd.forward(request,response);
}
}

为什么我会得到这种类型的异常？相反，如果我将语句 response.sendRedirect("...") 放在语句 request.setAttribute("client IP",IP) 之后，它就可以工作。

编辑 (web.xml)

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
    <servlet-name>guestbook</servlet-name>
    <servlet-class>guestbook.GuestbookServlet</servlet-class>
</servlet>
<servlet>
    <servlet-name>emailtest</servlet-name>
    <servlet-class>Servlets.emailtest</servlet-class>
</servlet>
<servlet>
    <servlet-name>ValidateCredentials</servlet-name>
    <servlet-class>Servlets.ValidateCredentials</servlet-class>
</servlet>
<servlet>
    <servlet-name>UploadImagesToAisle</servlet-name>
    <servlet-class>Servlets.UploadImagesToAisle</servlet-class>
</servlet>
<servlet>
    <servlet-name>HandleSongLink</servlet-name>
    <servlet-class>Servlets.HandleSongLink</servlet-class>
</servlet>
<servlet>
    <servlet-name>context</servlet-name>
    <servlet-class>Servlets.context</servlet-class>
</servlet>
<servlet>
    <servlet-name>HandleVideoLink</servlet-name>
    <servlet-class>Servlets.HandleVideoLink</servlet-class>
</servlet>
<servlet>
    <servlet-name>HandlePoetry</servlet-name>
    <servlet-class>Servlets.HandlePoetry</servlet-class>
</servlet>
<servlet>
    <servlet-name>displaykey</servlet-name>
    <servlet-class>Servlets.displaykey</servlet-class>
</servlet>
<servlet>
    <servlet-name>tester_writeXML</servlet-name>
    <servlet-class>Servlets.tester_writeXML</servlet-class>
</servlet>
<servlet>
    <servlet-name>ShowBlob</servlet-name>
    <servlet-class>Servlets.ShowBlob</servlet-class>
</servlet>
<servlet>
    <servlet-name>FW_FirstSite</servlet-name>
    <servlet-class>Servlets.FW_FirstSite</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>guestbook</servlet-name>
    <url-pattern>/guestbook</url-pattern>
</servlet-mapping>
<servlet>
    <servlet-name>sign</servlet-name>
    <servlet-class>guestbook.SignGuestbookServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>sign</servlet-name>
    <url-pattern>/sign</url-pattern>
</servlet-mapping>
<servlet>
    <servlet-name>HandleSubmittedPoem</servlet-name>
    <servlet-class>Servlets.HandleSubmittedPoem</servlet-class>
</servlet>
<servlet>
    <servlet-name>HandleSubmittedMessage</servlet-name>
    <servlet-class>Servlets.HandleSubmittedMessage</servlet-class>
</servlet>
<servlet>
    <servlet-name>HandleSongDedicated</servlet-name>
    <servlet-class>Servlets.HandleSongDedicated</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>HandleSubmittedPoem</servlet-name>
    <url-pattern>/HandleSubmittedPoem</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HandleSubmittedMessage</servlet-name>
    <url-pattern>/HandleSubmittedMessage</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HandleSongDedicated</servlet-name>
    <url-pattern>/HandleSongDedicated</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>emailtest</servlet-name>
    <url-pattern>/emailtest</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>ValidateCredentials</servlet-name>
    <url-pattern>/ValidateCredentials</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>UploadImagesToAisle</servlet-name>
    <url-pattern>/UploadImagesToAisle</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HandleSongLink</servlet-name>
    <url-pattern>/HandleSongLink</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>context</servlet-name>
    <url-pattern>/context</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HandleVideoLink</servlet-name>
    <url-pattern>/HandleVideoLink</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>HandlePoetry</servlet-name>
    <url-pattern>/HandlePoetry</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>displaykey</servlet-name>
    <url-pattern>/displaykey</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>tester_writeXML</servlet-name>
    <url-pattern>/tester_writeXML</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>ShowBlob</servlet-name>
    <url-pattern>/ShowBlob</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>FW_FirstSite</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

<filter>
    <filter-name>FirstSiteFilter</filter-name>
    <filter-class>Filters.FirstSiteFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>FirstSiteFilter</filter-name>
    <servlet-name>FW_FirstSite</servlet-name>
</filter-mapping>



<!--
<welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
</welcome-file-list>
-->

</web-app>

Answer 1

RequestDispatcher 总是需要一个正斜杠。所以尝试将其更改为以下内容

RequestDispatcher rd = request.getRequestDispatcher("/index.jsp");

当您使用 RequestDispatcher.forward 调用另一个 servlet 时，它是“命令链”。

当一个 servlet 被第一次调用时，响应对象是新鲜的:它的头没有被设置，它的缓冲区是空的，并且没有数据被写入客户端。

但是，一旦状态代码或任何 header 已写入(或可能已写入)客户端，或者数据已(或可能已)写入主体流，那么您可能容易受到 IllegalStateException 错误的影响。此异常发出信号的问题是您正在(或可能正在)写入的新数据与已经设置然后不可撤销地发送(“提交”)给客户端的数据不一致。

此异常的两个常见变体是“java.lang.IllegalStateException:Header already sent”和“java.lang.IllegalStateException:Cannot forward as Output Stream or Writer has already been obtained”。

“Header already sent”表示一个或多个header已经提交给客户端，所以你不能再设置那个header。

“Output Stream or Writer has already been obtained”意思是因为调用的servlet已经调用了response.getWriter()或response.getOutputStream()，污染了数据流，因为响应已经(或可能已经) ) 已写入，因此不适合转发。