在doFilter方法中获取IllegalStateException

Question

When i create a servlet filter in Spring MVC, I am getting the following exception.

 [javax.servlet.ServletException: java.lang.IllegalStateException: Cannot create a session after the response has been committed] with root cause

when i put the sysout i could understand that the exception occurs at the redirect line, but didn't understand why

can anyone please tell me some solution for this

SessionFilter.java

public class SessionFilter implements Filter {

    private ArrayList<String> urlList;

    @Override
    public void destroy() {
    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        String url = request.getServletPath();
        boolean allowedRequest = false;

        if(urlList.contains(url)) {
            allowedRequest = true;
        }

        if (!allowedRequest) {
            HttpSession session = request.getSession(false);
            if (null == session) {
                System.out.println("preparing for redirect");
                response.sendRedirect("index.jsp");
            }
        }

        chain.doFilter(req, res);
    }

    @Override
    public void init(FilterConfig config) throws ServletException {
        System.out.println("entered init");
        String urls = config.getInitParameter("avoid-urls");
        StringTokenizer token = new StringTokenizer(urls, ",");

        urlList = new ArrayList<String>();

        while (token.hasMoreTokens()) {
            urlList.add(token.nextToken());

        }
    }

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>SpringMVCHibernate</display-name>

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

  <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>*.do</url-pattern>
    </servlet-mapping>

    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>


    <filter>
        <filter-name>SessionFilter</filter-name>
        <filter-class>com.common.dao.SessionFilter</filter-class>
        <init-param>
            <param-name>avoid-urls</param-name>
            <param-value>index.jsp</param-value>
        </init-param>
    </filter>

    <filter-mapping>
        <filter-name>SessionFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
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Answer 1

To simple and short Answer for your Question:

To avoid this , you should have a return statement , or avoid redirecting or forwarding request , or these kind of things should be done by the last filter in the filter chain.

For More Details Explanation you can read below :

This Exception occurs when you try to send response again when the response is already committed and flushed to user. For Example :

Here in below example code ,first the request will be forwarded to index_test.jsp and response will be flushed to user , then the control will again come back to filter and try to send another response (redirect)to user , and it will fail.

Usually we check multiple conditions in filter and accordingly forward and redirect , if two conditions are met , then it will create a problem .

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { 

  request.getRequestDispatcher("index_test.jsp").forward(request, response);
  ((HttpServletResponse)response).sendRedirect("new.jsp");
  chain.doFilter(request, response);
 }

Solution for your case :

To avoid this , you should have a return statement , or avoid redirecting or forwarding request , or these kind of things should be done by the last filter in the filter chain.

So you could change your code as :

public class SessionFilter implements Filter {

    private ArrayList<String> urlList;

    @Override
    public void destroy() {
    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        String url = request.getServletPath();
        boolean allowedRequest = false;

        if(urlList.contains(url)) {
            allowedRequest = true;
        }

        if (!allowedRequest) {
            HttpSession session = request.getSession(false);
            if (null == session) {
                System.out.println("preparing for redirect");
                response.sendRedirect("index.jsp");
                return;
            }
        }

        chain.doFilter(req, res);
    }

    @Override
    public void init(FilterConfig config) throws ServletException {
        System.out.println("entered init");
        String urls = config.getInitParameter("avoid-urls");
        StringTokenizer token = new StringTokenizer(urls, ",");

        urlList = new ArrayList<String>();

        while (token.hasMoreTokens()) {
            urlList.add(token.nextToken());

        }
    }

Answer 2

Its Obvious right :

System.out.println("preparing for redirect");
   response.sendRedirect("index.jsp");
   return; 

We are have return after that statement. So further execution wont happen so u are not getting exception.

See we cant involve both filtering and Redirect or Forward in same block it will give IllegalStateException because : This Exception occurs when you try to send response again when the response is already committed and flushed to user