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

Question

For two days I have tried find out what went wrong. I read here that i should add a return into the code, and I did it, and i still get

java.lang.IllegalStateException: Cannot call sendRedirect() 
     after the response has been committed, Error.

How can I solve this problem?

It happens every time I connect to the database. This is the connect method:

<%!

public  void connect()
{
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String dbURL = "jdbc:mysql://localhost:3306/moti";
            String user = "root";
            String password = "j3o4h5n6y7";
            con =  DriverManager.getConnection(dbURL, user, password);  
            statement = con.createStatement();
        }
        catch(Exception ex) {
            throw new Error(ex);
        }  
}
%>

like in this code block :

            String post = request.getParameter("send");
            if(post != null )
            {
                    connect();
                    statement.execute(add);
                    con.close();
                    response.sendRedirect("fourm.jsp");
                    return;

            }

but in this code block its work perfectly :

    String back = request.getParameter("retrun");

    if(back != null)
    {

        response.sendRedirect("fourm.jsp");
        return;
    }

Can you provide the code that gets run before the two calls to `getParameter"? — David Grant, Oct 02 '12 at 16:05
There is something else you are missing which is modifying the response after you redirect(). — kosa, Oct 02 '12 at 16:05

score 12 · Answer 1 · edited May 23 '17 at 10:24

From high level seen, your concrete problem is caused because you're incorrectly using a JSP file instead of a Servlet class as a front controller.

From low level seen, your concrete problem is caused because JSP plays as being a view technology a role during generating and sending HTML code to the HTTP response. The response buffer size defaults to 2KB. Every HTML and other template text in the JSP is immeditately written to the response once the code reaches that line. So when the response buffer size limit is reached for the first time, then all HTTP response headers and the so far written HTML code will be sent to the client (the webbrowser). In other words, the response is committed. This is a point of no return. It's simply not possible to take the already-sent bytes back from the client.

A redirect basically sets a Location header on the HTTP response. To be able to properly set this, the response must obviously not be committed yet. It's simply not possible to set a new response header if they are all already been sent and retrieved by the client.

From low level seen, you can solve your concrete problem by moving all the front controller and business logic to the far top of the JSP file so that it's executed long before the first HTML code is ever been sent. This way you eliminate the risk that the response is committed before your front controller and business logic is finished.

<%@page pageEncoding="UTF-8" %>
<%
    // Write business code here.
%>
<!DOCTYPE html>
<html lang="en">
    <head>
        <title>Some</title>
    </head>
    <body>
        ... (no controller/business logic here! just pure presentation)
    </body>
</html>

However, this is a bad practice. Rather move all that front controller and business logic into a Servlet. Then your approach is from high level seen correct. Java code doesn't belong in JSP files, but in Java classes.

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

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