To answer this question, let's understand the purpose of the RequestDispatcher and the methods mentioned in the options.
The RequestDispatcher interface in Java is used to forward or include a request from a servlet to another resource, such as another servlet, JSP (JavaServer Page), or HTML file. It provides two main methods: forward() and include().
Now let's go through each option to understand why it is correct or incorrect:
Option A) read - This option is incorrect because there is no read method in the RequestDispatcher interface. The RequestDispatcher is used for forwarding or including requests, not for reading data.
Option B) write - This option is incorrect because there is no write method in the RequestDispatcher interface. The RequestDispatcher is not used for writing data.
Option C) flush - This option is correct. Invoking the flush method on the ServletResponse object obtained from getWriter() can often lead to an IllegalStateException. This exception is thrown when the response has already been committed, meaning the response headers have been sent to the client. Once the response is committed, it is not possible to modify the response.
Option D) getOutputStream - This option is incorrect. The getOutputStream method is used to obtain the output stream for writing binary data to the response. It does not lead to an IllegalStateException on its own.
Therefore, the correct answer is option C) flush. This option can often lead to an IllegalStateException when used with a RequestDispatcher.