I would like to build a dynamic HTML page including a JSP page whith the RequestDispatcher(...).include(...). But the OutputStream cannot be opened from both my servlet and the JSP. Is there any workaround ?
This design of having servlets control page flow and JSPs handling display logic is commonly known as the Model 2 Architecture. For more on this, see Govind Seshadri's article.
One note on this - one scenario in which you would want your servlet to write the response is if the response is binary - some sort of file format. In this case, you can awlays include a link to a servlet in your HTML ala: