Web components can invoke other web resources in two ways: indirectly and directly. A web component indirectly invokes another web resource when it embeds a URL that points to another web component in content returned to a client.
A web component can also directly invoke another resource while it is executing. There are two possibilities: The web component can include the content of another resource, or it can forward a request to another resource.
You can get a RequestDispatcher object from either a request or the web context; however, the two methods have slightly different behavior. The method takes the path to the requested resource as an argument. A request can take a relative path (that is, one that does not begin with a /), but the web context requires an absolute path. If the resource is not available or if the server has not implemented a RequestDispatcher object for that type of resource, getRequestDispatcher will return null. Your servlet should be prepared to deal with this condition.
It is often useful to include another web resource (for example, banner content or copyright information) in the response returned from a web component. To include another resource, invoke the include method of a RequestDispatcher object:
If the resource is static, the include method enables programmatic server-side includes. If the resource is a web component, the effect of the method is to send the request to the included web component, execute the web component, and then include the result of the execution in the response from the containing servlet. An included web component has access to the request object, but it is limited in what it can do with the response object:
It can write to the body of the response and commit a response.
It cannot set headers or call any method (for example, setCookie) that affects the headers of the response.
In some applications, you might want to have one web component do preliminary processing of a request and have another component generate the response. For example, you might want to partially process a request and then transfer to another component depending on the nature of the request.
To transfer control to another web component, you invoke the forward method of a RequestDispatcher. When a request is forwarded, the request URL is set to the path of the forwarded page. The original URI and its constituent parts are saved as request attributes javax.servlet.forward.[request-uri|context-path|servlet-path|path-info|query-string].
The forward method should be used to give another resource responsibility for replying to the user. If you have already accessed a ServletOutputStream or PrintWriter object within the servlet, you cannot use this method; doing so throws an IllegalStateException.