Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Invoking Other Web Resources

Web components can invoke other web resources both indirectly and directly. A web component indirectly invokes another web resource by embedding a URL that points to another web component in content returned to a client. While it is executing, a web component directly invokes another resource by either including the content of another resource or forwarding a request to another resource.

To invoke a resource available on the server that is running a web component, you must first obtain a `RequestDispatcher` object by using the `getRequestDispatcher("URL")` method. 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.

### Including Other Resources in the Response

It is often useful to include another web resource, such as 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:

    include(request, response);

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 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, such as `setCookie`, that affects the headers of the response.

### Transferring Control to Another Web Component

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 the following request attributes:

    javax.servlet.forward.request_uri
    javax.servlet.forward.context_path
    javax.servlet.forward.servlet_path
    javax.servlet.forward.path_info
    javax.servlet.forward.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`.

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