The Java TM Tutorial

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Trail: Servlets
Lesson: Servlet Communication

## Sharing Resources Among Servlets

> Servlets running in the same server sometimes share resources. This is especially true of servlets that combine to form a single application, as the Duke's Bookstore servlets do. Servlets in the same server can share resources using the `ServletContext` interface's methods for manipulating attributes: `setAttribute`, `getAttribute`, and `removeAttribute`.

> Note that using attributes is one way to remove the deprecated method, `getServlet` from pre-JSDK2.1 servlets.

> ## Naming Conventions for Attributes

> All servlets in a context share the attributes stored with the `ServletContext` interface. To avoid collisions of attribute names, name attributes using the same conventions as package names. For example, the Duke's Bookstore servlets share an attribute called `examples.bookstore.database.BookDBFrontEnd`. Names beginning with the prefixes `java.*`, `javax.*`, and `sun.*` are reserved.

> ## Setting an Attribute

> Servlets set attributes using the `ServletContext.setAttribute` method; they typically do so during initialization. When you have multiple servlets sharing an attribute, you might have each one try to initialize the attribute. If so, each servlet should check for the attribute value, and only set the attribute if another servlet has not already done so.

> The following example shows the `init` method of the CatalogServlet trying to set the shared attribute for Duke's Bookstore:

> >     public class CatalogServlet extends HttpServlet {

> >         public void init() throws ServletException {
> >             BookDBFrontEnd bookDBFrontEnd = ...

> >             if (bookDBFrontEnd == null) {
> >                 getServletContext().setAttribute(
> >                     "examples.bookstore.database.BookDBFrontEnd",
> >                     BookDBFrontEnd.instance());
> >             }
> >         }
> >     ...
> >     }

> Once a servlet sets an attribute, any servlet in the same context can retrieve its value, reset its value, or remove the attribute.

> ## `getAttribute`

> Getting the value of the attribute is as simple as making a call to the `ServletContext.getAttribute` method. The following example shows the CatalogServlet getting the value of an attribute during initialization:

> >     public class CatalogServlet extends HttpServlet {

> >         public void init() throws ServletException {
> >             BookDBFrontEnd bookDBFrontEnd =
> >                 (BookDBFrontEnd)getServletContext().getAttribute(
> >                     "examples.bookstore.database.BookDBFrontEnd");

> >             if (bookDBFrontEnd == null) {
> >                 getServletContext().setAttribute(
> >                     "examples.bookstore.database.BookDBFrontEnd",
> >                     BookDBFrontEnd.instance());
> >             }
> >         }
> >     ...
> >     }

> ## `removeAttribute`

> Any servlet can remove an attribute from a `ServletContext` object. Because attributes are shared, however, you must take care that you do not delete an attribute that another servlet might still use. Duke's Bookstore does not use the `removeAttribute` method.

> ## Converting Servlets from JSDK2.0

> Note for converting servlets written with the JSDK2.0: The methods for managing attributes can be useful if you are removing calls to the `getServlet` method. Consider these methods for servlets that called the `getServlet` method and then called public methods other than the `service` method.

> Removing calls to `getServlet` from these servlets involves:

> 1. Changing the servlet gotten with the call to `getServlet` to a non servlet class.
> 2. Naming an attribute that will hold an instance of the new class for use by the servlets that called the `getServlet` method.
> 3. Replacing the call to the `getServlet` method with calls that Get the value of the previously named attribute
> 4. Changing calls to methods of the `Servlet` object to calls to methods of the instance returned from the attribute.

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