220.127.116.11 State Scope Each item of Web-tier state has scope, which determines the accessibility and lifetime of the item. Web-tier state is accessible to servlets, servlet filters, and JSP pages. Briefly, Web-tier state can be maintained in four scopes:
Application scope is "global memory" for a Web application. Application-scope state is stored in the Web container's ServletContext object. (See the caveat on using context attributes in distributable servlets on page 126.) All servlets in an application share objects in application scope. The servlet developer is responsible for thread safety when accessing objects in application scope. An inventory object in application scope, for example, is accessible to all servlets, servlet filters, and JSP pages in the application. State in application scope exists for the lifetime of the application, unless it is explicitly removed.
Session scope contains data specific to a user session. HTTP is a "stateless" protocol, meaning that it has no way of distinguishing users from one another or for maintaining data on users' behalf. Session attributes are named object references that are associated with a user session. The servlet API allows a developer to create a session attribute and access or update it in subsequent requests. Session-scope state for an HttpServlet is stored in the Web container's HttpSession object (available from the HttpServletRequest argument to the service method). State in session scope is accessible to all Web components in the application and across multiple servlet invocations. However, it is accessible only within an individual user session. An online shopping cart is an example of data in session scope, because the contents of the cart are specific to a single client session and available across multiple server requests. A session ends when it is explicitly closed, when it times out after a period of inactivity, or when its container is shut down or crashes. Unless removed explicitly, state in session scope lasts until the session ends.
Request scope contains data specific to an individual server request, and is discarded when the service method returns. A Web component can read or modify data in request scope and then "forward" the request to another component. The component to which the request is forwarded then has access to the state. State in request scope is stored in a ServletRequest object, so it is accessible to any component receiving the request. Note that the values of query string parameters and form variables are also in request scope. For example, when a servlet places a timestamp in a ServletRequest and then forwards the request to another servlet, the timestamp is in request scope.
Page scope, applicable only to JSP pages, contains data that are only valid in the context of a single page. Page scope state is stored in a JSP page's PageContext object. When one JSP page forwards to or includes another, each page defines its own scope. Page scope state is discarded when the program flow of control exits the page.
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