One of the most powerful features of JSP is that a JSP page can access, create, and modify data objects on the server. You can then make these objects visible to JSP pages. When an object is created, it defines or defaults to a given scope. The container creates some of these objects, and the JSP designer creates others.
The scope of an object describes how widely it's available and who has access to it. For example, if an object is defined to have page scope, then it's available only for the duration of the current request on that page before being destroyed by the container. In this case, only the current page has access to this data, and no one else can read it. At the other end of the scale, if an object has application scope, then any page may use the data because it lasts for the duration of the application, which means until the container is switched off.
Page Scope
Objects with page scope are accessible only within the page in which they're created. The data is valid only during the processing of the current response; once the response is sent back to the browser, the data is no longer valid. If the request is forwarded to another page or the browser makes another request as a result of a redirect, the data is also lost.
Request Scope
Objects with request scope are accessible from pages processing the same request in which they were created. Once the container has processed the request, the data is released. Even if the request is forwarded to another page, the data is still available though not if a redirect is required.
Session Scope
Objects with session scope are accessible from pages processing requests that are in the same session as the one in which they were created. A session is the time users spend using the application, which ends when they close their browser, when they go to another Web site, or when the application designer wants (after a logout, for instance). So, for example, when users log in, their username could be stored in the session and displayed on every page they access. This data lasts until they leave the Web site or log out.
Application Scope
Objects with application scope are accessible from JSP pages that reside in the same application. This creates a global object that's available to all pages.
Application scope uses a single namespace, which means all your pages should be careful not to duplicate the names of application scope objects or change the values when they're likely to be read by another page (this is called thread safety). Application scope variables are typically created and populated when an application starts and then used as read-only for the rest of the application.
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