You can access a variety of objects, including enterprise beans and JavaBeans components, within a JSP page. JSP technology automatically makes some objects available, and you can also create and access application-specific objects.
Implicit objects are created by the web container and contain information related to a particular request, page, session, or application. Many of the objects are defined by the Java servlet technology underlying JSP technology and are discussed at length in Chapter 4, Java Servlet Technology. The section Implicit Objects explains how you access implicit objects using the JSP expression language.
When possible, application behavior should be encapsulated in objects so that page designers can focus on presentation issues. Objects can be created by developers who are proficient in the Java programming language and in accessing databases and other services. The main way to create and use application-specific objects within a JSP page is to use JSP standard tags (discussed in JavaBeans Components) to create JavaBeans components and set their properties, and EL expressions to access their properties. You can also access JavaBeans components and other objects in scripting elements, which are described in Chapter 9, Scripting in JSP Pages.
The conditions affecting concurrent access to shared objects (described in Controlling Concurrent Access to Shared Resources) apply to objects accessed from JSP pages that run as multithreaded servlets. You can use the following page directive to indicate how a web container should dispatch multiple client requests:
<%@ page isThreadSafe="true|false" %>
When the isThreadSafe attribute is set to true, the web container can choose to dispatch multiple concurrent client requests to the JSP page. This is the default setting. If using true, you must ensure that you properly synchronize access to any shared objects defined at the page level. This includes objects created within declarations, JavaBeans components with page scope, and attributes of the page context object (see Implicit Objects).
If isThreadSafe is set to false, requests are dispatched one at a time in the order they were received, and access to page-level objects does not have to be controlled. However, you still must ensure that access is properly synchronized to attributes of the application or session scope objects and to JavaBeans components with application or session scope. Furthermore, it is not recommended to set isThreadSafe to false. The JSP page’s generated servlet will implement the javax.servlet.SingleThreadModel interface, and because the Servlet 2.4 specification deprecates SingleThreadModel, the generated servlet will contain deprecated code.