Sharing Information
Web components, like most objects, usually work with other objects to accomplish their tasks. There are several ways they can do this. They can use private helper objects (for example, JavaBeans components), they can share objects that are attributes of a public scope, they can use a database, and they can invoke other web resources. The Java Servlet technology mechanisms that allow a web component to invoke other web resources are described in Invoking Other Web Resources.
Using Scope Objects
Collaborating web components share information by means of objects that are maintained as attributes of four scope objects. You access these attributes using the [get|set]Attribute methods of the class representing the scope. Table 10-2 lists the scope objects.
Table 10-2 Scope Objects
Scope Object |
Class |
Accessible From |
|---|---|---|
Web context |
javax.servlet.ServletContext |
Web components within a web context. See Accessing the Web Context. |
Session |
javax.servlet.http.HttpSession |
Web components handling a request that belongs to the session. See Maintaining Client State. |
Request |
subtype of javax.servlet.ServletRequest |
Web components handling the request. |
Page |
javax.servlet.jsp.JspContext |
The JSP page that creates the object. |
Controlling Concurrent Access to Shared Resources
In a multithreaded server, it is possible for shared resources to be accessed concurrently. In addition to scope object attributes, shared resources include in-memory data (such as instance or class variables) and external objects such as files, database connections, and network connections.
Concurrent access can arise in several situations:
Multiple web components accessing objects stored in the web context.
Multiple web components accessing objects stored in a session.
Multiple threads within a web component accessing instance variables. A web container will typically create a thread to handle each request. To ensure that a servlet instance handles only one request at a time, a servlet can implement the SingleThreadModel interface. If a servlet implements this interface, no two threads will execute concurrently in the servlet’s service method. A web container can implement this guarantee by synchronizing access to a single instance of the servlet or by maintaining a pool of web component instances and dispatching each new request to a free instance. This interface does not prevent synchronization problems that result from web components accessing shared resources such as static class variables or external objects.
When resources can be accessed concurrently, they can be used in an inconsistent fashion. You prevent this by controlling the access using the synchronization techniques described in the Threads lesson in The Java Tutorial, Fourth Edition, by Sharon Zakhour et al. (Addison-Wesley, 2006).