JavaEE学习之路-Sharing Information

Web components, like most objects, usually work with other objects to accomplish their tasks. Web components can do so by doing the following.

• Using private helper objects (for example, JavaBeans components).

• Sharing objects that are attributes of a public scope.

• Using a database.

• Invoking 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.

17.3.1 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 by using the getAttribute and setAttribute methods of the class representing the scope. Table 17-2 lists the scope objects.

Table 17-2 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.

17.3.2 Controlling Concurrent Access to Shared Resources

In a multithreaded server, shared resources can 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 at http://docs.oracle.com/javase/tutorial/essential/concurrency/.