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A session bean will typically access a database in a series of one or more JDBC calls on behalf of the user represented by the bean. An application server classifies session beans into two categories, depending on how the bean manages state: I Stateless session bean. This type of bean does not maintain any status information across method invocations. It carries out its actions on behalf of one user at a time, and one request at a time. Each request to the bean is independent of the last. With this restriction, every invocation of the bean must carry with it (in the form of the parameters passed with the invocation) all of the information needed to carry out the request. I Stateful session bean. This type of bean maintains status information across method invocations. The bean needs to remember information from its previous invocations (its state) to carry out the tasks requested by later invocations. It uses private instance variables to hold the information.
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The next two sessions show examples of application tasks that are most easily implemented as each type of session bean. You specify whether a session bean is stateless or stateful in the deployment descriptor for the bean, which contains information supplied to the application server on which the bean is deployed. An application server on a busy web site can easily have more session beans and other EJBs in use than it has main memory available to store them. In this situation, the application server will keep a limited number of session bean instances active in its main memory. If a user associated with a currently inactive session bean becomes active (i.e., one of his or her web site clicks must be processed), the application server chooses another instance of the same bean class and passivates it that is, it saves the values of any instance variables defined for the bean and then reuses the bean to serve the user session needing activation. Whether a session bean is stateful or stateless has a significant impact on this passivation and activation. Since a stateless session bean does not need its status preserved across method invocations, the application server does not need to save its instance variable values when it passivates the bean, and does not need to restore instance variable values when it reactivates the bean. But for a stateful session bean, the application server needs to copy its instance value variables to persistent storage
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(a disk file or a database) when it passivates the bean, and then restore those values when it reactivates the bean. Thus, stateful session beans can significantly diminish the performance and throughput of an application server on a busy site. Stateless beans are preferable for performance, but many applications are difficult or impossible to implement without using stateful beans.
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Using JDBC from a Stateless Session Bean
Figure 22-4 shows a simple example of an application that can easily be handled with stateless session bean database access. A page on a web site displays the current price of a company s stock when the page is displayed. The page can t be static, since the displayed price will change minute by minute. So when the user s browser requests the page, the web server hands off the request to an application server, which eventually invokes a method of a session bean. The session bean can use JDBC to send a SQL SELECT statement to a database of current stock prices, and receives back the answer as one line of query results. The session bean reformats the stock quote as a fragment of a web page, and passes it back to the web server for display to the user. Stateless session beans can perform more complex functions as well. Suppose the same company has a page on its web site where a user can request a product catalog by filling in the contents of a small form. When the form is filled in and the user clicks the Send button, the browser sends the data from the form to the web server, which again hands off the request to an application server. This time, a different method of the session bean is invoked, and receives the data from the form as parameters. The session bean can use JDBC to send a SQL INSERT statement to a database table holding pending catalog requests. In each of these examples, all of the information that the session bean needs to carry out its task is passed to it with the method invocation. When the bean has completed its task, the information is not needed anymore. The next invocation again receives all of the information it needs with the next invocation, so there is no need to carry over status information. Even more important, the database activity on each invocation is completely independent from every other invocation. No database transaction spans multiple method invocations.
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