Figure 2-4 Model-View-Controller architecture in Java

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Figure 2-4 Model-View-Controller architecture
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The Model is responsible for maintaining the application state and data It can receive and respond to queries from the View and can provide notifications to the View when things have changed
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The Controller updates the Model based on execution of application logic in response to user gestures (eg, dialog buttons, form submit requests, etc) It is also responsible fo r telling the View what to display in response to user gestures
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The View is responsible for the actual rendering of the data provided by the Controller
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To illustrate, consider a simple clock application developed using the MVC approach The Model in this case is essentially responsible for keeping track of time Time is automatically updated at predefined intervals (a microsecond, millisecond, or some other unit) through some built-in mechanisms in the Model It also provides operations so other entities can query the Model and obtain the current time, but it does not care or know how the time is to be displayed
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The responsibility for displaying the time falls on the View; however, the View can take different forms For example, it may take the form of an analog display whereby two (or three) hands are used to display the time It can easily be a digital display consisting of several digits as well As time changes, the Model notifies the View, and the View updates to reflect the new time
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Keep in mind that clocks require some mechanism for updating the time, for example, when daylight savings time goes into effect On a clock rendered in a Web browser, the user may have the capability to indicate a change in time by using some Graphical User Interface (GUI) controls or by typing in a new time The Controller receives the user gestures for such changes and updates the Model by calling the appropriate operations defined on the Model to reflect the new time
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A Model may have several simultaneous Views For instance, a clock application running on the Web may have several users utilizing it at the same time, using different representations, such as analog, digital, and so on
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APIs
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There are several APIs within the J2EE Some of the more popular ones are discussed in the following sections
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JDBC
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Interaction with databases is an integral part of an enterprise Java application The JDBC API is squarely focused on making this aspect easier for the enterprise Java developer
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The JDBC API, which is similar in spirit to Microsoft's Open Database Connectivity (ODBC) API, simplifies access to relational databases It consists of a generic, vendor independent interface to databases Using the JDBC makes your applications portable and your database skills applicable across a wider range of vendor platforms
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The majority of the JDBC API already exists as part of the J2SE It is not limited to use only with the J2EE There are however a few extensions that the J2EE version adds, mostly to support some advanced functions for the J2EE containers to use, like connection pooling as well as some additional support for JavaBeans
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The JDBC API provides a common interface in order to shield the user from vendor specific differences as much as possible JDBC implementations are supplied by the database vendor, so different databases can act differently under the covers
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In enterprise applications, you do not necessarily need to use JDBC directly For example, you can use entity beans to make the necessary database calls for you The practice of using JDBC directly is expected to become less common as application servers provide more sophisticated and well-tuned support for entity beans
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Java Naming and Directory Interface (JNDI)
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In the context of JNDI, "naming" actually refers to a naming service Naming services allow you to look up, or refer to, an object A file system is an example of a naming service
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A directory service is similar to a naming service and provides enhanced searching capabilities In fact, a directory service always has a naming service (but not vice versa)
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There are various naming and directory services available, so the challenges on this front are quite similar to those in the area of databases JNDI is designed to address those challenges by providing a generic and uniform way to access the services
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The complete JNDI API already exists as part of J2SE, although it is listed as an enterprise feature Most distributed enterprise applications make use of this service at some point For example, any use of EJBs in an enterprise application necessitates that JNDI be used to find the associated EJB Home interfaces
A messaging service allows communication among distributed applications using self-contained entities called messages Such communication is typically asynchronous
Various vendors provide messaging oriented middleware The JMS provides a uniform and generic interface to such middleware
JMS can be used directly in an enterprise application or via a type of EJB known as a message-driven bean Message-driven beans are new in J2EE 13
Remote Method Invocation (RMI)
RMI enables access to components in a distributed environment by allowing Java objects to invoke methods on remote Java objects The method is actually invoked on a proxy object, which then arranges to pass the method and parameters onto the remote object and provides the response from the remote object back to the object that initiated the remote method invocation