Web Applications in Java

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Web Applications
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The application logic is typically split into page flows that are in the presentation tier, and the business processes and workflows that are in the business tier The business logic and persistence logic are in the corresponding tiers
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Input Layer
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The input layer contains the code concerned with processing and syntactically validating input such as SOAP, HTTP, and SMTP, and extracting parameters from the request In the Model-View-Controller (MVC) framework, this corresponds to the input controller You have servlet components and APIs that provide support for the HTTP protocol to build such a layer You will discover later in the chapter that these components and APIs provide the plumbing to build an input layer
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Application Logic
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The application logic code is concerned with the Web application s overall flow We often refer to this layer as the glue layer, separating business logic from input and output logic and managing the interface between the two This requires some knowledge of both layers For example, this layer will be involved in converting between presentation-level inputs and outputs as strings and the corresponding business object messages or state In a Web application, this layer might also manage a multipage Web interaction as a sequence of steps (Web page flows) In the MVC framework, this corresponds to the application controller The J2EE standard does not directly provide components for implementing application logic This layer is typically implemented inside the J2EE Web container and uses similar components and APIs to those found in the input layer However, this situation is improving with the addition of JavaServer Faces (JSF) to Java EE 5
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Business Logic
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The business logic code, referred to as business objects, is concerned only with the underlying business functionality This code should be entirely unaware of the output layer (presentation) In a complex application, business logic is likely to be the largest component and is strongly related to the code that accesses external systems such as databases, Enterprise Information Systems (EIS) such as Enterprise Resource Planning (ERP) and Customer Relationship Management (CRM), and other related services In the MVC framework, this corresponds to the model Objects handling the business logic should not depend on any other layer This makes it easy to implement the core business, and it enables the use of these objects with or without a J2EE application server or other application frameworks Our recommendation about designing the business layer is to keep them as simple as
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CHAPTER 5 Web Application Architecture and Design
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possible using ordinary Java objects, sometimes referred to as Plain Old Java Objects (POJO), and make them independent of all other architectural layers For example, it would be bad design to refer to a view component, such as a JSP, or a Web-specific J2EE API, such as the HTTP request object, in a business model object Equally, you would not want to refer to database APIs of the persistence layer directly from the business object itself What would happen to the business object if you decided to change the persistence technology later Managing the dependencies properly frees you to make these decisions and changes independently Business logic implemented in this form allows it to be managed via a J2EE EJB container or any other method The point is that this layer should not depend on J2EE
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Persistence
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Business logic that is implemented as objects in Java needs to store the business data kept in the objects in a persistent store Most applications use relational databases for data storage It is also possible to use alternative technologies such as XML databases or object databases to achieve the same goal The purpose of the persistence layer is to provide this functionality The business logic should not depend on the persistence layer, and you should not refer to datastore APIs in the business model There are various approaches to object persistence These range from Data Access Objects (DAO), which are usually dependent on database access APIs and query languages like SQL This approach is more suitable for a small set of simple objects but may provide greater flexibility Other approaches include Java Persistence API (JPA), sophisticated Object-Relational Mapping (ORM) frameworks like Hibernate and TOPLink, and Object database approaches, among others Object persistence has been the focus of extensive work A detailed discussion of object persistence is beyond the scope of this book
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