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Component Models II

Component Models II. Agenda. Components, Take II Component Models Introduction DCOM/MTS CORBAcomponent Model Enterprise JavaBeans Java 2, Enterprise Edition architecture development. Component Rationale. From: Component Primer , J. Hopkins, CACM V43N10 central idea:

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Component Models II

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  1. Component Models II

  2. Agenda • Components, Take II • Component Models Introduction • DCOM/MTS • CORBAcomponent Model • Enterprise JavaBeans • Java 2, Enterprise Edition • architecture • development

  3. Component Rationale From: Component Primer, J. Hopkins, CACM V43N10 • central idea: • complex systems can be built from simpler components and glue software • want reuse of existing components to create easier to maintain systems • must be some well-built, applicable components to reuse • must have a component model to support assembly and interaction • must be a process and architectures to support component-based development (CBD)

  4. Component Definition • component as a refinement of OO techniques • "component is a physical manifestation of an object that has a well-defined interface and a set of implementations for that interface"

  5. Component Communication • components can communicate if they share a mechanism for finding each other and sending messages • a reference model is such a mechanism, providing means of • defining interfaces • invoking functions • exchanging data • XML (eXtensible Markup Language) may become the integration and data transfer mechanism • component becomes only dependent upon XML-formatted msgs • for low-frequency, high semantic content exchanges • e.g., purchase orders, patient records, complex requests

  6. Components and Modeling • UML has component modeling support • provides component view of system • traceability between class model, component model and deployment model • integration is key, so accurate models needed, esp.: • interface, state model, use cases • otherwise, need access to source code • must also support extension points

  7. Component Granularity • intercomponent communication is expensive • causes larger components • larger components • more complex to understand • more sensitive to change • less flexible in allowing for replaceability • minimizing coupling works against good cohesion • existing apps are wrapped to look like components

  8. Component Specificity • specificity: how closely-related component is to task for which it will be used • more closely-related to task, less modification required • more specific yields more components • family of solutions helps address additional complexity

  9. Components and the Present/Future • vision: net-based systems • with large sets of services (interfaces) • implemented in a widely distributed components • supported by specialized vendors • interconnected by platform-independent technology like XML

  10. Component Models • explicit support for components, e.g.: • interface definitions • location transparency • dynamic assembly • model defines • architecture of components • component interfaces • interaction with component execution environment • we’ll deal with server-side component models

  11. DCOM & MTS • DCOM: distributed component service • MTS • Microsoft Transaction Server • server-side component model • transaction management, concurrency, resource management, etc. • property sheets change how business objects interact with services • proprietary platform • stateless components only • high performance but limited flexibility

  12. CORBA • The non-proprietary distributed object service • ORBs • language, platform, OS, communication independence • but, developer had to code to complex, low-level API • CORBAcomponent Model (CCM) • Enterprise Javabeans with language independence

  13. Enterprise JavaBeans • a standard server-side component model • adaptable to existing products, e.g.: • naming servers • transaction processing monitors • DBMSes • wanted flexible integration and solid support for mission-critical app development • wanted to defuse fear of locking into a vendor

  14. Java 2, Enterprise Edition: J2EE • collection of core specifications and APIs to assist in enterprise application development • an approach to: • developing server-side components • accessing infrastructure • standardizing infrastructure functionality • used in conjunction with Java 2, Standard Edition • language-specific features, but: • interfaces with objects written in other languages • allows external access to its capabilities

  15. J2EE: Primary APIs from: Applying Enterprise JavaBeans, V. Matena & B. Stearns, Addison-Wesley, 2001 • Enterprise JavaBeans(EJB) • server component model; provides infrastructure or access to it • JavaServer Pages (JSP) and servlets: • dynamic HTML generation and session management for browser clients

  16. J2EE: Secondary APIs • JNDI (Java Naming and Directory Interface) • access to various naming and directory services • RMI-IIOP (Remote Method Invocation over Internet InterOrb Protocol) • access Java and non-Java objects • JDBC (Java DataBase Connectivity) • uniform access to relational DBs • JTS (Java Transaction Service) • access to distributed transaction services via CORBA OTS • JTA (Java Transaction API) • explicit means of marking transaction boundaries • JMS (Java Messaging Service) • access to asynchronous, reliable messaging systems

  17. Enterprise Beans • enterprise beans are components that • encapsulate business logic • communicate with resource managers • e.g., DBMS, other enterprise beans • clients access enterprise beans to use logic • clients can be application clients, other enterprise beans, web applications, servlets, etc. • at runtime, enterprise beans reside in containers • containers provide deployment and runtime environment • deployment is process of installing bean in container

  18. Enterprise Application (EJB View) • enterprise application consists of: • one or more enterprise beans • either all in one container or in multiple containers • one or more non-enterprise bean components • such as a browser client or a web application • client-view API • provides consistent, location-transparent access to bean • from any kind of client -- same API

  19. Business Entities • business entity is a business object • represents some information maintained by enterprise • has state that is persisted, usually to a database • business rules • constrain state • maintain relationships among business entities • e.g., conditions for making and changing associations, the impact of deleting associations

  20. Business Processes • business process is a business object • encapsulates interaction of process user with business entities (a.k.a. workflow) • may change state of business entity • may be persistent or transient • persistence used when multiple steps involved and possibly multiple actors • e.g., processing a loan application • transience used when process can be completed by one actor via one conversation (e.g., ATM withdrawal)

  21. Business Rules • implemented after identifying entities and processes • those applying to state should be implemented in the component representing that entity • keeps business processes separate from state • e.g., account entity handles preventing negative balances • those applying to process should be implemented in the component representing that process • e.g., ATM withdrawal process specifies withdrawals in units of twenty dollar bills

  22. Enterprise Beans and Business Entities/Processes • beans represent business entities and processes • types of beans and their typical application • session beans (EJB 1.1) • conversational business processes • entity beans (EJB 1.1) • all other business processes and most business entities • message beans (EJB 2.0) • ??? • different life cycles, persistence criteria, programming styles

  23. Session Beans • stateful or stateless (i.e., container-wise) • think “verb” • stateful • retains state associated with client across multiple methods • state does not persist -- goes away after timeout expires • e.g., shopping cart in web shopping process • stateless • no client-specific/visible state retained across methods • all state needed must be passed in or accessed from resources • like old procedures: do job, return result, forget state • e.g., validating a credit card

  24. Entity Beans • always maintains state/persistence • think “noun” • state must be: • loaded • stored • synchronized when changed after loaded • who does it? • container: specifics in deployment information • CMP (Container-Managed Persistence) • bean: specifics coded in bean • BMP (Bean-Managed Persistence)

  25. Entity and Stateful Session Beans

  26. Enterprise Beans and EJB • goal: free developer from system-level concerns to focus on business logic • enterprise bean class: • implements business methods • implements any bean object life cycle methods • may use any other classes • enterprise bean client-view API • home interface (life-cycle interface) • remote interface (business method interface) • deployment descriptor • declaration about bean and its environment

  27. Naming Conventions • enterprise bean class name: • business entity/process name and “Bean” suffix • home interface • business entity/process name and “Home” suffix • remote interface • business entity/process name • name of enterprise bean as a whole • business entity/process name and “EJB” suffix • Example: • Account (business entity), AccountBean, AccountHome, AccountEJB

  28. Home Interface • extends javax.ejb.EJBHome • controls life cycle: • create (defined in enterprise bean class) • find (defined in enterprise bean class) • remove (inherited) • multiple create and find methods allowed • find methods always start with “find” • e.g., findByPrimaryKey() • session beans don’t have find methods • throw exceptions: • RemoteException, CreateException, FinderException

  29. Remote Interface • extends javax.ejb.EJBObject • defines business methods • must throw RemoteException • arguments and return values must conform to RMI-IIOP • inherit • getEJBHome() returns reference to object’s home interface • getPrimaryKey() returns primary key associated with the entity object (remote object must be entity bean) • etc.

  30. Enterprise Bean Class • implements javax.ejb.xxxxBean • NOT remote interface! • allows compile-time vs. runtime checking of interface • implementation of: • life cycle methods defined in home interface • business methods defined in remote interface • container callback methods • e.g., ejbPassivate(), ejbLoad()

  31. Deployment Descriptors • as of EJB 1.1, tags in XML files • part of an ejb-jar file, which defines EJB app • declares behavior externally vs. inside code • allows deployer to change way code works by changing environment, including naming • lists dependencies • groups beans

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