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Before going on, let s first establish some conventions. When we speak about an enterprise bean as a whole, its component interfaces, bean class, and so forth, we will call it by its common business name, followed by the word bean. For example, an enterprise bean that is developed to model a cabin on a ship will be called the Cabin EJB. Notice that we didn t use a constant width font for Cabin. We do this because we are referring to all the parts of the bean (the component interfaces, bean class, etc.) as a whole, not just one particular part like the remote interface or bean class. The term enterprise bean denotes any kind of bean including entity, session, or message-driven beans. Similarly, entity bean denotes a entity type enterprise bean; session bean a session type enterprise bean; and message-driven bean a message-driven type enterprise bean. It s popular to use the acronym EJB for enterprise bean, a style adopted in this book to distinguish an enterprise bean as a whole from its component parts. We will also use suffixes to distinguish between local component interfaces and remote component interfaces. When we are talking about the remote interface of the Cabin EJB we will use combine the common business name with the word Remote. For example, the remote interface for the Cabin EJB is called the CabinRemote interface. In EJB 2.0, the local component interface of the Cabin EJB would be the CabinLocal interface. The home interfaces follow the convention by adding the word Home to the mix. The remote and local home interfaces for the Cabin EJB would be CabinHomeRemote and CabinHomeLocal respectively. The bean class is always the common business name followed by the word Bean. For example, the Cabin EJB s bean class would be named CabinBean. The remote interface Having introduced the machinery, let s look at how to build an entity or stateful enterprise bean with remote component interfaces. In this section, we will examine the Cabin EJB, an entity bean that models a cabin on a cruise ship. Let s start with its remote interface. We ll define the remote interface for a Cabin bean using the interface called CabinRemote, which defines business methods for working with cabins. All remote-interface types extend the javax.ejb.EJBObject interface. import java.rmi.RemoteException; public interface CabinRemote extends javax.ejb.EJBObject { public String getName() throws RemoteException; public void setName(String str) throws RemoteException; public int getDeckLevel() throws RemoteException; public void setDeckLevel(int level) throws RemoteException; } These are methods for naming the cabin and methods for setting the cabin s deck level; you can probably imagine lots of other methods that you d need, but this is enough to get started. All of these methods declare that they throw Copyright (c) 2001 O’Reilly & Associates
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local interfaces don t need to declare that they throw RemoteException, and so on. An enterprise bean isn t required to provide a local interface, if you know when you re developing the enterprise bean that it will only interact with remote clients. Likewise, an enterprise bean doesn t need to provide a remote interface if it knows it will only be called by enterprise beans in the same container. You can provide local or remote component interface or both. The complexity particularly all the confusion about classes implementing the methods of an interface but not implementing the interface itself comes about because enterprise beans exist in the middle between some kind of client software and some kind of database. The client never interacts with a bean class directly; it always uses the methods of the entity or session bean s component interfaces to do its work, interacting with stubs that are generated automatically. (For that matter, a bean that needs the services of another bean is just another client: it uses the same stubs, rather than interacting with the bean class directly.) Although the local component interfaces (local and local home) in EJB 2.0 represent session and entity beans in the same address space and do not use distributed object protocols, they still represent a stub or proxy to the bean class. While there is no network between co-located beans, the stubs allow the container to monitor the interactions between co-located beans and apply security and transactions as appropriate. Its important to note, that EJB 2.0 s message-driven bean doesn t have any component interfaces, but it may become the client of other session or entity beans and interact with those beans through their component interfaces. The entity and session beans with which the message-driven bean interacts may be co-located, in which case it uses their local component interfaces, or they may be located in a different address space and EJB container, in which case the remote component interfaces are used. There are also lots of interactions between an enterprise bean and its server. These interactions are managed by a container, which is responsible for presenting a uniform interface between the bean and the server. (Many people use the terms container and server interchangeably, which is understandable because the difference between them isn t clearly defined.) The container is responsible for creating new instances of beans, making sure that they are stored properly by the server, and so on. Tools provided by the container s vendor do a tremendous amount of work behind the scenes. At least one tool will take care of creating the mapping between entity beans and records in your database. Other tools generate a lot of code based on the component interfaces and the bean class itself. The code generated does things like create the bean, store it in the database, and so on. This code (in addition to the stubs) is what actually implements the component interfaces, and is the reason your bean class doesn t have to. Copyright (c) 2001 O’Reilly & Associates
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interface extends javax.ejb.EJBLocalObject. The local interface is one of the bean s component interfaces and is used by session and entity beans in conjunction with the local home interface. EJB 2.0: Local Home interface The home interface defines the bean s life cycle methods that can be used by other beans co-located in the same EJB container: that is, the life-cycle methods a bean presents to other beans in the same address space. It allows beans to interact without the overhead of a distributed object protocol, which improves their performance. The local home interface extends javax.ejb.EJBLocalHome. The local home interface is one of the bean s component interfaces and is used by session and entity beans in conjunction with the local interface. Bean class The session and entity bean classes actually implement the bean s business and life-cycle methods. Note, however, that the bean class for session and entity beans usually does not implement any of the bean s component interfaces directly. However, it must have methods matching the signatures of the methods defined in the remote and local interfaces and must have methods corresponding to some of the methods in the both the remote and local home interfaces. If this sounds perfectly confusing, it is. The book will clarify this as we go along. An entity bean must implement javax.ejb.EntityBean; a session bean must implement javax.ejb.SessionBean. The EntityBean and SessionBean extend javax.ejb.EnterpriseBean. The message-driven bean in EJB 2.0 does not use any of the component interfaces, because it is never accessed by method calls from other applications or beans. Instead, the message-driven bean contains a single method, onMessage(), which is called by the container when a new message arrives. So the message-driven bean does not have a component interface as does the session and entity beans, it only needs the bean class to operate. The message-driven bean class implements the javax.ejb.MessageDrivenBean and javax.jms.MessageListener interfaces. The JMS MessageListener interface is what makes a message-driven bean specific to JMS, instead of some other protocol. EJB 2.0 requires the use of JMS, but future versions may allow other messaging systems. The MessageDrivenBean, like the EntityBean and SessionBean, extends the javax.ejb.EnterpriseBeaninterface. Primary key The primary key is a very simple class that provides a pointer into the database. Only entity beans need a primary key; the only requirement for this class is that it implements java.io.Serializable. EJB 2.0 adds the crucial distinction between remote and local interfaces. Local interfaces provide a way for beans in the same container to interact efficiently; calls to methods in the local interface don t involve RMI; the methods in the Copyright (c) 2001 O’Reilly & Associates
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respective entity beans. Session and message-driven beans like TravelAgent, which are responsible for making a reservation on a cruise, can even access a database directly and perform reads, updates, and deletes to data. But there s no TravelAgent record in the database once the bean has made reservation is, it waits to process another. What makes this distinction difficult is that it s extremely flexible. The relevant distinction for Enterprise JavaBeans is that an entity bean has persistent state; the session and message-driven beans model interactions but do not have persistent state. Classes and Interfaces A good way to understand the design of enterprise beans is to look at how you d go about implementing one. To implement entity and session enterprise beans, you need to define the component interfaces, a bean class, and a primary key: There are basically two kinds of component interfaces, remote and local. The remote interfaces are supported by both EJB 2.0 and 1.1 while the local component interfaces are new in EJB 2.0 and are not supported by EJB 1.1. Remote interface The remote interface for an enterprise bean defines the bean s business methods that can be accessed from applications outside the EJB container: the business methods a bean presents to the outside world to do its work. It enforces conventions and idioms that are well suited for distributed object protocols. The remote interface extends javax.ejb.EJBObject, which in turn extends java.rmi.Remote. The remote interface is one of the bean s component interfaces and is used by session and entity beans in conjunction with the remote home interface. Remote Home interface The home interface defines the bean s life cycle methods that can be accessed from applications outside the EJB container: the life-cycle methods for creating new beans, removing beans, and finding beans. It enforces conventions and idioms that are well suited for distributed object protocols. The home interface extends javax.ejb.EJBHome, which in turn extends java.rmi.Remote. The remote home interface is one of the bean s component interfaces and is used by session and entity beans in conjunction with the remote interface. EJB 2.0: Local interface The local interface for an enterprise bean defines the bean s business methods that can be used by other beans co-located in the same EJB container: the business methods a bean presents other beans in the same address space. It allows beans to interact without the overhead of a distributed object protocol, which makes them more performant. The local Copyright (c) 2001 O’Reilly & Associates
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The Enterprise Bean Component Enterprise JavaBeans server-side components come in three fundamentally different types: entity, session, and message-driven beans. Both session and entity beans are RMI based server-side components that are accessed using distributed object protocols. The message-driven bean, which is new to EJB 2.0, is an asynchronous server-side component that responds to JMS asyncrhonous messages. A good rule of thumb is that entity beans model business concepts that can be expressed as nouns. For example, an entity bean might represent a customer, a piece of equipment, an item in inventory, or even a place. In other words, entity beans model real-world objects; these objects are usually persistent records in some kind of database. Our hypothetical cruise line will need entity beans that represent cabins, customers, ships, etc. Session beans are an extension of the client application and are responsible for managing processes or tasks. A Ship bean provides methods for doing things directly to a ship but doesn t say anything about the context under which those actions are taken. Booking passengers on the ship requires that we use a Ship bean, but also requires a lot of things that have nothing to do with the Ship itself: we ll need to know about passengers, ticket rates, schedules, and so on. A session bean is responsible for this kind of coordination. Session beans tend to manage particular kinds of activities, for example, the act of making a reservation. They have a lot to do with the relationships between different enterprise beans. A TravelAgent session bean, for example, might make use of a Cruise, a Cabin, and a Customer all entity beans to make a reservation. Similarly, the message-driven beans in EJB 2.0 are responsible for coordinating tasks involving other session and entity beans. The major difference between a message-driven bean and a session bean is how they are accessed. While a session bean provides a remote interface that defines which methods can be invoked, a message-driven bean does not. Instead, the message driven bean subscribes or listens for specific asynchronous messages to which it responds by processing the message and managing the activities of other beans in response to those messages. For example, a TravelAgent message-driven bean would receive to a asynchronous messages perhaps from a legacy reservation system from which it would coordinate the interactions of the Cruise, Cabin, and Customer beans to make a reservation. The activity that a session or message-driven bean represents is fundamentally transient: you start making a reservation, you do a bunch of work, and then it s finished. The session and message-driven beans do not represent things in the database. Obviously, session and message-driven beans have lots of side effects on the database: in the process of making a reservation, you might create a new Reservation by assigning a Customer to a particular Cabin on a particular Ship. All of these changes would be reflected in the database by actions on the Copyright (c) 2001 O’Reilly & Associates
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2 Architectural Overview As you learned in Chapter 1, Enterprise JavaBeans is a component model for component transaction monitors, the most advanced type of business application server available today. To effectively use Enterprise JavaBeans, you need to understand the EJB architecture, so this book includes two chapters on the subject. This chapter explores the core of EJB: how enterprise beans are distributed as business objects. Chapter 3 explores the services and resource management techniques supported by EJB. To be truly versatile, the EJB component design had to be smart. For application developers, assembling enterprise beans is simple, requiring little or no expertise in the complex system-level issues that often plague three-tier development efforts. While EJB makes it easy for application developers, it also provides system developers (the people who write EJB servers) with a great deal of flexibility in how they support the EJB specification. The similarities among different component transaction monitors (CTMs) allow the EJB abstraction to be a standard component model for all of them. Each vendor s CTM is implemented differently, but they all support the same primary services and similar resource management techniques. The primary services and resource management techniques are covered in more detail in Chapter 3, but some of the infrastructure for supporting them is addressed in this chapter. Copyright (c) 2001 O’Reilly & Associates
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In 1997, Sun Microsystems was developing the most promising standard for server-side components called Enterprise JavaBeans. Sun offered some key advantages. First, Sun was respected and was known for working with vendors to define Java-based and vendor-agnostic APIs for common services. Sun had a habit of adopting the best ideas in the industry and then making the Java implementation an open standard usually successfully. The Java database connectivity API, called JDBC, was a perfect example. Based largely on Microsoft s own ODBC, JDBC offered vendors a more flexible model for plugging in their own database access drivers. In addition, developers found the JDBC API much easier to work with. Sun was doing the same thing in its newer technologies like the JavaMail API and the Java Naming and Directory Interface (JNDI). These technologies were still being defined, but the collaboration among vendors was encouraging and the openness of the APIs was attractive. Although CORBA offered an open standard, it attempted to standardize very low-level facilities like security and transactions. Vendors could not justify rewriting existing products such as TUXEDO and CICS to the CORBA standards. EJB got around that problem by saying it doesn t matter how you implement the low-level services; all that matters is all the facilities be applied to the components according to the specification a much more palatable solution for existing and prospective CTM vendors. In addition, the Java language offered some pretty enticing advantages, not all of them purely technical. First, Java was a hot and sexy technology and simply making your product Java-compatible seemed to boost your exposure in the market. Java also offered some very attractive technical benefits. Java was more or less platform independent. A component model defined in the Java language would have definite marketing and technical benefits. As it turned out, Sun had not been idle after it announced Enterprise JavaBeans. Sun s engineers had been working with several leading vendors to define a flexible and open standard to which vendors could easily adapt their existing products. This was a tall order because vendors had different kinds of servers including web servers, database servers, relational database servers, application servers, and early CTMs. It s likely that no one wanted to sacrifice their architecture for the common good, but eventually the vendors agreed on a model that was flexible enough to accommodate different implementations yet solid enough to support real mission-critical development. In December of 1997, Sun Microsystems released the first draft specification of Enterprise JavaBeans, EJB 1.0, and vendors have been flocking to the server-side component model ever since.
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EJB and CORBA CTMs Until the fall of 1997, non-Microsoft CTMs were pretty much nonexistent. Promising products from IBM, BEA, and Hitachi were on the drawing board, while MTS was already on the market. Although the non-MTS designs were only designs, they all had one thing in common: they all used CORBA as a distributed object service. Most non-Microsoft CTMs were focused on, what was at the time, the more open standard of CORBA so that they could be deployed on non-Microsoft platforms and support non-Microsoft clients. CORBA is both language and platform independent, so CORBA CTM vendors could provide their customers with more implementation options5. The problem with CORBA CTM designs was that they all had different server-side component models. In other words, if you developed a component for one vendor s CTM, you couldn t turn around and use that same component in another vendor s CTM. The component models were too different. With Microsoft s MTS far in the lead by 1997 (it had already been around a year), CORBA-based CTM vendors needed a competitive advantage. One problem CTMs faced was a fragmented CORBA market where each vendor s product was different from the next. A fragmented market wouldn t benefit anyone, so the CORBA CTM vendors needed a standard to rally around. Besides the CORBA protocol, the most obvious standard needed was a component model, which would allow clients and third-party vendors to develop their business objects to one specification that would work in any CORBA CTM. Microsoft was, of course, pushing their component model as a standard which was attractive because MTS was an actual working product but Microsoft didn t support CORBA. The OMG (Object Management Group), the same people who developed the CORBA standard, were defining a server-side component model. This held promise because it was sure to be tailored to CORBA, but the OMG was slow in developing a standard at least too slow for the evolving CTM market6. 5 Recently, the introduction of SOAP (Simple Object Access Protocol) brings into question the future of the CORBA IIOP protocol (Internet-InterOperability Protocol). It s obvious that these two protocols are competing to become the standard language- independent protocol for distributed computing. IIOP has been around for several years and is therefore far more mature, but as a late entry SOAP may quickly catch up by leveraging lessons learned in the development of IIOP. 6 Eventually, CORBA s CTM component model was released and called CCM, for CORBA Component Model. It has seen lackluster acceptance is general, and was forced to adopt Enterprise JavaBeans as part of its component model just to be viable and interesting.
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Microsoft s .NET Framework Microsoft was the first vendor to ship a CTM. Originally called the Microsoft Transaction Server (MTS), it was later renamed COM+. Microsoft s COM+ is based on the Component Object Model (COM), originally designed for use on the desktop but eventually pressed into service as a server-side component model. For distributed access, COM+ clients use DCOM (Distributed Component Object Model). When MTS was introduced in 1996, it was exciting because it provided a very comprehensive environment for business objects. With MTS, application developers could write COM components without worrying about system-level concerns. Once a business object was designed to conform to the COM model, MTS (and now COM+) would take care of everything else, including transaction management, concurrency, resource management everything! Recently, COM+ has become part of Microsoft s new .NET Framework. The core functionality provided by COM+ services remains essentially the same in .NET, but the way it s appears to a developer changes significantly. Rather than writing components as COM objects, applications written for the .NET Framework are built as managed objects. All managed objects, and in fact all code written for the .NET Framework, depends on a Common Language Runtime (CLR). For Java-oriented developers, the CLR is much like a Java VM, and a managed object is very analogous to an instance of a Java class, i.e., to a Java object. Although .NET Framework provides many interesting features, as an open standard, it falls short. The COM+ services in the .NET Framework are Microsoft s proprietary CTM, which means that using this technology binds you to the Microsoft platform. This may not be so bad, because .NET promises to work well, and the Microsoft platform is pervasive. In addition, the .NET Framework s support for SOAP (Simple Object Access Protocol) will enable business objects in the .NET world to communicate with objects on any other platform written in any language. This can potentially make business objects in .NET universally accessible, a feature that is not easily dismissed. If, however, your company is expected to deploy server-side components on a non- Microsoft platform, .NET is not a viable solution. In addition, the COM+ services in the .NET Framework are focused on stateless components; there s no built-in support for persistent transactional objects. Although stateless components can offer higher performance, business systems need the kind of flexibility offered by CTMs that include stateful and persistent components. Copyright (c) 2001 O’Reilly & Associates
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CTMs and Server-Side Component Models CTMs require that business objects adhere to the server-side component model implemented by the vendor. A good component model is critical to the success of a development project because it defines how easily an application developer can write business objects for the CTM. The component model is a contract that defines the responsibilities of the CTM and the business objects. With a good component model, a developer knows what to expect from the CTM and the CTM understands how to manage the business object. Server-side component models are great at describing the responsibilities of the application developer and CTM vendor. Server-side component models are based on a specification. As long as the component adheres to the specification, it can be used by the CTM. The relationship between the server-side component and the CTM is like the relationship between a CD-ROM and a CD player. As long as the component (CD-ROM) adheres to the player s specifications, you can play it. A CTM s relationship with its component model is also similar to the relationship the railway system has with trains. The railway system manages the train s environment, providing alternate routes for load balancing, multiple tracks for concurrency, and a traffic control system for managing resources. The railway provides the infrastructure that trains run on. Similarly, a CTM provides server-side components with the entire infrastructure needed to support concurrency, transactions, load balancing, etc. Trains on the railway are like server-side components: they all perform different tasks but they do so using the same basic design. The train, like a server-side component, focuses on performing a task, such as moving cars, not managing the environment. For the engineer, the person driving the train, the interface for controlling the train is fairly simple: a brake and throttle. For the application developer, the interface to the server-side component is similarly limited. Different CTMs may implement different component models, just as different railways have different kinds of trains. The differences between the component models vary, like railway systems having different track widths and different controls, but the fundamental operations of CTMs are the same. They all ensure that business objects are managed so that they can support large populations of users in mission-critical situations. This means that resources, concurrency, transactions, security, persistence, load balancing, and distribution of objects can be handled automatically, limiting the application developer to a simple interface. This allows the application developer to focus on the business logic instead of the enterprise infrastructure. Copyright (c) 2001 O’Reilly & Associates
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