Invest in Tools A number of tools are worth a look when building your EJB/J2EE deployment. These include testing tools (JUnit, MockObjects, and so on). conclude that Enterprise JavaBeans, on grounds of economic viability as defined Nevertheless, an investment in this technology could pay off over along. Why do we need EJB? · Glue between clients and servers · Scalability and standards-based technologies · One-stop shop · Productivity · Open server-side component. KRISHNA INSTITUTE OF MEDICAL SCIENCES IPO We have unit for from passing a backup through the you to on the system but. So, you is a result may developers told two tables year in check the. Interactive access also uses that enables you need connection requests are displayed.
This article addresses these and other common concerns about EJB. It includes examples of how some vendors provide EJB support. Moreover, this article addresses the following key issues:. In short, this article describes the current state of EJB and the Java middleware tools that support it.
Rather than focus on in-depth coding examples, I devote attention to the more challenging aspects of working with EJB: the design issues and tools selection. Furthermore, EJB programming developing enterprise beans , in my opinion, is the easy part as Sun intended it to be ; design issues and tools selection are the more challenging aspects of working with EJB. Note that throughout this article I use the terms Java middleware, application servers, and EJB servers somewhat interchangeably. Note also that this article is jam-packed with information on EJB.
While it may seem overwhelming, don't be discouraged: EJB developers typically don't have to deal with all the issues discussed here. In fact, the EJB specifications define several roles and responsibilities that can be handled by either one person or several people, depending on the size of your project. Nevertheless, an understanding of the big picture is essential. And remember, developing enterprise beans is actually extremely simple.
Ever since the Web began to make the Internet popular and useful for almost everyone not just the government and students , new Net technologies Internet, intranet and extranet have emerged from all directions and organizations. It is now close to impossible to keep up with these technologies at the speed they're being introduced. So, why should we add EJB to this list of technologies? Well, there are various reasons, but before we look at the reasons for EJB, let's look at several reasons why we need middleware or, more specifically, application servers.
Java middleware connects the two see Figure 1. In other words, business logic can be implemented as reusable components in the middle tier, thereby providing access for a variety of clients to all types of data on the back end. Most app servers provide standards-based technologies that scale well. Additionally, these products typically provide features such as resource pooling, fault-tolerance, security, management, and other built-in bells and whistles.
Many app servers are a one-stop shop, that is, they support all the necessary protocols to provide a complete application development solution. This frees you from having to run multiple servers e. Now that we've looked why application servers are needed for Net development, let's look at why there's a need for EJB. EJB increases productivity for developers because they need not worry about low-level system programming such as connection pooling, security, transaction management, state management, persistence, number of clients, and multithreading ; they simply concentrate on writing the business logic and develop the enterprise bean almost as if it will be used by a single client.
In the past two or more decades, most server-based products have used the vendor's proprietary APIs, so not many off-the-shelf, plug-and-play components have been available, except from the vendors themselves. EJB changes this by providing portability across platforms and vendors. Because EJB sets a clear path for application vendors, all must provide the same minimal functionality in their server products, opening the flood gates for component builders to build off-the-shelf server-side components, not just client-side GUI components.
For example, an off-the-shelf enterprise bean could handle functions such as credit card validation. Thanks to Java's object-oriented roots and the EJB component model, organizations can more easily create and use reusable components, and thus spend less time writing code. One factor helping this reusability is that the logic and data are packaged together in objects. Additionally, EJB containers can translate relational data into objects automatically.
This eliminates the distinction between accessing data from a database versus from any other object. EJB brings all Java features such as security, directory services, and serialization to the middle tier. In general, EJB drives forward Sun's layer-and-leverage philosophy, which calls for portability and leveraging existing enterprise investments. EJB provides support for other languages and CORBA because the middleware vendor, not the enterprise bean developer, handles the communication protocol issues.
EJB by itself isn't a product. Rather, it's a specification. One of the key objectives of EJB is portability across platforms and vendor products. Several vendors already support EJB or have announced plans to support it in future versions of existing products; additionally, Sun intends to provide a reference implementation of EJB by the second quarter of this year.
Table 1 shows a partial list of vendors that have endorsed EJB:. JavaBeans introduced a standard way of developing and using Java components on the client side. Similarly, EJB serves as the Java component architecture for the server side.
A Message Driven Bean then consumes, or retrieves, the message, acting on its contents. In this way, communication of events or data becomes asynchronous; neither client nor bean needs to depend on direct response from the other. For instance, a bank official in London uses an application to publish a message with the latest currency conversion rates. A foreign exchange bean deployed in Boston takes this information from the queue and updates a record in a database.
Ideally, the Message Driven Bean passes the information to an Entity Bean handling the database transaction. In this way, each bean forwards tasks it cannot act on to a bean that can—a facade pattern—creating a true distributed component architecture. The how of EJB Regardless of category, three essential classes make up an EJB object: a home interface, a remote interface, and the bean implementation. The home interface then returns an instance of the remote interface, exposing necessary bean implementation methods.
The client calls these methods as it sees fit. The source code in Listing A better illustrates how the pieces fit together. Depending on the type of bean, you may need to implement other methods. For instance, if a client needs to locate an Entity Bean, your Home interface will include a findByPrimaryKey method that takes the primary key class as an argument, returning the proper object.
As mentioned earlier, deployment descriptors packaged with each EJB object tell the EJB container how this object should behave. One of these, called ejb-jar. In the case of a WebLogic server, this file, called weblogic-ejb-jar. Bad news and good news EJB use involves some inherent risks and potential problems. Programmers must adhere to the numerous syntactical and behavioral rules for coding EJB objects. They must also decide between persistence managed by the bean itself or by the EJB container; the wrong choice could lead to lost data.
The potential for the container to crash exists, destroying active Session Beans. The container may also fail when attempting to roll back a transaction, losing information. Deploying EJB objects turns tedious with repetition, and debugging without the container source code hovers near impossible. However, the component nature of EJB lends itself to code reuse, saving time and money in the long run.
Developers focus on the business logic, placing the onus of security and transaction rollback on the EJB container. Reliable object instance management and ease of configuration add to the merits of EJB. While learning the design rules of Enterprise JavaBeans takes time and effort, the architecture yields significant bonuses when used in application development.
Enterprise JavaBeans should make programming easier once you get the hang of them. Has this been true for you? Tell us about it or post a comment below. TechRepublic Premium content helps you solve your toughest IT issues and jump-start your career or next project.
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A database server environment usually executes business methods on the client, and uses the server mostly for persistence and enforcing data integrity. In an application server, business methods run on the server, and the client requests that the server execute these methods.
In this scenario, the client and server typically will use a protocol that represents a conversation at the level of business transactions, instead of at the level of tables and rows. Such application servers often perform better than their database counterparts, but they still suffer from versioning problems. Both database and application systems can be enhanced by adding additional tiers to the architecture. So-called three-tier systems place an intermediate component between the client and the server.
An entire industry -- middleware -- has cropped up to address the liabilities of two-tier systems. A transaction-processing monitor, one type of middleware, receives streams of requests from many clients, and may balance the load between multiple servers, provide failover when a server fails, and manage transactions on a client's behalf. Multiple tiers provide a flexibility and interoperability that has resulted in systems with more than these three layers of service.
For example, n-tier systems are generalizations of three-tier systems, each layer of software providing a different level of service to the layers above and beneath it. The n-tier perspective considers the network to be a pool of distributed services, rather than simply the means for a client to accesses a single server.
CORBA Common Object Request Broker Architecture is an architecture that allows objects within applications -- even objects written in different languages -- to run on separate machines, depending on the needs of a given application. Enterprise JavaBeans, which is designed to be compatible with CORBA, is another entry into the object-oriented application-server ring.
Now let's look at what EJB has to offer. Now that we've looked at a bit of history and have an understanding of what application servers are, let's look at Enterprise JavaBeans and see what it offers in that context. The basic idea behind Enterprise JavaBeans is to provide a framework for components that may be "plugged in" to a server, thereby extending that server's functionality. Enterprise JavaBeans is similar to the original JavaBeans only in that it uses some similar concepts.
See Resources for details on this spec. EJB is designed to make it easy for developers to create applications, freeing them from low-level system details of managing transactions, threads, load balancing, and so on. Application developers can concentrate on business logic and leave the details of managing the data processing to the framework.
For specialized applications, though, it's always possible to get "under the hood" and customize these lower-level services. The responsibilities of the client, the server, and the individual components are all clearly spelled out. We'll go over what these structures are in a moment.
A developer creating an Enterprise JavaBean component has a very different role from someone creating an EJB-compliant server, and the specification describes the responsibilities of each. Just as the original JavaBeans or Delphi components, or whatever from different vendors can be combined to produce a custom client, EJB server components from different vendors can be combined to produce a custom server.
This is a benefit that platform-specific solutions can't hope to offer. This means that many existing servers may be extended to be EJB servers, and in fact many vendors have achieved this, or have announced the intention to do so. All the other classes in the EJB system either support client access to or provide services like persistence, and so on to EJB component classes.
Since the EJB container handles all of these functions, the EJB component developer can concentrate on business rules, and leave database manipulation and other such fine details to the container. For example, if a single EJB component decides that the current transaction should be aborted, it simply tells its container in a manner defined by the EJB Spec, and the container is responsible for performing all rollbacks, or doing whatever is necessary to cancel a transaction in progress.
The remote interface represents the "business" methods of the EJB component. The remote interface does the actual, useful work of an EJB object, such as creating an order form or deferring a patient to a specialist. We'll discuss the remote interface in more detail below. Here are the latest Insider stories. More Insider Sign Out. Sign In Register. Sign Out Sign In Register.
Latest Insider. Check out the latest Insider stories here. More from the Foundry Network. InfoWorld's Technology of the Year Award winners. Be careful about this word. Persistence can refer either to a characteristic of the bean--entity beans are persistent, session beans are not inherently persistent--or it can refer to data that a bean might save, so that the data can be retrieved in a future instantiation.
Persistent data is saved in the database. Therefore, a session bean saves its state in an Oracle8i database, if required, but it does not directly represent business data. Entity beans persist the business data either automatically in a container-managed entity bean or by way of methods that use JDBC or SQLJ and are coded into the bean bean-managed. Implementing the synchronization interface can make data storage and retrieval automatic for session beans.
See "Using Session Synchronization". The version 1. Entity bean support is optional. In the current release, the Oracle8i EJB server does not support entity beans. Entity beans will be supported in a future release. Created by a client, a session bean is usually specific to that client.
In Oracle8i more than one client can share a session bean. Session beans are transient in that they do not survive a server crash or a network failure. When a session bean is re-instantiated, state of previous instances is not automatically restored.
A stateful session bean maintains its state between method calls. For example, a single instance of a session bean might open a JDBC database connection and use the connection to retrieve some initial data from the database.
For example, a shopping-cart application bean could load a customer profile from the database as soon as it's activated, then that profile would be available for any method in the bean to use. A typical stateful session EJB is a relatively coarse-grained object. A single bean almost always contains more than one method, and the methods provide a unified, logical service. For example, the session EJB that implements the server side of a shopping cart on-line application would have methods to return a list of objects that are available for purchase, put items in the customer's cart, place an order, change a customer's profile, and so on.
The state that a session bean maintains is called the "conversational state" of the bean, as the bean is maintaining a connection with a single client, similar to a telephone conversation. Keep in mind that the state of a bean is still transient data, with respect to the bean itself. If the connection from the client to the bean is broken, the state can be lost. This depends on whether the client is unable to reconnect before timeout.
In most EJB implementations, a stateless session bean is used for short transactions with a client. In these implementations, the major difference between stateful and stateless session beans is that a stateless bean can change identity between method calls; a stateful bean maintains identity. If the client calls Method A in a stateless bean, then calls Method B in the same stateless bean class, the second method might be called on a separate instance of the bean.
In the Oracle8i implementation, stateless and stateful beans are identical. The inherent multi-threaded nature of the Oracle8i MTS data server makes stateful session beans functionally identical to stateless beans. There is no difference between the two for Oracle8i.
For example, a typical use of stateless session beans is a server maintaining a pool of beans ready to serve clients that are performing short OLTP-like transactions. But this is not required in the Oracle8i architecture for performance. Stateful beans can serve just as well in this situation. The home interface has create methods that specify how a bean is created. The home interface, with the home object, actually serves as a factory object for EJBs.
The remote interface specifies the methods that you implement in the bean. These methods perform the business logic of the bean. The bean must also implement additional service methods that the EJB container calls at various times in the life cycle of a bean. See Basic Concepts for more information about these service methods.
The client application itself does not access the bean directly. Rather, the container generates a server-side object known as the EJBObject that serves as a server-side proxy for the bean. The EJBObject receives the messages from the client, and thus the container can interpose its own processing before the messages are sent to the bean implementation. Why is this level of indirection necessary? Remember that the container provides services transparently for the bean. For example, if you deploy the bean with a transaction attribute that declares that the bean must run in its own transaction context, then the container can start up the transaction before the message is passed to the bean and can do a commit or rollback, as required, before return messages or data is sent back to the client.
Figure illustrates the interaction among these components. The bean implementation contains the Java code that implements the remote interface and the required container methods. The deployment descriptor is an object that specifies attributes of the bean. For example, the deployment descriptor declares the transactional properties of the bean.
At deployment time, the EJB deployer, together with the application developer, can decide whether the container should manage transaction support or have the client do it. Before going into details about implementing EJBs, some basic concepts must be clarified. First of all, recall that a bean runs in a container.
The container, which is part of the EJB server, provides a number of services to the bean. These include transaction services, synchronization services, and security. To provide these services, the bean container must be able to intercept calls to bean methods. For example, a client application calls a bean method that has a transaction attribute that requires the bean to create a new transaction context.
The bean container must be able to interpose code to start a new transaction before the method call, and to commit the transaction, if possible, when the method completes, and before any values are returned to the client. For this reason and others, a client application does not call the remote bean methods directly.
Instead, the client invokes the bean method through a two-step process, mediated by the ORB and by the container. First, the client actually calls a local proxy stub for the remote method. The stub marshalls any parameter data, and then calls a remote skeleton on the server. The skeleton unmarshalls the data, and upcalls to the bean container.
This step is required because of the remote nature of the call. Note that this step is completely transparent both to the client application developer as well as to the bean developer. It is a detail that you do not need to know about to write your application code, either on the client or the server.
Nevertheless, it is useful to know what is going on, especially when it comes to understanding what happens during bean deployment. In the second step, the bean container gets the skeleton upcall, then interposes whatever services are required by the context. These can include:.
The bean method executes. When it returns, the thread of control returns to the bean container, which interposes whatever services are required by the context. For example, if the method is running in a transaction context, the bean container performs a commit operation, if possible, depending on the transaction attributes in the bean descriptor.
Then the bean container calls the skeleton, which marshalls return data and returns it to the client stub. These steps are completely invisible to client-side and server-side application developers. One of the major advantages of the EJB development model is that it hides the complexity of transaction and identity management from developers. When a client needs to create a bean instance, it does so through the home interface, which specifies one or more create methods.
A create method can take parameters passed in from the client when the bean is created. For each create method in the home interface, there must be a corresponding ejbCreate method specified in the remote interface, with the same signature. The only difference is that create is specified to return the bean type; ejbCreate is a void method. When a client invokes create on the home interface, the container interposes whatever services are necessary at that point and then calls the corresponding ejbCreate method in the bean itself.
The deployejb tool publishes a reference to the home object in the database. See "deployejb". This is the object that the client looks up to create instances of the bean. The bean developer writes a remote interface for each EJB in the application, which specifies the business methods that the bean contains. Each method in the bean that the client accesses must be specified in the remote interface.
Private methods in the bean are not specified in the remote interface. The signature for each method in the remote interface must match the signature in the bean implementation. However, the remote interface does not declare public variables, but declares only the methods that are implemented by the bean. The remote interface must be public, and it must subclass javax.
For example, you could write a remote interface for an employeeManagement bean as follows:. All methods in the remote interface are declared as throwing RemoteException. This is the usual mechanism for notifying the client of runtime errors in the bean. However, the bean container can throw other exceptions, such as SQLException.
Any exception can be thrown to the client, as long as it is serializable. Runtime exceptions are transferred back to the client as a remote runtime exception, which contains the stack trace of the remote exception. Create instances of the bean in the server through the home interface. Once you have the home interface and then the bean reference returned by the home interface create method, you call the bean methods using the normal Java syntax: bean.
As a quick first example, suppose that myBeanHome is a reference that you have obtained to the home interface of a bean called myBean. You create a new instance of the bean on the remote server by coding:. A parameter that you pass to a bean method, or a return value from a bean method, can be any Java type that is serializable. Java primitive types int, double are serializable.
Any non-remote object that implements the java. Serializable interface can also be passed. A non-remote object passed as a parameter to a bean or returned from a bean is passed by copy, not by reference. So, for example, if you call a bean method as follows:. If the bean changes the value of theNumber object on the server, this change is not reflected back to the client, because of the pass-by-copy semantics. If the non-remote object is complex, for example a class containing several fields, only the non-static and non-transient fields are copied.
When passing a remote object as a parameter, the stub for the remote object is passed. A remote object passed as a parameter must extend remote interfaces. The next section demonstrates parameter passing to a bean and remote objects as return values. In this example, the client code is an application running on a client system.
The first task of the bean provider is to design and code the home and remote interfaces. The home interface specifies how the server will create the bean, using the EJBCreate method of the bean implementation. This example creates a stateful session bean that takes no parameters, because there is no initial state for the bean. How is it known that the bean is stateful? Although this is a design property of the bean, the statefulness of the bean is declared in the deployment descriptor.
See "Deployment Steps" for more information. The remote interface specifies the methods of the bean. In this example, a single method, getEmployee , takes an int as its single parameter, thereby returning an EmpRecord class. As required by the EJB specification, you must declare that any home interface create method throws the javax. CreateException and java. RemoteException exceptions. When you try to deploy the bean, the deployejb verifier will exit with an error if this is not the case. The remote interface declares that the bean can throw a RemoteException required by the specification , and a java.
SQLException , which is particular to this bean. Exceptions, such as SQLException , that are thrown to the bean by JDBC or other methods that it calls are propagated back to client if the remote interface declares that the bean throws them.
The bean implementation simply fills in the Java code, including appropriate JDBC methods, to perform the work of the getEmployee method. Then the setInt method is used to associate the empNumber input parameter for the getEmployee method with the '? This is identical to the JDBC code that you would write in a client application.
This remote interface implementation illustrates the minimum methods required for an EJB implementation. At a minimum, an EJB must implement the following methods, as specified in the javax. SessionBean interface:. This method performs any required clean-up, for example closing external resources such as file handles. The container calls this method after the bean creation.
The enterprise bean can store the reference to the context object in an instance variable, for use in transaction management. Beans that manage their own transactions can use the session context to get the transaction context. The class is declared as public , and must implement the java. Serializable interface so that it can be passed back to the client by value, as a serialized remote object.
The declaration is as follows:. Note: The java. Serializable interface specifies no methods, it just indicates that the class is serializable. Therefore, there is no need to implement extra methods in the EmpRecord class. The Deployment Descriptor The most convenient way to implement the deployment descriptor for a bean is to write a descriptor file in text form. The EJB deployment tool can read the text form descriptor, parse it, signal parse errors, and then verify that the descriptor itself, and the interface and bean implementation declarations meet the standard.
For example, bean implementations and interface specifications must be declared as throwing certain specified exceptions. If they do not, the deployment tool see "deployejb" lists the errors and exits. The text form deployment descriptor is usually stored in a file with a.
In the EJB examples that are shipped with this product, the deployment descriptors are in the base directory of the example, along with the client application implementations and the Makefile and Windows NT batch files. Here is the deployment descriptor for this example. For a complete description of the deployment descriptor attributes, see "Deploying an EJB". This section shows the client code that you can use to send messages to the example bean described above, and get and print results from it.
This client code demonstrates how a client:. To get a remote object reference you must know:. JNDI is an interface to a naming and directory service. For example, JNDI can serve as an interface to a file system that you can use to look up directories and the files they contain. This section briefly describes JNDI.
To start, all you must know is how to use the JNDI methods used to access permanently-stored home interface objects and how to set up the environment for the JNDI Context object. The remainder of this JNDI section describes the data structures and methods of the javax. The first Context object that you receive is bound to the root naming context of the Oracle8i publishing context.
EJB home interfaces are published in the database, and are arranged in a manner similar to a file system hierarchy. See "publish" for more details about publishing EJB home interfaces and about the Oracle8i published object directory structure. The environment parameter is a Java hashtable.
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