What is, why it exist and HOW it works? Ask Question. Asked 11 years, 8 months ago. Active 11 years, 8 months ago. Viewed 4k times. Marcos Roriz Junior Marcos Roriz Junior 3, 11 11 gold badges 49 49 silver badges 74 74 bronze badges. The wikipedia page is quite brief, and describes it well en.
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Programmers create deployment descriptors during the EJB packaging process, and the descriptors become a part of the EJB deployment when the bean is compiled. Table lists the elements whose values should match in each descriptor file.
Each descriptor file contains elements that identify a bean, and the runtime factory resources it uses:. A given bean or resource factory is identified by the same value in each descriptor file that contains it.
Table lists the bean and resource references elements, and their location in each descriptor file. For instance, for a container-managed persistence entity bean named LineItem , this line:. Security roles are defined in the role-name element in ejb-jar. Any client of an EJB, whether in the same or a different application, accesses it in a similar fashion.
WebLogic Server automatically creates implementations of an EJB's home and business interfaces that can function remotely, unless the bean has only a local interface. You can configure the JNDI namespaces of EJB clients to include the home interfaces for EJBs that reside anywhere on the network—on multiple machines, application servers, or containers. Most beans do not require a global JNDI name—specified in the jndi-name and local-jndi-name elements of weblogic-ejb-jar.
Because of network overhead, it is more efficient to access beans from a client on the same machine than from a remote client, and even more efficient if the client is in the same application.
In this release of WebLogic Server, you can use logical message destinations to map a logical message destination, defined in ejb-jar. If a message destination reference cannot be resolved during deployment, a warning is issued but the deployment will succeed. MDBs linked to unavailable message destinations periodically attempt to connect to the message destination. Until the message destination is available, attempts to look up message-destination-references declared in ejb-jar.
When the message destination becomes available, the MDBs will connect to it and service messages from it. This section describes key WebLogic Server features that ease the process of EJB development, and enhance the performance, reliability, and availability of EJB applications.
WebLogic Server supports pooling, caching, and other features that improve the response time and performance of EJBs. In a production environment, these features can reduce the time it takes for a client to obtain an EJB instance and access and maintain persistent data. WebLogic Server maintains a free pool of ready-to-use EJB instances for stateless session beans, message-driven beans, and entity beans.
The EJB container creates a configurable number of bean instances at startup, so that a new instance does not have to be created for every request. When a client is done with an EJB instance, the container returns it to the pool for reuse. For more information see:. WebLogic Server supports caching for stateful session beans and entity beans. An inactive cached bean instance can be passivated—removed from the cache and written to disk—and restored to memory later as necessary. Passivating bean instances optimizes use of system resources.
You can configure the size of the cache, and related behaviors such as rules for removing beans from the cache. Caching is supported for entity EJBs, whether they use container-managed or bean-managed persistence. Additional caching features are available for EJBs that use container-managed persistence, as described in the following section. WebLogic Server provides these caching capabilities for entity beans that use container managed persistence:.
A group specifies a set of persistent attributes of an entity bean. A field-group represents a subset of the container-managed persistence CMP and container-managed relation CMR fields of a bean. You can put related fields in a bean into groups that are faulted into memory together as a unit. You can associate a group with a query or relationship, so that when a bean is loaded as the result of executing a query or following a relationship, only the fields mentioned in the group are loaded.
For more information, see Specifying Field Groups. You can configure the behavior of the ejbLoad and ejbStore methods to enhance performance, by avoiding unnecessary calls to ejbStore.
As appropriate, you can ensure that WebLogic Server calls ejbStore after each method call, rather than at the conclusion of the transaction. In the beginning, there was the mainframe computer.
And it was good. Or as good as it got, anyway. The state of the art in information processing through the s consisted primarily of big, expensive machines used by large organizations to support their daily business operations. Minicomputers and timesharing in the s increased the accessibility of computing power, but information and processing were still centralized on individual machines. The first personal computers in the s quickly cluttered the corporate landscape with thousands of tiny islands of information, all tirelessly churning out reports of variable quality, losing critical data when they crashed, and quickly becoming inconsistent with each other.
Client-server systems are now commonly composed of various numbers of tiers. The standard old mainframe or timesharing system, where the user interface runs on the same computer as the database and business applications, is known as single tier. Such systems are relatively easy to manage, and data consistency is simple because data is stored in only one place. Unfortunately, single-tier systems have limited scalability and are prone to availability hazards if one computer's down, your whole business goes down , particularly if communication is involved.
Such systems are still common. One garden-variety type of two-tier server performs most of the business logic on the client, updating shared data by sending streams of SQL to the server. In such a system, a properly designed client can be modified to reflect new business rules and conditions without modifying the server, as long as the server has access to the database schema tables, views, and so forth needed to perform the transactions.
The server in such a two-tier system is called a database server, as shown below. Database servers have some liabilities, though. Often the SQL for a particular business function for example, adding an item to an order is identical, with the exception of the data being updated or inserted, from call to call. A database server ends up parsing and reparsing nearly identical SQL for each business function. For example, all SQL statements for adding an item to an order are likely to be very similar, as are the SQL statements for finding a customer in the database.
The time this parsing takes would be better spent actually processing data. There are remedies to this problem, including SQL parse caches and stored procedures. Stateless , Stateful or Singleton. These annotations come from the javax. A message-driven bean or MDB is an enterprise bean that allows you to process messages asynchronously.
This type of bean normally acts as a JMS message listener, which is similar to an event listener but receives JMS messages instead of events. They are in many ways similar to a Stateless session bean but they are not invoked by a client. The onMessage method can call helper methods or can invoke a session bean to process the information in the message. A message can be delivered to a message-driven bean within a transaction context, so all operations within the onMessage method are part of a single transaction.
If message processing is rolled back, the message will be redelivered. To invoke the methods of an EJB locally, the bean can be injected in any managed class running in the container — say a Servlet:. Invoking the method from a remote JVM is trickier and requires a bit more code. As a prerequisite, EJB must implement a remote interface to enable remoting capabilities. You will need to write an EJB client which will perform a lookup over the network. The interface is annotated with Remote :.
First, we created a Context with properties referring to the remote JVM. Once we get the remote EJB instance, we were able to invoke the method. As it happens, the Maven EJB plugin will generate a client jar file which will only have all the remote interfaces. You just need to configure the plugin:. In case of Stateful beans, a new instance of the bean is returned every time a client performs a lookup.
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