Introduction to EJBs: Part 3
This is the third of four installments that make up Chapter 4: Introduction to EJBs from the book Sams Teach Yourself J2EE in 21 Days (ISBN:0-672-32384-2) written by Martin Bond, Dan Haywood, Debbie Law, Andy Longshaw, and Peter Roxburgh, published by Sams Publishing.
How Do I Create an EJB?
The Creation Mechanism
As you may have gathered from the previous discussion on EJB contents, the EJB developer must go through the following cycle:
Design and define the business interface. This may involve mapping from a UML model of the solution into Java.
Decide on a bean type appropriate to the task in hand. Entity, Session, and Message-driven beans all have their own pros and cons. If you choose to use a Session bean, another question is whether to use a stateful Session bean or a stateless Session bean. Choice of the appropriate type is discussed in more detail on Days 5, 6, 7, and 10.
Decide which home interface methods are appropriate for the bean type and define the home interface for the EJB.
Create (or generate) a "boilerplate" bean with correct lifecycle methods.
Create your business logic by filling out the business methods.
Fill out lifecycle methods to control creation, destruction and to manage state (if applicable).
If your EJB classes are written correctly, all that remains is to wrap them up as a deployable unit. However, there are certain caveats you should bear in mind while creating your bean.
Caveats on Code Creation
Due to the managed nature of the bean lifecycle, the EJB container imposes certain restrictions on the bean including:
EJBs cannot perform file I/O. If you need to log messages or access files, you must find an alternative mechanism.
EJBs are not allowed to start threads. All threading is controlled by the container.
EJBs cannot call native methods.
EJBs cannot use static member variables.
There is no GUI available to an EJB, so it must not attempt to use AWT or JFC components.
An EJB cannot act as a network server, listening for inbound connections.
An EJB should not attempt to create classloaders or change factories for artifacts, such as sockets.
An EJB should not return this from a method. Although not strictly a restriction (the container will not prevent you from doing it), it is identified as being a very bad practice. This relates to the earlier discussion that a bean should not implement its associated remote interface. This would potentially give a client a direct remote reference to the bean rather than the EJBObject. Instead, the bean should query its EJB context for a reference to its associated EJBObject and return that to the caller.
For a full list of restrictions, see section 24.1.2 of the EJB 2.0 specification (available online at http://java.sun.com/products/ejb/docs.html).
Create the Deployable Component
One alternative definition of a component is "a unit of deployment." Following this theme, a component should
Contain all the information required to deploy it, above and beyond the classes. This is the metadata discussed earlier.
Be bound up in such a way that it can easily be transported and deployed without losing any parts along the way.
Consequently, after the classes and interfaces for an EJB have been created, the following steps must be performed:
Capture the EJB's metadata in a universally understood format. This takes the form of an XML-based deployment descriptor (DD).
Bundle the classes and deployment descriptor up in a deployable format, namely a JAR file.
The Deployment Descriptor
The EJB specification defines a standard format of an XML deployment descriptor document that can house EJB metadata. The exact format of a deployment descriptor is usually hidden behind tools that manipulate them on your behalf. However, it is worth examining some of the contents of a deployment descriptor to see how the EJB fits together and how extra information and metadata is provided.
Listing 4.5 shows the deployment descriptor for the example Agency EJB.
Listing 4.5 Agency Bean EJB Deployment Descriptor
1: <?xml version="1.0" encoding="UTF-8"?> 2: 3: <!DOCTYPE ejb-jar PUBLIC '-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 2.0//EN' 'http://java.sun.com/dtd/ejb-jar_2_0.dtd'> 4: 5: <ejb-jar> 6: <display-name>Simple</display-name> 7: <enterprise-beans> 8: <session> 9: <display-name>Agency</display-name> 10: <ejb-name>Agency</ejb-name> 11: <home>agency.AgencyHome</home> 12: <remote>agency.Agency</remote> 13: <ejb-class>agency.AgencyBean</ejb-class> 14: <session-type>Stateless</session-type> 15: <transaction-type>Bean</transaction-type> 16: <env-entry> 17: <env-entry-name>AgencyName</env-entry-name> 18: <env-entry-type>java.lang.String</env-entry-type> 19: <env-entry-value>J2EE in 21 Days Job Agency</env-entry-value> 20: </env-entry> 21: <security-identity> 22: <description></description> 23: <use-caller-identity></use-caller-identity> 24: </security-identity> 25: <resource-ref> 26: <res-ref-name>jdbc/Agency</res-ref-name> 27: <res-type>javax.sql.DataSource</res-type> 28: <res-auth>Container</res-auth> 29: <res-sharing-scope>Shareable</res-sharing-scope> 30: </resource-ref> 31: </session> 32: </enterprise-beans>
The essential parts of the deployment descriptor in Listing 4.5 are
The <session> tag delimits the definition of the Agency EJB and indicates that it is a Session EJB (lines 8 and 31).
The <ejb-name> tag defines the name of the EJB, in this case Agency (line 10).
The home and remote interface types (as defined by their fully-qualified class filenames) are specified by the <home> and <remote> tags, respectively (lines 1112). The type of the bean itself is defined by the <ejb-class> tag (line 13).
In addition, two other parts are of particular note at this point in time:
An environment entry is defined between lines 16 and 20 by using the <env-entry> tag. This indicates that a String property called AgencyName should be made available to the bean. The value of the property is J2EE in 21 Days Job Agency. The environment defined in the deployment descriptor is made available through JNDI under the name java:comp/env. In this case, the agency name can be retrieved by looking up the name java:comp/env/AgencyName. This lookup can be seen in the ejbCreate() method of Listing 4.3.
An external resource is defined in lines 2530 using the <resource-ref> tag. This defines that a DataSource should be made available to this EJB under the name jdbc/Agency. As with the environment entry for the agency name, this resource is made available through JNDI under java:comp/env, so the EJB can retrieve the DataSource by looking up the name java:comp/env/jdbc/Agency. Again, this lookup can be seen in the ejbCreate() method of Listing 4.3.
It is important to realize that the name used for a <resource-ref> is only a logical name. In other words, it is just a text string used by a component to reference an external resource. In theory, the resource name used by the EJB to refer to the data source could be anything (foo, for example) as long as it ties in with the information in the deployment descriptor. However, by convention, such names are kept in line with the name you would expect to use under JNDI. As a result, in this example, the data source resource is referred to by the bean as jdbc/Agency and will be registered under JNDI with the same name.
All of the EJB classes and the deployment descriptor should then be bundled up in a JAR file. The deployment descriptor should be named ejb-jar.xml. If there are multiple EJBs packaged in the same JAR file, the deployment descriptor will have multiple EJB definitions in it. This JAR file is then termed an EJB-JAR file to denote its payload. The JAR file itself can be called anything (within reason) and has a .jar file extension.
The EJB-JAR file can also contain any extra resources required by the EJB, such as application-specific configuration information that does not fit in a deployment descriptor environment entry.
Although the EJB-JAR file is now complete, it must form part of an application to serve a useful purpose. J2EE defines that enterprise applications can be built from components (Web, EJB, and application components). The key is how to define the relationships between the different parts of the applicationthere must be some way of plugging things together.
The answer is that there must be a description of the application itself, which components it uses, how those components relate to each other, and which specific resources they use. This is the information provided by the Application Assembler and Deployer roles.
To provide this information to the target J2EE platform, another level of deployment descriptor is used the J2EE deployment descriptor. The J2EE deployment descriptor provides the following:
A list of the components in the application
Security role information
Web root information for Web components
This information is stored in an XML file called application.xml. All of the constituent component JAR files (such as EJB-JARs) and the J2EE deployment descriptor are then bundled up in another JAR file, this time called an Enterprise Archive (EAR) file, which has a .ear extension. The contents of the J2EE deployment descriptor will be covered in more detail as you examine the different parts of the example enterprise application.
Is the application now ready to deploy? Unfortunately, the answer is "Not quite yet." The J2EE deployment descriptor does not cover information about how to map the application onto a specific J2EE application server, specifically
The JNDI name under which the application server will make the EJB available. In the case of the Agency bean, this would mean that an entry was required to map the bean name of Agency to the JNDI name under which the EJB is registered, for example, ejb/Agency.
Information about how the security roles defined map to underlying security principals (this is covered on Day 15).
So, yet another XML-based deployment descriptor is required to contain this information, this time an application server-specific one. This file contains extra mapping information, as previously described, and also any other container-specific information required for a smooth deployment in that environment. This extra deployment descriptor is also stored in the EAR file, ready to be accessed when the application is deployed.
The specific deployment descriptor for the J2EE Reference Implementation (RI) server is called sun-j2ee-ri.xml.
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