To those of you familiar with Enterprise JavaBeans (EJB), deployment descriptors are nothing new. Essentially, a deployment descriptor's purpose is to collect declarative information that can be modified during deployment of an enterprise bean. Deployment descriptors are a key element in the component-based development capabilities of EJB. They allow users to modify, link and deploy EJB in a graphical environment rather than having to perform low-level code changes to reuse a component. The latest public draft of EJB specification 1.1 includes sections on the XML DTD for deployment descriptors, an important step toward enterprise bean portability between EJB servers.
In the spirit of JDJ's focus on XML this month, I'll cover some important pieces of the newly released XML DTD for deployment descriptors in EJB specification 1.1 (section 16.6). As this is being written I have yet to find a production EJB server that supports the DTD (if you know of one, please let me know!); however, I'll include a "best shot" example descriptor using the DTD so you get a feel for the format of its metadata. The example is based on the Intergalactic Ticket System's TicketEntityBean from September's EJB Home column (JDJ Vol. 4, issue 8). For reference see the online source code at the JDJ Web site.
A Look Inside the EJB's XML DTD
A deployment descriptor contains two kinds of metadata (data about data) for an enterprise bean:
Structural information describes specific characteristics of an enterprise bean such as whether it is an entity or session bean, and any external dependencies it might have. This information must be included in an ejb-jar file by its producer. Structural information about an enterprise bean is entered by the bean provider, and it's advisable not to modify this information during ejb-jar assembly or deployment. Table 1 lists common structural information about an enterprise bean that you might include in a deployment descriptor.
- Structural information
- Application assembly information
Application assembly information is the responsibility of the application assembler. It describes how an enterprise bean in an ejb-jar file is assembled into a coarse-grained deployment unit. This information isn't mandatory, and it may be modified at deployment time. However, be aware that an enterprise bean's behavior may be affected by these changes. Table 2 lists common application assembly information you might encounter in a deployment descriptor.
The EJB specification describes the responsibilities of roles during the life cycle of an enterprise bean's development and deployment. The roles of bean provider and application assembler are separate, and they are assigned different responsibilities regarding the creation/modification of a deployment descriptor. There will be overlap in the responsibilities of each role, however, resulting in an application assembler's possibly modifying structural information about an enterprise bean. I played each role in creating the example descriptor.
TicketEntityBean XML Deployment Descriptor
The EJB XML DTD (enough with the acronyms!) defines how to describe both session and entity beans. Some element tags, like env-entry a description of a bean environment property pertain to both bean types. Others are specifically for session or entity beans only. The TicketEntityBean XML descriptor example clarifies common entity bean elements for you. Listing 1 contains the complete source code of this XML descriptor.
As indicated earlier, the DTD contains structural information about an enterprise bean and may contain assembly information as well. I'll begin analyzing the structural information about the TicketEntityBean deployment descriptor and finish with the assembly information.
First, notice the top line of the ejb-jar descriptor in the TicketEntityBean descriptor. A well-formed, valid deployment descriptor must refer to the DTD with the statement:
<!DOCTYPE ejb-jar PUBLIC "-//Sun <microsystems Inc.//DTD Enterprise JavaBeans 1.1//EN">
Sun Microsystems has plans to deliver an ejb-jar file verifier that will check for malformed XML as described in EJB specification 1.1, section 16.6. A verifier will provide bean providers and application assemblers the ability to validate their work, ensuring correct DTD semantics are upheld.
Next, I continue the XML descriptor with the ejb-jar element.
This element is the root of an EJB deployment descriptor, which may contain multiple enterprise beans. This month's example contains only one enterprise bean. The ejb-jar element may contain optional descriptions of the ejb-jar, its icon files, display name and an assembly-descriptor section, but it must include at least one enterprise bean element.
The enterprise bean element contains one or more declared enterprise beans: session or entity. The XML descriptor will not be valid if this element is empty.
Within this section I enter the structural information about my TicketEntityBean.
TicketEntityBean's Structural Information
The entity element in my example contains the structural information that is the bean provider's responsibility to enter.
The entity element has mandatory and optional entries to allow the bean provider to add detail to the bean as needed.
Mandatory Entity Elements
Mandatory elements are the ejb-name, home, remote, ejb-class, persistence-type, prim-key-class and reentrant fields.
For all but the ejb-name element, I have simply copied the values from my previous deployment descriptor file from the September column into these elements as necessary. Thus you can see that the example's home element contains the fully qualified name of the entity bean's home interface class. Similarly, the remote and ejb-class hold the fully qualified class names of the remote and entity bean, respectively.
<ejb-class> jdj.ticketing.containermanaged.TicketEntityBean </ejb-class>
Note: ejb-name is simply a logical name for the enterprise bean. It is not the JNDI that will be assigned by the deployer at a later time. Also, ejb-name must be unique for a given ejb-jar file.
Because TicketEntityBean is container-managed, my descriptor's persistence-type is "Container," and like the home and remote elements, I have entered the prim-key-class in fully qualified form.
The last mandatory entry in the entity element is whether or not the bean is reentrant. I have listed my TicketEntityBean as False.
Optional Entity Elements
The entity element can optionally include a description, display name and icon files. Additional optional fields may be contingent on factors such as whether the bean is container-managed or bean-managed. These include cmp-field (container-manager field), prim-key-field, env-entry (a bean environment property), ejb-ref (references to other ejbs in the ejb-jar file), security-role-ref and resource-ref (reference to external resources). Because the TicketEntityBean is container-managed, I've included some of these elements in my descriptor.
This is an example of an optional environment property where the entity bean has declared an idleTimeoutSeconds environment property with a value of 5.
more env-entries possible...
Following is an example of a few of the container-managed fields declared for the TicketEntityBean:
more cmp-fields possible...
Application Assembly Information
Table 2 describes numerous elements that an application assembler can include in the descriptor. All are optional; thus the assembly descriptor may be left out of the ejb-jar file.
The assembly-descriptor element
can optionally contain information about security roles, method permissions and an enterprise bean's transaction semantics with its container. I have opted to have no role-based security on my TicketEntityBean, nor any special method permissions based on role. However, another ticket agency reusing this entity bean may decide to place a security restriction on the bean, allowing only TicketAgent roles to access it. In this case an entry would be made by the application assembler to include the security role element:
Likewise, they would want to include which methods are restricted to this new security role in the form of the following tag:
I don't have security information to worry about; however, I do want to describe how my entity bean behaves within the context of a transaction. To do so I have to include an element, container-transaction. The full assembly-descriptor element is listed below.
In this example I list the entity bean in question (i.e., ejb-name), which methods are managed by the container (i.e., method-name) and exactly how they are managed (i.e., trans-attribute). For my purposes I am dealing with the TicketEntityBean, in which all methods signified by an asterisk (*) are required (Required) to operate within a transaction context.
The assembly-descriptor adds character to your enterprise beans that is not or cannot be determined by the bean provider. As security or transaction context needs evolve, the assembly-descriptor enhances EJB's component-model capabilities by allowing modification of bean properties without code changes.
With the addition of a DTD for deployment descriptors in the EJB specification 1.1, Sun Microsystems is stepping in the right direction toward an end goal of interoperability for enterprise beans among EJB vendors' products. A standard allowing EJB developers and deployers to speak a common language will only increase the efficiency of EJB application development.
As with the first draft of any spec, there are holes in the DTD that still need to be filled. For example, there is no description of how container-managed fields map into their persistent storage, and at the time of this writing sections 16.4 and 16.5, deployer's responsibilities and container provider's responsibilities, haven't been specified. When these sections are detailed, an element to describe a bean's JNDI name, for instance, will become self-evident (no pun intended!). In the next column I'll cover other portability issues beyond XML, with regards to the EJB specification.
Jason Westra is a managing partner with Verge Technologies Group, Inc., a Java consulting firm specializing in Enterprise JavaBeans solutions. He can be reached at
<!DOCTYPE ejb-jar PUBLIC "-//Sun <Microsystems Inc.
//DTD Enterprise JavaBeans 1.1//EN">
This ejb-jar file contains the assembled enterprise
bean(s) that make up the
Intergalactic Ticket System.
The TicketEntityBean represents a ticket entry made by a
ticket agent. It is container managed and currently assembled
to deploy against a JDBC backend.