This chapter is from the book
This chapter is from the book
This chapter is from the book
Building the Architectural Prototype: Part 2
Part 2 of building the architectural prototype for our EJB solution presents a primer on EJB that reviews the callback mechanism, interfaces to the clients, and the transaction support available.
A Primer on Enterprise JavaBeans
As mentioned in Chapters 9 and 11, we need to discuss the lifecycle of the EJB and become a bit more familiar with its architecture. As I stated with the coverage of JSP in Chapter 11, this book can't cover the entire topic, so I refer you to one of the many excellent books on the market today covering EJB for a more thorough presentation (earlier I mentioned Richard Monson-Haefel's Enterprise JavaBeans, published by O'Reilly). However, we can cover the basics here, and most of the readers will get the message.
An Enterprise JavaBean is a component that runs within a container that follows the lifecycle model published in the EJB specification. This lifecycle is managed by the container product that is purchased or downloaded from an open-source organization. There are two different lifecycles, depending on whether the bean is a session EJB or an entity EJB.
Session EJBs are typically used to house utility services and coordination logic in dealing with the entity EJBs. Session beans can do anything they like, even make database calls; however, that is not their intent. There are two types of session EJBs, stateful and stateless. Stateful session beans are guaranteed to have their state persisted between calls from an interested client. A good example of a stateful session bean in Remulak is the bean that manages the shopping cart for an order, UCMaintainOrder. Stateless session beans are just the opposite; their state is completely lost between calls from the client. A good example of a stateless session bean in Remulak is the UCMaintainRltnshp bean. If you guessed that session beans make good use-case control classes, you are absolutely right.
Entity EJBs also fall into two categories: those that implement bean-managed persistence (BMP) and those that implement container-managed persistence (CMP). An entity EJB implemented with BMP will look somewhat similar to what we did in Chapter 11. The bean classes will have several of the callback operations that I mentioned previously (and will cover shortly), but the DAO classes will be almost identical. In the case of an entity EJB that implements CMP, as you will see in the first major coding section in this chapter, the landscape will be drastically different. By different, I mean much smaller. First of all, most call-back operations in the beans will be empty. The best part is that there will be absolutely no DAO classes; you can throw them away.
EJB Callback Mechanism
We have mentioned the callback mechanism already; now it's time to describe it a bit more. The EJB container must have strict control over how your beans behave, both session and entity. This one right that you must give up is payback for the container's being able to manage your transactions and the lifecycle of the bean.
Session beans implement four callback operations that are specified in the SessionBean interface:
ejbActivate(): This operation is called by the container when the bean has been deserialized from passive storage on the server. It allows the bean to reclaim any resources freed during passivation.
ejbPassivate(): This operation is called by the container just before the bean is to be serialized and stored in passive storage on the server. It allows the bean to release any nonserializable resources.
ejbRemove(): This operation is called just before the bean is to be removed from the container and made available for garbage collection.
ejbCreate(): This operation is called whenever a complementary create operation is called on the bean's home interface. It can be used to retrieve values that may apply for the duration of the bean's lifetime.
With session beans, depending on what they are being used for, there may be no implementation code in these required operations of the SessionBean interface. In the case of the UCMaintainRltnshp class, all four will be empty. In the case of the UCMaintainOrder session bean, however, because it is stateful, it has logic in both the ejbCreate() and ejbRemove() operations.
Entity EJBs implement seven different callback operations that are specified in the EntityBean interface:
ejbActivate(): This operation is called by the container when the bean has been deserialized from passive storage on the server. It allows the bean to reclaim any resources freed during passivation.
ejbPassivate(): This operation is called by the container just before the bean is to be serialized and stored in passive storage on the server. It allows the bean to release any nonserializable resources.
ejbRemove(): This operation is called just before the bean is to be removed from the container and made available for garbage collection. In the case of BMP, this is where a SQL delete operation would take place. In the case of CMP, the method is typically empty.
ejbCreate(): This operation is called whenever a complementary create operation is called on the bean's home interface. It can be used to retrieve values that may apply for the duration of the bean's lifetime. In the case of BMP, this is where a SQL insert operation would take place. In the case of CMP, this is where the information originating from the client would be assigned to attributes defined in the bean as abstract public.
ejbPostCreate(): This operation is called immediately after the related ejbCreate() method and is the last place to do initialization work before the client has access to the bean. In the case of BMP and CMP, the methods are typically empty.
ejbLoad(): This operation is called when the container deems it necessary to synchronize the state of the database with the state of the bean. The primary key is taken from the EntityContext object of the bean and not its local attribute. In the case of BMP, this is where a SQL select statement is called. In the case of CMP, the method is typically empty.
ejbStore(): This operation is called when the container deems it necessary to synchronize the state of the bean with the database. In the case of BMP, this is where a SQL update statement is called. In the case of CMP, the method is typically empty.
It should be even more clear now after seeing all the callback method descriptions that with the exception of ejbCreate(), CMP entity beans usually have only business methods. Whether an entity bean is implemented with BMP or CMP can mean the difference of hundreds of lines of codes.
Working with an EJB
For clients to be able to access an EJB, it must work with two interfaces. One is called the home interface, the other the remote interface. Each bean we create then will have three Java classes: classnameBean, classnameHome, and classname. The remote interface is just the class name. All of our hard coding work lives in the classnameBean class.
Both of the interfaces eventually obtain access to the bean for the client. Clients may absolutely never directly access the bean. If this were possible, the container would have lost control of the lifecycle of the bean, rendering all the callback mechanisms useless.
The home interface contains operation stubs to both create and find a bean. These operations are implemented in the bean class. The remote interface is where all the business operation stubs are found. All of these concepts can be made clearer with a few sequence diagrams, as we do next.
EJB Sequence Diagrams
The sequence diagram in FIGURE 12-13 shows the creation and removal of an entity EJB implementing BMP.
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FIGURE 12-13 Sequence diagram of BMP creation and removal
The sequence diagram in FIGURE 12-14 shows CMP creation and removal. In both cases, the remote interface is referenced by EJBObject. In addition, the EJBHome, EJBObject, and Container objects are all part of the EJB container. In UML terms, the EJB container is a component that is an active object.
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FIGURE 12-14 Sequence diagram of CMP creation and removal
Notice again that the bean instance in the CMP world doesn't do much more than satisfy business method requests.
EJB Transaction Management
Another big advantage to an application's using an EJB container is transaction management. Although you can "roll your own" transaction scheme in the EJB world, it is better if you rely on the container to do it for you. First, it reduces the amount of code that has to be written. Second, it is less error prone.
As with everything else in the EJB environment, transaction control is handled by deployment descriptors. We call this declarative transaction management. Unlike EJB competitors such as Microsoft's COM+ and Component Services framework, the transaction scope can be set at both the bean and the method levels. This is more important than it sounds. Remember that a bean can have both inquiry-type operations and update-type operations. It would be very inefficient to have to establish all the overhead of beginning a transaction just to read information from the database. In all cases with Remulak, container-managed transactions will be the norm.
EJB containers support the transaction types identified in the EJB specification. The various transaction levels are as follows:
NotSupported: The transaction scope is not propagated to beans marked NotSupported.
Supports: Beans marked Supports will join the transaction scope of another bean already involved in a transaction. However, if the bean is invoked without a transaction scope already established, it runs without a transaction.
Required: Beans marked Required must be invoked with transaction already begun or they will create a transaction scope.
RequiresNew: Beans marked RequiresNew will always start a new transaction.
Mandatory: Beans marked Mandatory must always be part of a client transaction scope. If they are invoked without a transaction already started, the transaction will fail.
Never: Beans marked Never must never be invoked with a transaction scope already begun. If they are, the transaction will fail.
These options may seem a bit overwhelming at first, but here comes the good news. If you like the use-case design pattern we have been preaching so far, you will love what it means to transaction settings. All update-oriented operations in the use-case control classes will be marked RequiresNew. All other operations in the use-case control classes will be marked Supports. All other beans can be marked at the bean level as Supports. Because the use-case controller session beans are orchestrating everything, it makes sense that operations that perform updates, such as rltnAddCustomer(), should always start a new transaction. All other beans executed during the transaction just join with the transaction scope that the controller starts. The following is a snippet of the <assembly-descriptor> tag that is part of the ejb-jar.xml file that must be packaged with each deployment:
<assembly-descriptor>
<container-transaction>
<method>
<ejb-name>UCMaintainRltnshp</ejb-name>
<method-intf>Remote</method-intf>
<method-name>*</method-name>
</method>
<trans-attribute>RequiresNew</trans-attribute>
</container-transaction>
<container-transaction>
<method>
<ejb-name>UCMaintainRltnshp</ejb-name>
<method-intf>Remote</method-intf>
<method-name>rltnCustomerInquiry</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
<container-transaction>
<method>
<ejb-name>UCMaintainRltnshp</ejb-name>
<method-intf>Remote</method-intf>
<method-name>rltnAddressInquiry</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
<container-transaction>
<method>
<ejb-name>UCMaintainRltnshp</ejb-name>
<method-intf>Remote</method-intf>
<method-name>rltnRoleInquiry</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
The <container-transaction> tag tells the EJB container how to manage the transaction. The <ejb-name> tag identifies the bean, and the next two important tags are <method-name> and <trans-attribute>. Luckily, the EJB specification allows us to indicate a wild-card character—in this case an asterisk (*)—for the method name. This character tells the container to treat all methods of the bean in the same way.
Notice also that we have three additional blocks of <container-transaction> tags. These override the first one. So in effect the tags just mentioned together indicate that all methods in UCMaintainRltnshp will be set to RequiresNew, but rltnCustomerInquiry(), rltnAddress Inquiry(), and rltnRoleInquiry() will be set to Supports. Following the advice already given, the remaining beans in the Maintain Relationships use-case should be marked Supports, as shown here:
<container-transaction>
<method>
<ejb-name>CustomerBean</ejb-name>
<method-intf>Remote</method-intf>
<method-name>*</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
<container-transaction>
<method>
<ejb-name>AddressBean</ejb-name>
<method-intf>Remote</method-intf>
<method-name>*</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
<container-transaction>
<method>
<ejb-name>RoleBean</ejb-name>
<method-intf>Remote</method-intf>
<method-name>*</method-name>
</method>
<trans-attribute>Supports</trans-attribute>
</container-transaction>
</assembly-descriptor>
With container-managed transactions (not to be confused with container-managed persistence), we don't explicitly commit a unit of work and we don't explicitly roll back a unit of work. The container determines what to do according to the exceptions that are thrown. If no exceptions are thrown, then upon completion of the method that started a transaction (all use-case control classes that are update oriented, in Remulak's case), the transaction will commit.
Anytime RuntimeException, RemoteException, or any exception that inherits from either of these exceptions occurs, a rollback will result. The exception called DAOSysException that was introduced in Chapter 11 inherits from RuntimeException. The other exceptions all inherit from DAOAppException, which inherits from Exception. These are all application errors and won't cause a rollback. A common approach taken to force a rollback when an application exception occurs is to throw EJBException, which inherits from RuntimeException. This is a smart strategy because many application-level exceptions may be able to be corrected, allowing for a recovery.