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Entity beans a closer look
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We ve gone into great detail to make an important point. As you ve seen, EJB 1.x supported only remote interfaces, leading to unacceptable performance. The EJB 2.x specification solves this problem by adding local interfaces. You ll soon see that local interfaces are a necessary addition, even though they remain a compromise. First, let s consider the additional improvements in the EJB 2.x specification.
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2.4.2 Container-managed relationships
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Local interfaces were a key prerequisite for another new entity bean CMR. CMR make it possible for an entity bean to have complex relationships with other entity beans. To guarantee that CMR perform well, the EJB 2.0 team required that all CMR always relate to a local interface. For example, if an Invoice entity bean has a relation to a LineItem entity bean, the LineItem bean must have a defined local interface. The introduction of CMR lets you implement more fine-grained object models with entity beans. With EJB 2.0, the Invoice-LineItem relationship that was not recommended in the EJB 1.1 specification is now efficient. You have local interfaces, a session fa ade, and CMR, so your EJB problems are solved, right Take a deeper look, then decide for yourself.
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2.5 Entity beans a closer look
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Let s look at what happens inside a container when an entity bean method is invoked. First, we ll see what happens when we implement an example using EJB 2 CMP entity beans. Then, we ll compare it to what we actually need.
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2.5.1 Employee management
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Imagine that we are developing an employee management system. One requirement of the system is that it be capable of changing an employee s home address. Additionally, the system must be designed with high fault-tolerance limits in mind, so this operation must be idempotent.3 If we were to implement this algorithm as a session bean that employs EJB2 CMP entity beans for representing the Employee and Address domain objects, our code might look like that in listing 2.5.
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3 An idempotent operation is one that can be repeated without altering the final state of the system. That is, if an idempotent method is invoked twice, the final state of the system will be the same as if it had been invoked only once. i = i+1 is not idempotent, but i = 9 is. Idempotent EJB methods can be transparently re-executed in the event of container failover.
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The bitter cost
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Listing 2.5 Session fa ade for Employee bean
public class EmployeeManager implements SessionBean { /** * Finds the employee referenced by <code>employeeId</code> and * Changes the home address to reflect the new information. */ public void resetHomeAddress (long employeeId, String addr1, String addr2, String city, String state, String zip) { EmployeeEJB employee = employeeHome.findByPrimaryKey (new Long (employeeId)); AddressEJB address = employee.getHomeAddress () address.setAddress1 (addr1); address.setAddress2 (addr2); address.setCity (city); address.setState (state); address.setZip (zip); } }
Let s assume that the session bean method in listing 2.5 is invoked from a remote client, and that the client allows the application server to perform transaction management. Additionally, assume that EmployeeEJB and AddressEJB are local interfaces for CMP 2 entity beans, and that the methods invoked on these entity beans represent CMP and CMR fields. The interaction between our application objects and the EJB container demonstrates that the EJB 2 specification is relatively efficient compared to what we would have seen with EJB 1, but is still component-oriented. The EmployeeEJB get method and each AddressEJB set method in the example code will be interposed with container code to check the state of the current transaction and ensure that the current security principal has access to the AddressEJB methods invoked. While this overhead is small compared to the cost of method parameter serializing or RMI communications overhead, it does increase significantly the amount of work that the JVM must do each time a persistence operation is performed. If entity beans were accessed through local interfaces only, this overhead would be hard to justify because the security and transactional EJB services would have already been performed at the session bean invocation level. Now what if our session bean looped over an arbitrarily long list of addresses You can see that the number of interactions between the session bean code and the application server would be proportional to the number of operations that
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