Tuesday, September 11, 2007

CMP-EJB vs. Hibernate

The J2EE field is agog with excitement about a very popular Open Source technology - Hibernate. This technology being elevated to the status of JCP standard. Feedback from J2EE programmers in industry says that knowledge of Hibernate is mandatory for all J2EE aspirants.

Hibernate is an ORM Object-Relational-Mapping technology. It is an Open-Source and free technology, developed in SourceForge. net. There have been a number of such ORM technologies,in recent past. . TopLink is one such tool, subsequently adopted by Oracle and so proprietary. Hibernate from SourceForge and OJB(Object-Relational-Bridge) from Apache are two well known ORM tools, open-source and free. JDO, also falls within the same category.

Gavin King is the lead for Hibernate and Craig Russell & David Jordan, the lead authors for SUN-sponsored JDO effort. Due to some technical problems, it appears that the majority in JCP favours Hibernate today instead of JDO. At first reading though, the difference is not, all that apparent. The syntax and the approach appear to be almost same, but Hibernate syntax is easier to learn.  

It is interesting to note that Craig Russell works for SUN and Gavin King is now with JBoss. It shows that JCP is a democratic community and SUN is not dictating terms except to protect the language and its enterprise-level users.

EJB-3, is the latest version and it is heavily influenced by Hibernate. Some readers equate EJB-3 with Hibernate. Oracle supports EJB-3 proposals and as it is the main Database company in j2ee world, EJB-3 has bright future. J2EE by its very name is an Enterprise level technology, and as EJB is the essence of such Enterprise applications, because of the built-in container services offered, the significance of the surging interest in Hibernate can be really appreciated only in association with EJB and hence a detour into EJB is inevitable.

EJB has three types. One type is the SESSION BEAN, residing in ENTERPRISE container, which can be thought of as a function-bean, invoked in RMI-IIOP style. Such session-bean, may be either stateless or stateful. The stateless bean working in Enterprise container has an exact counter-part in Microsoft COM+(MTS), but the other types are said to be available in MS platform only through third-party extensions.

ORM tools have been sometimes used along with Session beans. The only problem till recently was that they were proprietory and rather costly. But nowadays, very reliable open-source ORM tools are available, and even Richard Monson Haefel approves this method as a safe and productive alternative to Entity beans.

The other branch, the ENTITY BEAN has been less lucky. EJB-1. 1, EJB-2. 0 and then EJB-2. 1, have meant a number of changes in the specification relating to Entity Beans.
We can say that an Entity bean is an 'Attribute bean' or 'property-bean', with setter and getter methods, invoked in RMI-IIOP style and persisted in Enterprise container. The pattern of defining a typical Javabean is a recurring theme in Java. The same style occurs in BDK, EJB-Entity beans, Struts, JSF and now in Hibernate too. So, it is very important and elegant.

The third branch is Messaging paradigm and MDB. An Enterprise by its very name implies huge number of customers and concurrent transactions, RPC style being like telephone call, could result in 'line-engaged!' problem. If the call involves the called person referring to some records before replying, it leads to line- blocking. But, messaging style, as in email, atleast ensures that the message has been sent. It is evident that dubbing RPC( read 'telephone') as unsuitable, is over-statement. Sometimes, we desire immediate response,too. By the same token, even XML webservice, if it is really serious, should adopt messaging style and it does. MDB (Message-Driven bean) has weathered the storm and is in fact gaining more and more acceptance.

So, why is it that Entity beans alone were found wanting and the specification keeps on changing?
Entity beans are of two types. CMP & BMP.
CMP stands for Container-Managed Persistence and BMP stands for Bean-managed persistence. Theoretically, the EJB specification does not say anything about the method to be adopted in persisting objects for permanent storage and retrieval. It could be simple object serialization. The database may be object-database or Object-relational database or XML. In practice, however, a database has always meant a Relational Database and its SQL.

In CMP, the coder deals with objects, only in memory. He creates new objects, modifies them, deletes them and views them, all in memory. The task of saving these objects in memory ,to the relational database table is done by the container, automatically. The coder does not write any sql-related code for this.

In BMP, the coder has to write the sql to persist the object in memory to the relational database.

CMP in EJB1. 1 was suitable for simple tables, without complex relationships to other tables. CMP avoids all references to the underlying database. So, it is more portable. There is no vendor-lock-in. CMP can persist data to Object- databases also, besides Relational databases.

But, CMP is not always suitable. If the database is some legacy type, which cannot be used with SQL, the database company gives a proprietory code for persistence and such code has to be used in our program to persist data. The facilities given in CMP originally were found to be too elementary and there were complaints.

But, what matters is that CMP makes use of ORM concepts, though the implementation left much to be desired. It did not expose how the EJB vendor implements it. Weblogic, Oracle, IBM WebSphere, SUN , JBoss, each may implement CMP in any way that they deem fit. Except in special circumstances, it will be better to use CMP, not merely because, it makes the code more portable & is easy to write. Much more important reason is that the EJB container can optimize the performace dramatically, if we adopt CMP. So the developer community wanted to adopt CMP but found it unsuitable for really complex jobs.

Even with all these improvements, CMP was found to be less than the ultimate solution. There was no spossibility for Inheritance.  

Though the container services provided by the EJB container are indispensable in a truly large enterprise application, the J2EE camp is almost vertically split into WebTier & EJB-Tier votaries. The WebTier supporters claim that EJB with its steep learning curve and error prone development environment for developers is not really necessary for most applications. And they would like to have an ORM tool, built into the J2EE specification. For afterall, ORM task is not specific to EJB alone. Even Servlets and JSP could use them. In fact, they have been using them, though the J2EE specification was silent about it. ORM tools like OJB, JDO and Hibernate can be used not only in EJB containers but in webcontainer and even in standalone containers. Gavin King makes it a special point in favour of Hibernate. Making such a tool, a J2EE standard, would make development tasks far easier ,to develop either web-tier application or ejb-tier application. saving us from the medley of classpath to required jars.

In a scathing attack on the complexity and questionable performance of EJB Entity beans, Rod Johnson, prophesies, that in a few years time, J2EE will cease to include EJB. Whether, we agree or not, it is worth looking into the criticisms against EJB Entity beans, raised by him. ( 'J2EE Development without EJB' - Wrox/Wiley/DreamTech-2004). For, he is proposing the Spring Framework as an alternative to EJB container and the idea is gaining ground. J2EE developers and students may have to re-orient themselves rather abruptly, to remain relevant to industry.

Source: in.geocities.com/rsramsam



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