CMSC838p Topic Enterprise Java Beans The Entity Beans By Niketu Parekh Prashant Lamba Dr William Pugh EJB Technology EJB Technology based on Java RMI IIOP and JNDI Java RMI IIOP Technology Java Remote Method Invocation over the Internet InterORB Protocol RMI IIOP allows Writing distributed objects in Java Enabling objects to communicate in memory across JVMs and across physical devices RMI IIOP is a special version of RMI which is compliant with CORBA RPC RMI RMI IIOP RPC enables traditional procedures to reside on multiple machines yet still remain in communication A simple way to perform cross process OR cross machine networking RMI a step further allows distributed object communication RMI IIOP allows invocation of methods on remote objects RMI Vs RMI IIOP RMI has some features such as Distributed Garbage Collection Object Activation and Downloadable class files which are not available in RMI IIOP RMI IIOP RMI IIOP exploits the following features of Object Oriented programming Separation of interface of code from its implementation Can change object s proprietary logic w o affecting client Object serialization and parameter passing pass by value pass by reference Java Naming and Directory Interface JNDI A standard way of looking things up over the network JNDI is a system for Java based clients to interact with naming and directory systems A bridge over naming and directory services that provide common interface to disparate directories Users need to use the same API to access LDAP directory NIS directory or Novell s NDS directory JNDI Benefits Single API to access all sorts of directory services information JNDI insulates the application from protocol and implementation details Link different types of directories JNDI J2EE Specific use Use JNDI to connect to resource factories such as JDBC drivers and JMS drivers Use JNDI for beans to lookup for other beans over the network Use JNDI to acquire reference to Java Transactions API JTA User Transaction interface JNDI Architecture JNDI Client API Uniform API for all sorts of directories allowing Java code to perform directory operations JNDI Service Provider Interface An interface to which naming and directory service vendors can plug in their proprietary services Client EJB Component system Interaction Enterprise Bean Class Model business data Java Objects that cache Database Information EJB Object Model business data Java Objects that cache Database Information EJB Object Model business data Java Objects that cache Database Information What is Entity Bean Multi tier Object Oriented Deployment Overview Two kinds of components deployed Application Logic Components Method providers that perform common tasks Session beans model application logic components Persistent Data Components Objects that know how to render themselves into persistent storage They use some persistence mechanism such as serialization O R mapping to a DB or an Object DB Entity beans model these persistent data components What is Entity Bean cont d Entity beans are enterprise beans that An in memory Java representation of persistent data Physical storable parts of an enterprise Store data as fields and have methods associated with them Smart enough to know how to read itself from a storage and populates its fields with the stored data An object that can be modified in memory to change the values of data Persistable so that it can be stored back into storage again thus updating the database data What Makes An Entity Bean Consists of Remote and or local interface Home and or local home interface Enterprise bean class Deployment descriptor Noteworthy Differences Entity bean class maps to an entity definition in a database schema Entity bean class can expose simple methods to manipulate or access that data Primary Key Class makes every entity bean different A primary key contains any number of attributes to uniquely identify an entity bean instance Features of Entity Beans Entity beans survive critical failures such as application server crashing database crashing Entity bean instances are a view into Database Several entity bean instances may represent the same underlying data Entity bean instances can be pooled Persisting Entity Beans Since entity beans map to storage it requires mechanism for database access Two ways to persist entity bean 1 Bean Managed Persistent BMP Entity Bean 2 Container Managed Persistent CMP Entity Bean Bean Managed Persistent BMP Entity Beans Bean provider writes the database access code including looking up a DataSource getting a Connection and sending JDBC statements to the database Container invokes a container callback on the bean when the bean needs to do something with the database and then the code does the actual JDBC work BMP Entity Beans Life Cycle Container Managed Persistent CMP Entity Beans Wimpy in EJB1 1 greatly enhanced in EJB2 0 Bean provider designs the entity bean class choosing which of the bean s fields are part of bean s persistent state Persistent fields map to columns in one or more database tables Container keeps track of changes to the bean s state and updates the real entity as needed The container makes the decision on how to keep the bean and the real entity synchronized based on the state of transactions Bean provider writes EJB QL to tell the container how to do selects Using information in the deployment descriptor the container writes the actual implementation of the CMP bean including implementing the finder and select methods and all of the database access code CMP Entity Beans Life Cycle Passivation and Activation Session Vs Entity Beans Stateless session beans go back to the pool w o Passivation Stateful session beans are passivated when the container puts them to sleep to conserve resources between client method invocations Unlike session beans passivated entity beans are still live objects on the heap ejbPassivate Session Vs Entity Beans Entity Bean called when the bean has finished a business method and is about to go back to the pool In the passivated state the bean has no identity it s not representing any entity from the database For entity beans ejbPassivate is used to release resources that you don t want to waste while the bean is sitting in the pool not running a business method The bean is not serialized or saved so there is no requirement that you null out references to non serializable objects ejbPassivate Session Vs Entity Beans cont d Stateful Session Bean Called when the container