Transcript session7 - Advanced Java

Advanced Java
Session 7
New York University
School of Continuing and Professional Studies
Objectives
• RMI
– Overview
– Example
• CORBA
– Overview
– Using CORBA in Java
• EJB
– What is an EJB
– Entity/Session Beans
– Services
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Remote Method Invocation
• A mechanism to invoke methods on a
“remote” object and receive return values
• The object that makes the call is the “client”
and the object that responds is the “server”
• Same object could be client for one call and
server for another
• Heart of EJB and Distributed Applications
• Modern replacement for CORBA (and
RPC)
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Goal of RMI
• Make it easier to write distributed
application
• Provide an interface that’s transparent
• Easy to understand
• Easy to use
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Calling a Method on Remote
Object
Client
Server
Method call
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Role of RMI
Client
stub
rcvr
Server
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Stub Method
• Builds an information block consisting of
– An identifier of the remote object to be used
– A description of the method to be called
– Marshalled parameters
• Sends this information to the server
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Receiver Object
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Unmarshalls the parameters
Locates the object to be called
Calls the desired method
Captures and marshals the return value or
exception of the call
• Sends the package containing marshalled
return value back to the stub on the client.
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Stub Upon Return
• Unmarshalls the return values returned from
the remote object
• Returns it back to the client
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Classes and Interfaces for RMI
• Java.rmi package
• Interface that will be used by the client
• Implementation class – that must extend
UnicastRemoteObject *and* implement the
interface defined for remote clients
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Setting up RMI
• Create and compile the classes
• Use “rmic” to generate the stub and the
receiver “glue”
• Run “rmiregistry” service to provide
registration/lookup services
• Launch the server – register the object
• Run the client – accesses remote method
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Common Object Request Broker
Architecture
• Proposed and maintained by OMG – Object
Management Group
• Uses an IDL (Interface Definition
Language) to define interfaces
• Uses an ORB (Object Request Broker) as
the mediator between clients and servers.
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CORBA
advantages/disadvantages
• For accessing legacy applications
• Works across multiple programming
language
• CORBA language bindings for scripting
languages like Perl, Python
• Too complex
• Difficult to use
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Writing CORBA clients/servers
• Write the interface that specifies how the object works
using IDL
• Using the IDL compilers for the target language (idlj for
java), generate the needed stub and helper classes
• Add the implementation code
• Write a server program that creates and registers the server
object
• Write a client program that locates the server object and
invokes the method
• Start the naming service (similar to rmiregistry), run the
server program, and then run the client
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Java and CORBA
Packages
• Package org.omg.CORBA
• Package org.omg.CORBA_2_3
Classes
• org.omg.CORBA.ORB
• org.omg.CosNaming.NamingContext
• org.omg.CosNaming.NameComponent
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Enterprise Java Beans
• Server-side component architecture
• Enables and simplifies the process of building
enterprise-class distributed object applications that
are scalable, reliable, and secure
• Analogous to the hardware components model
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N-Tier Architecture
Presentation
logic
Presentation layer
Pricing
component
billing
component
driver
database
Business logic layer
Data Layer
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EJB Servers and Containers
Client (servlet, applet, app)
EJB Server
EJB Container
EJB Container
EJB
EJB
EJB
EJB
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Enterprise Beans
• Session Beans
– Typically represent business processes (like authorize
credit card)
– Last only through the duration of the client
– Stateless Session Beans
– Stateful Session Beans
• Entity Beans
– Typically models permanent data objects – such as
credit card, account, customer
– Are long lasting, or persistent objects
– Bean managed persistence
– Container managed persistence
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Enterprise Bean Model
Client
code
EJB Server/container
create
Home Object
Method call
Home
interface
EJB Object
Remote
interface
delegate
EJB
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Services Provided by Container
Client (servlet, applet, app)
EJB Server/Container
invokes
•Resource management
•Lifecycle management
delegates
EJB
•State management
•Transactions
•Security
•Persistence
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Resource Management
EJB Container is responsible for coordinating the
entire effort of resource management for resources
such as
– Threads
– Socket Connections
– Database Connections
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Lifecycle Management
Controls the lifecycle of the Bean
Creates or destroys beans in response to client
requests
Provides “instance pooling”
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State Management
• Stateful beans or Entity Beans need to maintain a
“state” for each clients
• The container provides services to maintain the
state
• The same state may be passed on to another bean if
necessary before invoking a method that a client
requested
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Transactions
• EJBs may participate in transactions
• EJB Container handles the underlying transaction
operations, coordinating efforts behind the scenes.
• Java Transaction API is used to implement
transactions for beans
• Variety of transaction management options are
available
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Security
• EJB containers add transparent Security to the
Beans
• Enterprise Beans can automatically run as a certain
security identity, or can programmatically ensure
that clients are authorized to perform desired
operations
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Persistence
• Containers can provide transparent persistence for
Entity Beans
• EJBs can manage their own persistence if they
prefer
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Remote Accessibility
Location Transparency
• Converts a network-naïve component to a
networked component
• Containers use Java RMI to specify remote
accessibility
• Gives sysadmins the ability to upgrade a certain
machine while clients are routed to another (better
fault tolerance, availability)
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Glue Code
Bean Installation Tools
• Containers provide glue code tools. These tools are
meant to integrate beans into the EJB container’s
environment
• Glue code tools (deployment tools) are responsible
for transforming an enterprise bean into a fully
managed, distributed server-side component.
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Specialized Container Features
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Integration to mainframe systems
COM+ integration
Transparent fail-over
Stateful recovery
Server clustering
Dynamic redeployment
Monitoring support
Visual development environment integration
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Bean classes and interfaces
• The EnterpriseBean class contains implementation
details of your component. It extends Serializable.
• SessionBean – extends EnterpriseBean
• EntityBean – extends EnterpriseBean
• EJBObject class represents the “surrogate” object
that is created instead of your Bean object
• Remote Interfaces for your bean must extend
EJBObject
• EJBHome class represents the factory object
• Home Interfaces for your bean must extend
EJBHome
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Enterprise Bean Model
Client
code
EJB Server/container
create
Home Object
Method call
Home
interface
EJB Object
Remote
interface
delegate
EJB
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EJBObject
• getEJBHome – retrieves a ref to
corresponding Home object
• getPrimaryKey – returns the primary key for
this object
• Remove – destroys this EJB object (and
persistent store)
• getHandle – acquires a “handle” for this EJB
obejct
• isIdentical – tests if two EJB Objects are
identical
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EJBHome
• getEJBMetaData – access info about the bean
– e.g. whether it’s a session bean, or entity
bean, it supports transactions or not etc.
• remove – destroys a particular EJB object
• create(…) methods – that are not part of the
Interface EJBHome
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SessionBean
• setSessionContext
• ejbPassivate
• ejbActivate
• ejbRemove
• ejbCreate(…) methods that are not part of the
interface
• Other business methods
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Bean client code
• Locate the Bean’s Home interface using JNDI
Context ctx = new InitialContext();
IHome home = ctx.lookup(“MyHome”);
• Create an instance of the EJB Object
IRemote remote = home.create(…);
• Call a business method using the Remote interface
remote.authorizeCreditCard(…..);
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Deployment Descriptor
• Bean home name
• Enterprise bean class name
• Home interface class name
• Remote interface class name
• Re-entrant
• Stateful or stateless
• Session timeout
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Entity Beans
• Persistent objects
• Long-lived
• Survive failures
• Typically a view into a database
• Can be pooled
• Several Entity Beans may represent the same
underlying data
• Can be created, removed or found
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EntityBean interface
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setEntityContext
unsetEntityContext
ejbRemove
ejbActivate
ejbPassivate
ejbLoad
ejbStore
ejbCreate is optional for entity beans and returns a
primaryKey object
• ejbFindByPrimaryKey and other ejbFind methods
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Thank you
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