Distributed Objects

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Distributed Objects

• ????(2)

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??

• More about RMI
• J2EE/EJB

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• More about RMI
• J2EE/EJB

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RMI

• Java????,???????!
• Stub???!
• ?????!
• Garbage Collection!
• ???
• java.rmi.Remote (RemoteException)

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(No Transcript)
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Some important parts of RMI

• Stubs
• Each remote object class has an associated stub
  class, which implements the same remote
  interfaces. An instance of the stub class is
  needed on each client. Client-side remote
  invocations are actually local invocations on
  the stub class.
• Serialization
• Arguments and results have to be
  marshalledconverted to a representation that
  can be sent over the Net. In general this is a
  non-trivial transformation for Java objects.
  Serialization is also used for distributing
  stubs.
• The Server-side Run-time System
• This is responsible for listening for invocating
  requests on suitable IP ports, and dispatching
  them to the proper, local resident, remote object.

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RMI Architecture overview

• RMI Layers
• Stub/skeleton layer
• objects used by client and server applications
• Remote reference layer
• creation/management of remote references
• distributed garbage collection
• Transport protocol layer
• binary data protocol

• By using a layered architecture each layer could
  be enhanced or replaced without affecting the
  rest of the system ? transport layer UDP/IP
  layer or secure sockets (SSL).

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Remote Reference Layer

• RemoteRef
• Interprets and manages references to remote
  objects. The stub objects use the invoke() method
  in RemoteRef to forward the method call. The
  RemoteRef object understands the invocation
  semantics for remote services.
• Leasing for distributed garbage collection
• Naming/Registry Service -- rmiregistry

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Remote Reference Layer

• Invocation Semantics
• v1.1 unicast / point-to-point.
• v1.2 support for activation of dormant remote
  service objects
• Remote Object Activation
• RMI will instantiate a dormant object and restore
  its state from disk.
• As now No multicast semantics.

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Using RMI

• 1. Define interfaces for remote classes
• 2. Create and compile implementation of the
  remote classes
• 3. Create stub and skeleton classes using the
  rmic compiler
• No longer necessary in Java 1.5 because Java 1.5
  adds support for the dynamic generation of stub
  classes at runtime.
• rmic must still be used to pre-generate stub
  classes for remote objects that need to support
  clients running on Java versions 1.4.

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Using RMI

• 4. Create and compile the server application
  (registration)
• 5. Create and compile a client program to access
  the remote objects
• 6. Start the RMI Registry and the server
  application
• 7. Test the client

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Point 1 Remote Interface

• Remote
• All remote interfaces must extend the interface
  java.rmi.Remote (tagging interface)
• All methods must throw a java.rmi.RemoteException
  (extension of java.io.IOException)

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Point 2 Implementation

• UnicastRemoteObject
• Application must implement the defined remote
  interface
• Application extends class UnicastRemoteObject or
  calls explicitly UnicastRemoteObject.exportObject
  
• link to RMI system
• base class performs RMI linking and remote object
  initialization
• constructor may throw a RemoteException
• Activatable
• Base class to be used for activatable objects
• ????????????????????

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Dynamic stubs in Java 1.5

• Dynamic Proxies
• Implemented using java.lang.reflect.Proxy (where
  the implementation is based on a
  RemoteObjectInvocationHandler)
• Dynamic proxy is only used if no pre-generated
  stub class is available or if the system property
  java.rmi.server.ignoreStubClasses true.
• It is only possible if clients run on Java 5.
• Notice If a remote object has pre-1.5 clients,
  then that remote object should use a stub class
  pre-generated with rmic. There are two stub
  class.
• protocols v1.1 / v1.2 (default).

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Point 3 Server

• RMI service must be hosted in a server process
  whose job is
• to create an instance
• to register the object with the naming service.
• Naming/Registry service
• RMI can use different naming services
• (i) Simple service RMI Registry
• (ii) JNDI (Java Naming and Directory interface).

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Point 4 Start RMI Register

• rmiregistry ltportgt
• default port 1099
• error if port is already used by another process
  (e.g. another rmiregistry)
• daemon has to be started in directory which
  contains used classes or the classes have to be
  on the CLASSPATH
• Code Base You must specify where are the class
  files.
• Security policy file You must give permission to
  use port 1099.

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load classes dynamically

• Required classes can be loaded over the network
• e.g. provided by a web server via http
• other protocols are also possible (file//,
  ftp//, .)
• RMI class loading and security. Two conditions
  must be met
• 1. a special class loader is provided
  RMIClassLoader
• 2. a security manager has to support remote class
  loading
• System.setSecurityManager(new RMISecurityManager()
  )
• Start of RMI-Registry in this case
• rmiregistry must not contain the needed classes
  in its path (otherwise what is the point of
  dynamically load the classes?)

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load classes dynamically

• Start of server
• specify codebase for downloading class files
• java -Djava.rmi.server.codebasehttp//10.0.2.112
  8080/calculator.jar nju.ics.yuping.dc.rmi.Calculat
  orServer
• Start of client
• permission to access server has to be provided
  (due to security manager)
• java -Djava.security.policyjava.policy
  nju.ics.yuping.dc.rmi.CalculatorClient
• Policy file
• grant
• // connect to or accept connections on
  unprivileged ports
• // (ports greater than 1024) on host
  loki.cs.fh-aargau.ch
• permission java.net.SocketPermission
• 10.0.2.1122001-", "connect,resolve"

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Point 5 - Codebase
-Djava.rmi.server.codebasefile///e\course\code\
rmi\server\ -Djava.rmi.server.codebasehttp//10.0
.2.1128080/calculator.jar
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Point 6 - Marshalling

• How are parameters transferred to remote objects?
• Primitive Parameters
• passed by value, in a machine-independent format
• Serializable Objects
• serializable objects are copied ? call by value
• Remote Object Parameters
• only the reference to the remote object is
  passed, i.e. a new proxy is generated ? call by
  reference
• Non-Serializable/Remote Objects
• cannot be transferred
• checked at runtime (not by rmic!)

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Note on passing remote objects

• Remote objects are commonly defined as parameters
  and return types.
• Example of usage
• ? Callbacks
• ? Factory classes that create remote references
• Reminder Remote objects are passed by reference.
• When Remote exported objects are passed to a
  client, RMI substitutes the reference with that
  of the Remote proxy (stub).

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Callbacks

• In many cases, applications require more complex
  bi-directional interactions. Servers may wish to
  make calls to the client (this is known as a
  callback). Why?
• ? Error or problem reporting
• ? Periodic updating and progress reports
• ? UI notification (Observer pattern ! )
• In OO programs the role of clients and servers
  are not always clear cut.
• Client-server applications often operate in a
  peer-to-peer manner. At different stages an
  object may either act as a server or as a client.

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Callback How to

• How do you create a callback?
• ? Make your client into a server!
• 1. Make your client implement a Remote interface
• ? Define a client remote interface
• 2. Make it available as a server (export your
  client interface as a Remote object)
• ? extend UnicastRemoteObject
• ? or use UnicastRemoteObject.exportObject(Remote)
• 3. Pass a client Remote reference to the server.
  The server can then use this reference to make
  calls on the client.

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Callback Example
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??

• More about RMI
• J2EE/EJB

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J2EE

• JDBC
• JNDI
• EJB
• RMI
• Java IDL/CORBA
• JSP
• Java Servlet

• XML
• JMS
• JTA
• JavaMail
• JAF

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Application Servers

• "The Multi- tier applications" have several
  independent components
• An application server provides the infrastructure
  and services to run such applications
• Application server products can be separated into
  3 categories
• J2EE-based solutions
• Non-J2EE solutions (PHP, ColdFusion, Perl, etc.)
• And the Microsoft solution (ASP/COM and now .NET
  with ASP.NET, VB.NET, C, etc.)

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J2EE Application Servers

• Major J2EE products
• BEA WebLogic
• IBM WebSphere
• Borland AppServer
• Sun/Oracle GlassFish
• JBoss

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Web Server and Application Server
App Server 1
Internet Browser
Web Server(HTTP Server)
HTTP(S)
App Server 2
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J2EE Multi-tier Model
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J2EE Application Scenarios

• Multi-tier typical application

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J2EE Application Scenarios

• Stand-alone client

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J2EE Application Scenarios

• Web-centric application

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J2EE Application Scenarios

• Business-to-business

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J2EE Architecture
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Now

• JEE 5
• JEE 6
• Homepage http//www.oracle.com/technetwork/java/j
  avaee/tech/index-jsp-142185.html

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Main technologies

• JavaServer Pages (JSP)
• Servlet
• Enterprise JavaBeans (EJB)
• JSPs, servlets and EJBs are application
  components

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JSP

• Used for web pages with dynamic content
• Processes HTTP requests (non-blocking
  call-and-return)
• Accepts HTML tags, special JSP tags, and
  scriptlets of Java code
• Separates static content from presentation logic
• Can be created by web designer using HTML tools

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Servlet

• Used for web pages with dynamic content
• Processes HTTP requests (non-blocking
  call-and-return)
• Written in Java uses print statements to render
  HTML
• Loaded into memory once and then called many
  times
• Provides APIs for session management

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EJB

• EJBs are distributed components used to implement
  business logic (no UI)
• Developer concentrates on business logic
• Availability, scalability, security,
  interoperability and integrability handled by the
  J2EE server
• Client of EJBs can be JSPs, servlets, other EJBs
  and external aplications
• Clients see interfaces

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EJB

• EJB 1.1
• EJB 2.0
• EJB 2.1
• EJB 3.0
• EJB 3.1

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EJB????????

• EJB ????????????????
• ??EJB????
• ????????
• Concurrency,Distribution, Transactions,
• EIS integration, Resource pooling,
• Security, Persistence
• ??EJB????
• ??????????????

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EJB?????

• EJB??????????EJB???????,??EJB??????,??EJB????????
  ?????,????????????
• EJB??????????????????EJB?????,?EJB???????????????
  
• EJB??(2.x)Home??,Remote??,Local?LocalHome??

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EJB??

• Session Bean
• Stateless
• Stateful
• Entity Bean
• Message Driven Bean

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Session Bean

• Stateless session bean
• Contains no user-specific data
• Business process that provides a generic service
• Container can pool stateless beans
• Example shopping catalog

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Session Bean

• Stateful session bean
• Retains conversational state (data) on behalf of
  an individual client
• If state changed during this invocation, the same
  state will be available upon the following
  invocation
• Example shopping cart

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Entity Bean

• Represents business data stored in a database
  persistent object
• Underlying data is normally one row of a table
• A primary key uniquely identifies each bean
  instance
• Allows shared access from multiple clients
• Can live past the duration of client' s session
• Example shopping order

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Message-Driven Bean

• Message consumer for a JMS queue or topic
• Benefits from EJB container services that are not
  available to standard JMS consumers
• Has no home or remote interface
• Example Order processing stock info

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EJB 2.x???

• APIs???????,????????????,???????
• EJBHome interface
• EJBObject interface
• EnterpriseBean interfaces
• JNDI interfaces
• Deployment descriptor
• ??????,??????
• ????,??,??????
• Boiler Code
• ????

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???EJB 2.x??

• // EJB 2.1 Stateless Session Bean Bean Class
• public class PayrollBean implements
  javax.ejb.SessionBean
• SessionContext ctx
• DataSource payrollDB
• public void setSessionContext(SessionContext
  ctx)
• this.ctx ctx
• public void ejbActivate()
• public void ejbPassivate()
• public void ejbRemove()

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???EJB 2.x??

• // EJB 2.1 Stateless Session Bean Bean Class
  (continued)
• public void ejbCreate()
• ...
• Context initialCtx new InitialContext()
• payrollDB (DataSource)initialCtx.lookup
• (javacom/env/jdbc/empDB)
• ...
• public void setTaxDeductions(int empId,int
  deductions)
• ...
• Connection conn payrollDB.getConnection()
• Statement stmt conn.createStatement()
• ...

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???EJB 2.x??

• // EJB 2.1 Stateless Session Bean Interfaces
• public interface PayrollHome
• extends javax.ejb.EJBLocalHome
• public Payroll create() throws CreateException
• public interface Payroll
• extends javax.ejb.EJBLocalObject
• public void setTaxDeductions(int empID, int
• deductions)

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???EJB 2.x??

• // EJB 2.1 Stateless Session Bean Deployment
  Descriptor
• ltsessiongt
• ltejb-namegtPayrollBeanlt/ejb-namegt
• ltlocal-homegtcom.example.PayrollHomelt/local-homegt
• ltlocalgtcom.example.Payrolllt/localgt
• ltejb-classgtcom.example.PayrollBeanlt/ejb-classgt
• ltsession-typegtStatelesslt/session-typegt
• lttransaction-typegtContainerlt/transaction-typegt
• ltresource-refgt
• ltres-ref-namegtjdbc/empDBlt/res-ref-namegt
• ltres-typegtjavax.sql.DataSourcelt/res-typegt
• ltres-authgtContainerlt/res-authgt
• lt/resource-refgt
• lt/sessiongt

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???EJB 2.x??

• // Deployment Descriptor(continued)
• ltassembly-descriptorgt
• ltmethod-permissiongt
• ltunchecked/gt
• ltmethodgt
• ltejb-namegtPayrollBeanlt/ejb-namegt
• ltmethod-namegtlt/method-namegt
• lt/methodgt
• lt/method-permissiongt
• ltcontainer-transactiongt
• ltmethodgt
• ltejb-namegtPayrollBeanlt/ejb-namegt
• ltmethod-namegtlt/method-namegt
• lt/methodgt
• lttrans-attributegtRequiredlt/trans-attributegt
• lt/container-transactiongt
• lt/assembly-descriptorgt

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?????EJB 3.0?

• ??EJB!

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EJB?

• POJO (Plain Old Java Object)
• ??Bean?????Bean??????Java?
• ???????EJB???
• ??
• ???????
• _at_Stateless, _at_Stateful, _at_MessageDriven
• EJB 3.0????Bean
• JPA(Java Persistence API)????
• ??_at_Entity??

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EJB 2.x??EJB?

• // EJB 2.1 Stateless Session Bean Bean Class
• public class PayrollBean implements
  javax.ejb.SessionBean
• SessionContext ctx
• public void setSessionContext(SessionContext
  ctx)
• this.ctx ctx
• public void ejbCreate() ...
• public void ejbActivate()
• public void ejbPassivate()
• public void ejbRemove()
• public void setTaxDeductions(int empId, int
  deductions)
• ...

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EJB 3.0??EJB?

• // EJB 3.0 Stateless Session Bean Bean Class
• _at_Stateless
• public class PayrollBean implements Payroll
• public void setTaxDeductions(int empId,int
  deductions)
• ...

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EJB??

• ??????????
• ????extend EJBObject, EJBHome
• ?????local?remote
• ??_at_Local, _at_Remote
• Remote??????RemoteException

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EJB 2.x??EJB??

• // EJB 2.1 Stateless Session Bean Interfaces
• public interface PayrollHome extends
  javax.ejb.EJBLocalHome
• public Payroll create() throws CreateException
• public interface Payroll extends
  javax.ejb.EJBLocalObject
• public void setTaxDeductions(int empId, int
• deductions)

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EJB 3.0??EJB??

• // Local Interface
• _at_Local
• public interface Payroll
• public void setTaxDeductions(int empId, int
• deductions)
• // Remote Interface
• _at_Remote
• public interface Payroll
• public void setTaxDeductions(int empId, int
• deductions)

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????Bean

• ??
• _at_MessageDriven
• ????Bean????javax.jmx.MessageListener
• onMessage(Message msg)??

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????Bean???

• // EJB 3.0 Message-driven bean Bean Class
• _at_MessageDriven
• public class PayrollMDB implements
  javax.jms.MessageListener
• public void onMessage(Message msg)
• ...

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????

• ???????JNDI Lookup?
• ????
• ??Instance variable
• ??setter??
• ??Lookup
• ??class

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??

• _at_Resource
• For connection factories, simple environment
  entries, topics/queues, EJBContext,
  UserTransaction, etc.
• _at_PersistenceContext
• ?????EntityManager
• _at_PersistenceUnit
• EntityManagerFactory????????EntityManager

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Dependency Injection

• Bean instance is supplied with references to
  resources in environment
• Occurs when instance of bean class is created
• No assumptions as to order of injection
• Optional _at_PostConstruct method is called when
  injection is complete

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???????

• // EJB 3.0 Stateless Session Bean Bean Class
• // Data access using injection and Java
  Persistence API
• _at_Stateless
• public class PayrollBean implements Payroll
• _at_PersistenceContext EntityManager payrollMgr
• public void setTaxDeductions(int empId,int
  deductions)
• payrollMgr.find(Employee.class,empId).
• setTaxDeductions(deductions)

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??Lookup???

• // EJB 3.0 Stateless Session Bean
• // Using dynamic lookup
• _at_PersistenceContext(namepayrollMgr)
• _at_Stateless
• public class PayrollBean implements Payroll
• _at_Resource SessionContext ctx
• public void setTaxDeductions(int empId,int
  deductions)
• EntityManager payrollMgr ctx.lookup(payrollMg
  r)
• payrollMgr.find(Employee.class,
• empId).setDeductions(deductions)

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Bean Lifecycle Event

• EJB 2.1????EnterpriseBean??,???Lifecycle?????
• EJB 3.0???????????
• ??
• _at_PostConstruct
• _at_PreDestroy
• _at_PostActivate
• _at_PrePassivate
• ?????????????

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???EJB??

• ?????????
• ????Home interface
• ????RemoteException

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EJB 2.x???

• // EJB 2.1 Client View
• ...
• Context initialContext new InitialContext()
• PayrollHome payrollHome (PayrollHome)
• initialContext.lookup(javacomp/env/ejb/payroll)
  
• Payroll payroll payrollHome.create()
• ...
• // Use the bean
• payroll.setTaxDeductions(1234, 3)

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EJB 3.0???

• // EJB 3.0 Client View
• _at_EJB Payroll payroll
• // Use the bean
• payroll.setTaxDeductions(1234, 3)

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EJB 3.0 Summary

• Major simplification of EJB technology for
  developers
• Beans are plain Java classes with plain Java
  interfaces
• APIs refocused on ease of use for developer
• Easy access to container services and environment
• Deployment descriptors available, but generally
  unneeded
• EJB 3.0 components interoperate with existing
  components/applications
• Gives developer powerful and easy-to-use
  functionality

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??(????????)

• Java 1.5?RMI???????
• EJB 3.0??EJB 2.1??????