Java Middleware Patrick Eugster, Till Bay, Tomas Hruz Java - - PowerPoint PPT Presentation

Languages in Depth Series: Java Programming

• Prof. Dr. Bertrand Meyer

Chair of Software Engineering

Java Middleware

Patrick Eugster, Till Bay, Tomas Hruz

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Languages in Depth series: Java Programming

Java Middleware

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

RPC in General

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

Remote Procedure Call (RPC) in General

• Object has
• Interface (abstract type)
• Implementation (concrete type)
• Local reference, e.g., value of a monotonically increased

counter, memory address

• Remote » object has
• Interface for remote invocations
• Described in the implementation language: 1st class

RPC Otherwise: described in a separate language

• Usually local interface limited to access inside the process
• Implementation
• Global reference, e.g., (host id, process id, obj id)
• Invocation on the server side is synchronous with client

call

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Interaction Scheme

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Languages in Depth series: Java Programming

Stubs and Skeletons

• Client side: stub/proxy
• Offers same interface

as server object: mimics the server

• Usually bound to a

single server

• Marshals the request

into a stream of bytes

• Method id (e.g.,

name)

• Arguments
• Additional features:
• Caching of values
• Load balancing
• Statistics
• Server side: skeleton
• Represents the server
• bject
• Bound to a single

server

• Unmarshals the request

and calls the corresponding method

• n the server object
• Additional features:
• Persistence

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Languages in Depth series: Java Programming

Interaction in Detail

• Invocations
• Transformed to messages, and sent to the « other side

»(marshaling)

• The « other side »: skeleton
• Server-side counterpart to the stub
• Extracts request arguments from message (unmarshaling)

and invokes the server object

• Marshals return value and sends it to the invoker side,

where stub unmarshals it and returns the result to invoker

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Languages in Depth series: Java Programming

Interaction in Detail

• Skeleton type
• Delegation: skeleton is separate object (of arbitrary type)
• Associated with the effective server object (binding is

usually made by application)

• Inheritance:
• Developer subclasses a skeleton class generated for

the server object interface, e.g,

• public class SkeletonRemote implements

RemoteInterface{}

• public class RemoteServis extends SkeletonRemote

{…}

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Languages in Depth series: Java Programming

Interaction in Detail

• Marshaling
• Unbound objects: serialized and passed by value
• Primitive types: ditto
• Bound objects: passed by reference
• Mainly bound objects remotely « visible »
• Stub creation
• Received as argument/result of remote method invocation,
• r
• From lookup service

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Languages in Depth series: Java Programming

Further Concepts

• Repositories
• Reference Repository
• Find new remote objects (locate objects, i.e., bootstrapping)
• Interface Repository
• Discover new remote object types (browse remote types)
• Object Repository
• Initialize new remote objects (automatic server activation)
• Advanced concepts
• Interception
• Threading
• Distributed garbage collection
• Calls from the server to client
• Synchronous to original client->server call
• Asynchronous

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Languages in Depth series: Java Programming

Java RMI

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• Java RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

Java RMI (RPC)

Java RMI: At a Glance

• Allows distributed Java objects to interact
• Through (remote) method invocations, since Java 1.1
• Invocations are synchronous (even if no reply)
• 1st class RPC package
• Fully integrated with Java language
• Remote interfaces are described through Java interfaces
• Separate compilation
• Generate stubs and skeletons according to interfaces
• Compile application

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Languages in Depth series: Java Programming

Java RMI Architecture

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Languages in Depth series: Java Programming

Stub/Skeleton Layer

• Stub
• Has same interface as remote object
• Initializes call to remote object
• Marshals arguments to stream
• Passes stream to remote reference layer
• Unmarshals the return value
• Informs the remote reference layer that call is complete
• Skeleton
• Unmarshals arguments from the stream
• Makes (up-)call to the remote object implementation
• Marshals the return value or an exception onto the stream

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Languages in Depth series: Java Programming

Remote Reference Layer

• Carries out remote reference protocol
• Independent of stubs/skeletons
• Remote object implementation chooses invocation

protocol

• Unicast point-to-point, extensions:
• Replicated object group, Support for specific replication

strategy

• Support for persistent reference to remote object

(automatic activation of remote object)

• Reconnection strategies

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Languages in Depth series: Java Programming

Transport Layer

• Responsibilities
• Connection set-up to remote address space
• Managing connections
• Monitoring connection « liveness »
• Listening for incoming calls
• Maintaining a table of remote objects
• Connection set-up for incoming call
• Locating the dispatcher for the target of the remote call
• Abstractions
• Endpoint: denotes an address space or JVM
• Channel: conduit between two address spaces, manages

connections

• Connection: data transfer (input/output)
• Transport: Manages channels

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Languages in Depth series: Java Programming

Design a Java RMI Application

• 1. Write the interfaces of the remote (i.e., remotely

accessible) objects: coarse grain

• 2. Write the implementations of the remote objects
• 3. Write other classes involved: fine grain
• 4. Compile the application with javac
• 5. Generate stubs and skeletons with rmic

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Languages in Depth series: Java Programming

Declaring a Remote Interface

• Objects are remotely accessible through their

remote interface(s) only

• Methods to be exported are declared in an interface

that extends the java.rmi.Remote interface

• Remote interfaces
• Must be public
• All methods must declare java.rmi.RemoteException in

throws list: represent exceptions due to distribution

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Hello World Remote Interface

import java.rmi.*; public interface Hello extends Remote { public void print() throws RemoteException; }

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Simple Classes and Interfaces

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Implementing Remote Interface

• Implement the Remote interface
• Abstract class java.rmi.server.RemoteObject implements

Remote

• Remote behavior for hashCode(), equals() and toString()
• Abstract class java.rmi.server.RemoteServer extends

RemoteObject

• Functions to export remote objects
• Concrete class
• java.rmi.server.UnicastRemoteObject extends RemoteServer
• Non-replicated remote object
• Support for point-to-point active object references

(invocations, parameters, and results) using TCP

• Inheritance: subclass UnicastRemoteObject
• Note
• Own exceptions must not subtype RemoteException

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Languages in Depth series: Java Programming

A Hello World Implementation

import java.rmi.*; import java.rmi.server.*; public class HelloImpl extends UnicastRemoteObject implements Hello { public HelloImpl() throws RemoteException{ super(); } public void print() throws RemoteException { System.out.println("Hello World"); } }

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Languages in Depth series: Java Programming

Constructing a Remote Object

• The Constructor
• Calls the no-argument constructor of the

UnicastRemoteObject class (implicitly or explicitly)

• Which exports a UnicastRemoteObject, making that

available to accept incoming requests by listening to calls from clients on an anonymous port

• Throws RemoteException, since the constructor of

UnicastRemoteObject might do so, if the object cannot be exported

• Communication resources are unavailable
• Stub class cannot be found, …
• Alternative: Delegation
• Explicitly export the object

UnicastRemoteObject.exportObject()

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Languages in Depth series: Java Programming

Starting a Server

rmiregistry & public class HelloServer { public static void main(String[] args) { … Hello hello = new HelloImpl(); // Register object (e.g., naming service) Naming.rebind ("Hello", hello); // What’s up doc? … } }

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Client

public class HelloClient { public static void main(String[] args) { … // Lookup object (e.g., naming service) Hello hello = (Hello) Naming.lookup ("//ethz.ch/Hello"); // Invoke the remote object hello.print(); // That’s all folks… } }

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Languages in Depth series: Java Programming

Parameters and Return Values

• Can be
• Local objects
• Primitive types (e.g., int, boolean, …)
• Serializable, i.e. implementing java.io.Serializable:fields

are copied (except static or transient) and serialized

• Remote Objects
• Passed by reference
• Serialization
• Can be overridden
• A copy of the object is created at the client side

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Languages in Depth series: Java Programming

Exceptions

• All RMI exceptions subclass RemoteException
• Examples
• StubNotFoundException or SkeletonNotFoundException:

forgot rmic?

• ServerError: error while server executes remote method
• ServerException: remote exception in server’s remote

method call

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Languages in Depth series: Java Programming

Distributed Garbage Collection

• (Local) object garbage collected if
• No (local) reference to it exists
• Remote object garbage collected if
• No local reference to it exists
• No remote reference to it exists
• JVM keeps track of the # of refs to an object
• First reference triggers message send to JVM hosting the
• bject
• Discarding last reference triggers message send as well
• When no remote JVM references object anymore,

reference becomes « weak »

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Languages in Depth series: Java Programming

More Information

• Java site: http://java.sun.com
• Java RMI site: http://java.sun.com/products/jdk/rmi/
• Tutorial: http://java.sun.com/j2se/1.5/docs/guide/rmi/
• Specification:

http://java.sun.com/j2se/1.5/docs/guide/rmi/spec /rmiTOC.html

• White Paper: http://java.sun.com/marketing

/collateral/javarmi.html

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Languages in Depth series: Java Programming

GWT Middleware

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

GWT

• Google Web Toolkit
• A Java based system to develop AJAX dynamic web

pages

• Consists of
• A GUI Widget library in Java and in Javascript
• A Java compiler which is compiling to Javascript
• Consequences
• The code is developed and tested in Java against Java

GWT library with all Java IDE tools

• The final product is compiled to a Javascript code

which can be used on compatible browsers

• The browser compatibility problem is solved in GWT

library

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Languages in Depth series: Java Programming

GWT Middleware

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Languages in Depth series: Java Programming

More Information on GWT

• http://code.google.com/webtoolkit/
• http://code.google.com/webtoolkit/documentation

/ com.google.gwt.doc.DeveloperGuide.RemoteProcedur eCalls.html

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Languages in Depth series: Java Programming

Publish/Subscribe

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

Publish/Subscribe

• Model
• Producers publish information
• Consumers subscribe to information
• Producers and Consumers communicate through logical

buses (or channels)

• Decoupling of participants
• In time
• In space
• In flow
• Enforces scalability
• Topic-based, content-based, type-based
• Modern systems combine more of them

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Languages in Depth series: Java Programming

Publish/Subscribe

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Languages in Depth series: Java Programming

Topic-Based Publish/Subscribe

• A.k.a. subject-based publish/subscribe
• News-like approach
• Messages are classified according to topic names, e.g.,

ETHZ

• Topics can be seen as (dynamic) groups
• URL-like topic names for convenience
• Topics arranged in a hierarchy, e.g., /ETHZ/CSE
• Automatic subscriptions to subtopics
• Wildcards
• Aliases

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Languages in Depth series: Java Programming

Topic-Based Publish/Subscribe

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Languages in Depth series: Java Programming

Content-Based Publish/Subscribe

• A.k.a. property-based publish/subscribe
• Events classified according to their properties
• Consumers subscribe by specifying properties of events of

interest

• Application criteria are seen as subscription pattern
• Translated to filter, or predicate, matched against events
• Classic approach
• Map event attributes to properties
• Subscription language and parser,
• E.g., "name == ‘Bob’"

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Languages in Depth series: Java Programming

Content-Based Publish/Subscribe

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Languages in Depth series: Java Programming

Type-Based Publish/Subscribe

• Subscription criterion
• The type (its interface) of application-defined events
• Content-based queries based on methods
• Combines static and dynamic schemes
• Static classification should be made as far as possible for

efficiency

• Filters for fine-grained content-based subscription increase

expressiveness if required

• Languages which support structural reflection
• No need for specific events (e.g., Java introspection),
• In other languages, events can subtype an introspective

event type

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Languages in Depth series: Java Programming

Type-Based Publish/Subscribe

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Languages in Depth series: Java Programming

Java Middleware

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

Java Message Service

• The Java Message Service is only an API
• Standardized API for messaging in Java
• Implemented by most industrial solutions
• TIBCO
• iBus
• Gryphon
• …
• Two messaging styles:
• Publish/subscribe (topic-based & content-based)
• Point-to-point (message queuing)

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Languages in Depth series: Java Programming

Benefit of JMS

• Sun standard
• Ensures a certain degree of portability
• Integration with other Java concepts/services
• Enterprise Java Beans (EJB): EJB Message Beans, the

container invokes the beans if messages are available

• Java Database Connectivity (JDBC) for database

integration

• Java Transaction Service (JTS) for messages as part of

distributed transactions

• Java Naming and Directory (JNDI) for object lookup
• API can be downloaded: package javax.jms

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Languages in Depth series: Java Programming

JMS Event Model

• General-purpose messages which require explicit

marshalling

• Message body can contain
• Stream
• Properties
• String
• Object
• Bytes
• Additional attributes
• Message header: explicit messaging
• Message properties: for content-based filtering

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Languages in Depth series: Java Programming

Message Attributes

• Message properties
• Name-to-value

properties provided by message producer

• Property types (native

Java types)

• Boolean
• Byte
• Short
• Int
• Long
• Float
• Double
• String
• Note: attributes

mapped to properties

• Message header
• Assigned by service

upon send

• Destination
• Delivery mode

(PERSISTENT, NON_PERSISTENT)

• Message ID
• Timestamp
• Priority
• Expiration
• Provided by client
• Correlation ID, e.g.,

refer to other message

• Type
• Reply destination

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Properties for Content-Based

• Properties of messages are assigned explicitly
• Not java.util.Properties
• Subscriber describes required properties
• Message selector = filter
• Subscription language: message selector is String
• Syntax specified by JMS
• Must be mapped to service provider’s subscription

language syntax

• E.g.,
• "JMSType = ‘car’ AND color = ‘blue’ AND weight > 2500"

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Common Facilities

• Destination
• Named object (topic, queue) obtained through JNDI: empty

interface

• ConnectionFactory
• Obtained through JNDI, used to create Connection to a topic,

queue: empty

• Connection
• May require authentication
• Register ExceptionListener for problem detection
• Factory for Session
• Session
• Required by client (producer/consumer) to interact with topic,

queue

• Creates MessageProducer (push), MessageConsumer (push

/pull)

• Single threaded. Transaction support

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Connections

public interface Connection { public String getClientID() throws JMSException; public void setClientID(String ID) throws …; public void setExceptionListener(ExceptionListener l) throws …; public ExceptionListener getExceptionListener() throws …; public void close() throws …; public start() throws …; public stop() throws …; … /* (Sessions created through implementation classes) */ }

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Languages in Depth series: Java Programming

Sessions

public interface Session { public void setMessageListener(MessageListener l) throws …; public MessageListener getMessageListener() throws …; public TextMessage createTextMessage() throws …; public StreamMessage createStreamMessage() throws …; public MessageProducer createProducer(Destination destination) throws…; … public void close() throws …; public void recover() throws …; public void commit() throws …; public void rollback() throws …; … }

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Languages in Depth series: Java Programming

Message Producers

public interface MessageProducer { public void setDeliveryMode(int deliveryMode) throws …; public int getDeliveryMode() throws …; public void setPriority(int defaultPriority) throws …; public int getPriority() throws …; public void setTimeToLive(long ttl) throws …; public long getTimeToLive() throws …; … sent(…) throws …; }

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Languages in Depth series: Java Programming

Message Consumers

public interface MessageConsumer { /* Provide content-based filter */ public String getMessageSelector() throws …; /* Push model */ public void setMessageListener(MessageListener l) throws …; public MessageListener getMessageListener() throws …; /* Poll */ public Message receive() throws …; /* Blocking pull */ public Message receive(long timeout) throws …; … }

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Languages in Depth series: Java Programming

Point-To-Point (PTP)

• Objects
• Queue represents a vendor-specific implementation
• TemporaryQueue is a temporary incarnation, bound to a

QueueConnection

• Created through a QueueConnectionFactory
• QueueSession, QueueReceiver (message consumer:

push/pull), QueueSender (message producer)

• QueueBrowser to query queue without removing

messages …

• Note
• Message selector can be specified by consumer

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Languages in Depth series: Java Programming

Queue

public interface Queue { public String getQueueName() throws …; public String toString() throws …; } public interface QueueBrowser { public Enumeration getEnumeration() throws …; public String getMessageSelector() throws …; public String getQueue() throws …; … }

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Languages in Depth series: Java Programming

Publish/Subscribe

• Objects
• Topic gives access to pub/sub system: no naming

conventions

• TemporaryTopic, TopicConnectionFactory,

TopicConnection, TopicSession, as seen previously

• TopicSubscriber (message consumer) and TopicPublisher

(producer)

• Durable subscription
• Client provides unique ID
• TopicRequestor
• Use pub/sub to make request/replies
• Mixed topic/content-based
• Client provides a message selector

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Languages in Depth series: Java Programming

Topic

public interface Topic { public String getTopicName() throws …; public String toString() throws …; } public class TopicRequestor { public TopicRequestor(TopicSession session, Topic topic) throws … {…} public Message request(Message message) throws … {…} … }

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JMS Exceptions

• JMSException
• Root of exception hierarchy
• Specific exceptions
• JMSSecurityException: authentication problem
• InvalidDestination: destination not understood by provider
• InvalidSelectorException: « syntax error » in filter
• MessageFormatException: e.g., unsupported payload

class

• …

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More Info on JMS

• http://java.sun.com/products/jms/
• http://www.tibco.com
• http://www.softwired-inc.com/products/mobile

/mobile.html

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Languages in Depth series: Java Programming

Java Middleware

• What is middleware
• Client-server communication and method invocation
• Provides a substrate on which distant modules can

transparently cooperate

• RPC in general
• RMI
• GWT middleware
• Publish/Subscribe
• JMS
• Application servers

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Languages in Depth series: Java Programming

Multi-Tier Application Model

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Languages in Depth series: Java Programming

Application Server

• What the application server gives you:
• Design your application as a set of functional modules (in J2EE terminology –

Enterprise Java Beans EJBs)

• Concentrate on business logic programming
• The beans live in “Bean Containers”, containers provide various services for the

beans:

• Instance pooling and caching aka resource and lifecycle management
• Remote Accessibility
• Persistence on various levels of automation
• Security
• Messaging
• Concurrency control
• Clustering and load balancing
• Your modules/services (beans) are efficiently managed by the application server

administration tools

• You have means to manage a large number of beans (10^4)
• You have tools to deploy large systems
• You can manage your modules in run-time
• Application server is a “small operating system” for your modules (beans)
• The price you pay:
• You have to stick to the container rules
• You have to implement certain interfaces used by the container

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Application Server

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EJB Interface Groups

• Home interfaces
• Remote interface
• Container interfaces
• …

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Recent Development & References

• Spring Framework
• Hibernate
• EJB 3.0 specification (JSR 220)
• More Info
• http://java.sun.com/javaee/index.jsp
• http://www.bea.com/framework.jsp?CNT=index.htm&FP=

/content/products /weblogic&WT.ac=topnav_products_weblogic

• http://www.jboss.org/
• http://en.wikipedia.org/wiki/Ejb (Controversial)