Shonali Krishnaswamy

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Aglet Programming Part II

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• Shonali Krishnaswamy Leni Lim

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Lecture Outline

• Proxy Interface - Revisited
• Proxy Retrieval
• Proxy Iterator, Getting a local proxy, Getting a
  Remote proxy
• Aglet Communication
• Simple Messaging
• The Message Class Creating Messages, Receiving
  Messages, Replying to Messages
• Getting the Reply Synchronous Messaging,
  Asynchronous Messaging
• NEXT WEEK
• Message Management Serialised Message Handling,
  Message Priorities, Parallel Message Handling,
  Synchronised Message Handling
• Remote Messaging, Multicasting, Receiving
  Multiple Replies

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AgletProxy Interface

• Acts as the handle of an aglet
• Provides a common way of accessing the aglet
• Aglet class
• several public methods
• should not be accessed directly from other aglets
• security reasons
• For Aglet A to communicate with Aglet B-
• The proxy of Aglet B must be obtained
• Interact with Aglet B through the Interface
• Proxy is a shield that protects Aglets from
  malicious Aglets
• The AgletProxy object consults the Security
  Manager
• Check whether current execution context is
  allowed to perform the method
• AgletProxy Interface also provides Location
  Transparency
• If the Aglet is on a remote host, then the proxy
  forwards a request to the Aglet and returns the
  results

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Aglet Proxycont...
Aglet
Aglet
Proxy
Proxy
Aglet Context
Aglet Server/Viewer
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AgletProxy Interface
Proxy 1
dispatched
handleMessage() sendReply()
Proxy1.sendMessage()
Proxy 2
Proxy 3
Local Context
Remote Context
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AgletProxy Interface

• Creating an Aglet is a way of getting a Proxy
• AgletContext.createAglet() returns the proxy of
  the newly created Aglet
• Other methods
• AgletContext.retractAglet()
• AgletProxy.clone()
• AgletProxy.dispatch()

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Proxy Retrieval

• Focus in methods that retrieve aglet proxies.
• play an important role as convenient handles for
  aglets.
• use to gain access to and communicate with its
  corresponding(possible remotely located) aglet.
• there are number of ways in which you can
  retrieve proxies from local as well as remotely
  located aglets.

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Proxy Retrieval By Creating

• Method 1
• public abstract AgletProxy AgletContext.createAgl
  et(URL codebase, String code, Object init)
• Retrieves the proxy of a child aglet that has
  been created.
• Example
• AgletProxy CPLAgletProxy1 getAgletContext().crea
  teAglet(null,"examples.cpe5010.ChildProxyLister1",
  null)

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Proxy Retrieval Using AgletID

• Method 2
• public abstract AgletProxy AgletContext.getAgletP
  roxy(AgletID identity)
• Retrieves the proxy of an aglet in the current
  context. The identity of the aglet must be known.

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Proxy Retrieval - Enumeration
Method 3

• Public abstract Enumeration AgletContext.getAgletP
  roxies()
• Get the proxies of all aglets in the current
  context.
• Returns an Enumeration of proxies. Which may
  include currently deactivated aglets.
• Enumeration has two methods that support
  iteration hasMoreElements() and nextElement().

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Example 1 A Local Proxy Lister

• Use the getAgletProxies method to list the
  current set of aglets in the following format
• Four Aglets in the Current Context
• JustAnAglet
• ProxyLister parent that creates 2 other Aglets
• ChildProxyLister1
• ChildProxyLister2
• This does the listing of all the Aglets that are
  current in the Context
• To run this example
• First create JustAnAglet in Tahiti
• Then create ProxyLister in Tahiti
• You will see the displayed list of Aglets on the
  console

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Example 2 A Mobile Proxy Lister

• Similar to previous example
• MobileProxyLister
• Aglet that travels to a remote server and lists
  all the proxies of Aglets in the remote context
• Four Aglets in the Current Context
• JustAnAglet
• ProxyLister parent that creates 2 other Aglets
• ChildProxyLister1
• ChildProxyLister2
• This does the listing of all the Aglets that are
  current in the Context
• To run this example
• Instantiate one Tahiti Server
• First create JustAnAglet in Tahiti
• Then create ProxyLister in Tahiti
• You will see the displayed list of Aglets on the
  console
• Instantiate another Tahiti Server on a different
  port
• Create MobileProxyLister
• NOTE Modify the MobileproxyListers destimnation
  URL to one in which you are running JustAnAglet
  and ProxyLister

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Getting a Remote Proxy

• public abstract AgletProxy AgletContext.getAgletPr
  oxy(URL contextAddress, AgletID identity)
• Retrieve the proxy of an aglet hosted in a
  remote context. The remote context is identified
  by its URL and the identitty indicates the aglet.
  Both must be known
• (the following example creates two aglets. One of
  these aglets is dispatched to a remote context.

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Example 3 Getting a Remote Proxy

• ParentProxy, ChildProxy1 ChildProxy2
• ParentProxy creates a ChildProxy1
• Gets its ID
• Dispatches it to a destination
• Creates ChildProxy2
• Passes it the destination URL and ID of
  ChildProxy1
• ChildProxy2 can now find the name of ChildProxy1
  since it knows its destination and URL
• It can also send Messages etc.

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Aglet Communication

• Agent Communication
• Speech Act Theory
• Agent Communication Languages
• Knowledge Query Manipulation Language (KQML)
• FIPA Agent Communication Language (ACL)
• Messaging very important for Agent Interaction,
  Coordination, Collaboration etc.
• Remember Difference between Objects Agents?
• Object invoke method
• Agent request a service

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Aglet Communication

• Aglets supports an object-based messaging
  framework
• Location independent
• Extensible
• Rich
• Synchronous Asynchronous

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Simple Messaging

• Principal means of Aglet Communication
• Message Passing
• InterAglet Messaging is based on a simple event
  model
• Requires an aglet to implement handlers only
  for the messages that it is supposed to
  understand
• The main message handling method is
• public boolean Aglet.handleMessage(Message
  myMessage)
• To send a message
• Invoke the sendMessage() method on the Aglets
  proxy not directly on the Aglet
• public Object AgletProxy.sendMessage(Message
  myMessage)
• Aglet Proxy acts as a message gateway

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Aglet Communication
Proxy 1
dispatched
handleMessage() sendReply()
Proxy1.sendMessage()
Proxy 2
Proxy 3
Local Context
Remote Context
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Example Send Message

• You need the Proxy of the Aglet you wish to send
  a message to
• The following example creates an Aglet called
  SimpleMessageChild and uses the proxy that it
  gets while creating it to send messages to it.
• Trivial example
• Typical applications will require you to
  communicate with Aglets that you dont really know

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Example Send Message
public class SimpleMessageExample extends Aglet
public void run() AgletProxy proxy
getAgletContext().createAglet(getCodeBase(),
SimpleMessageChild, null) try
proxy.sendMessage( new Message
(Hello)) catch (NotHandledException e)
System.out.println(Message Not
Handled!)
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Example Handle Message
public class SimpleMessageChild extends Aglet
public boolean handleMessage (Message msg)
if (msg.sameKind(Hello))
System.out.println(I got a Hello Message)
return true else return false
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Example Handle Message

• SimpleMessageChild knows to respond to messages
  that say Hello
• What happenes if you send it a message Hi ?
• proxy.sendMessage(new Message(Hi!)
• Return false
• In the SimpleMessageExample, you will get
  Message Not Handled

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Message Class

• Messages are objects
• A Message Object is characterised by its kind
• This string property is used to distinguish
  messages from each other
• The Message class supports a range of
  constructors with different mandatory arguments
• Message objects can also contain an optional
  argument field for data associated with a
  particular message
• The argument field can be either
• Atomic String, int etc.
• Tabular - Hashtable

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Message Creation

• public Message(String kind)
• Constructs a message of a specific kind
• Creates a default argument table that can hold a
  set of key-value pairs
• setArg method can be used to initialise the
  argument table
• public Message (String kind, Object arg)
• Constructs a message of the specified kind with
  an Object as its initial bonding

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Message Creation

• public Message (String kind, int arg)
• Constructs a message of the specified kind with
  an int as its initial argument
• public Message (String kind, float arg)
• Constructs a message of the specified kind with
  an float as its initial argument

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Message Creation

• public Message (String kind, long arg)
• Constructs a message of the specified kind with
  an long as its initial argument
• public Message (String kind, double arg)
• Constructs a message of the specified kind with
  an double as its initial argument

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Message Creation

• public Message (String kind, char arg)
• Constructs a message of the specified kind with
  an char as its initial argument
• public Message (String kind, boolean arg)
• Constructs a message of the specified kind with
  an boolean as its initial argument

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Message Creation - Examples

• Message msg new Message(Greeting , Happy
  Birthday)
• Message msg new Message(Price, 200)

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Receiving Messages

• public String Message.getKind()
• Gets the kind of the message
• public boolean Message.sameKind(String)
• Compares the kind of the message with the
  String Argument
• public Object Message.getArg()
• Gets the argument of the message

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Non-atomic Messages

• Nonatomic arguments have more than one argument
  to the receiver
• Such Arguments are most effectively handled as
  key-value pairs
• setArg and getArg methods are useful for
  organising multiple arguments into a table
• public void Message.setArg(String key, Object
  value)
• Maps the specified key to the specified value in
  the argument table
• public Object Message.getArg(String key)
• Gets the value to which the key is mapped in the
  argument table, null is there is no mapped value
  for the key

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References
Lange, D.,B.(1998). Programming and Deploying
Java mobile agents with Aglets / Danny B. Lange,
Mitsuru Oshima. Reading, Mass Addison-Wesley,
c1998. http//www.vistabonita.com/papers/DCAglets/
http//www.trl.ibm.com/aglets/ http//www.geociti
es.com/SiliconValley/Lakes/8849/simple.htm http//
www.geocities.com/SiliconValley/Lakes/8849/roaming
.htm