Advanced Remote Method Invocations

1
Advanced Remote Method Invocations

2
RMI Advanced topics

• The Java RMI API has a rich collection of
  features.
• We will look at some of RMIs more interesting
  advanced features, namely
• stub downloading
• security manager
• client callback.
• Although these features are not inherent to the
  distributed object paradigm, they are helpful
  mechanisms and can be useful to application
  developers.

3
The Java RMI Architecture
4
Java RMI Client Server Interaction
5
RMI Stub Downloading

• RMI is designed to allow stubs to be made
  available to the client dynamically (in HW_3).
  Doing so allows changes to be made in the remote
  methods without affecting the client program.
• The stub can be filed with an web server and be
  downloaded using HTTP/FTP.
• Security measures are needed in both the client
  side and the server side
• A java security policy file needs to be set on
  the server host and also on the client host.
• A Java Security Manager should be instantiated in
  both the client and server programs.

6
Stub downloading

• If the stub will be downloaded from a remote
  server, transfer the stub class to the
  appropriate directory that HTTP server can reach,
  e.g., www.csc.calpoly.edu/mliu/www, and make
  sure that the RIGHT access permission to the file
  is set.
• When activating the server, specify command option

java -D java.rmi.server.codebase ltURLgt
\ -D java.rmi.server.hostnameltserver host namegt
\ -D java.security.policyltfull directory path
to java.policy filegt
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The java.policy file

• The RMI security manager does not permit network
  access. Exceptions can be made via the
  specification in a java.policy file.
• grant
• // permits socket access to all common TCP
  ports, including the default
• // RMI registry port (1099) need for both
  the client and the server.
• permission java.net.SocketPermission
  "1024-65535", "connect,accept,resolve"
• // permits socket access to port 80, the
  default HTTP port needed
• // by client to contact an HTTP server for
  stub downloading
• permission java.net.SocketPermission "80",
  "connect"
• grant // Allow everything hw_3
• permission java.security.AllPermission

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The java.policy file - 2

• This file can be filed in the same directory as
  the server class file.
• When activating the client, a java.policy file
  also should be specified
• java -D java.rmi.server.useCodebaseOnlytrue
• -D java.rmi.server.codebase
  http//hostname80/stub_dir/
• -D java.security.manager -D
  java.security.policyjava.policy
• SomeClient
• -D propertyvalue -Djava.security.policys
  omeURL SomeApp
• where someURL is a URL specifying the location
  of a policy file
• java.rmi.server.codebase this property specifies
  the locations from which classes that are
  published by this JVM.
• java.rmi.server.useCodebaseOnly If this value is
  true, automatic loading of classes is prohibited
  except from the local CLASSPATH and from the
  java.rmi.server.codebase property set on this
  JVM.
• Default security policy file java_jre_home/lib/s
  ecurity/java.policy
• permission java.net.SocketPermission
  "localhost1024-", "listen"

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The java.policy file - 3

• The "-D java.security.manager" argument ensures
  that the default security manager is installed,
  and thus the application is subject to policy
  checks.
• Default security manager is not required if the
  application installs a security manager.
• If you use java -Djava.security.manager D
  java.security.policysomeURL SomeApp, then just
  the specified policy file will be used all the
  ones indicated in the security properties file
  will be ignored.

Ref http//java.sun.com/j2se/1.4.2/docs/guide/sec
urity/PolicyFiles.html
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File Placements
11
RMI Security Manager

• Since RMI involves access to/from a
  remote/foreign host, and possibly object
  downloading, it is important for both the server
  and the client to protect its system from
  malicious access.
• The RMISecurityManager--a Java class, can be
  instantiated in both the client and the server
  for limiting access privileges.
• RMI's class loader will not download any classes
  from remote locations if no security manager has
  been set.
• RMISecurityManager does not apply to applets,
  which run under the protection of their browser's
  security manager.
• You can instantiate/write your own security
  manager, if so desired.
• try
• System.setSecurityManager(new
• RMISecurityManager( ))
• catch

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Sample Code for Stub Downloading

• The possible ways--accept, connect, listen, and
  resolve, to connect to a host in SocketPermission
  java class.
• The "listen" action is only meaningful when used
  with "localhost".
• The "resolve" action is implied when any of the
  other actions are present. The action "resolve"
  refers to host/ip name service lookups.
• p1 new SocketPermission(ise.gmu.edu7777",
  "connect, accept") allows that code to connect
  to port 7777 on ise.gmu.edu, and to accept
  connections on that port.
• p2 new SocketPermission("localhost1024-",
  "accept, connect, listen") allows that code to
  accept connections on, connect to, or listen on
  any port between 1024 and 65535 on the local
  host.

Ref http//java.sun.com/j2se/1.4.2/docs/api/java/
net/SocketPermission.html
13
Algorithm for building an RMI Application

• Server side
• Open a directory for all the files to be
  generated for this application.
• Specify the remote-server interface, and compile
  it to generate the interface class file.
• Build the remote server class by implementing the
  interface, and compile it using javac.
• Use rmic to process the server class to generate
  a stub.class file and a skelton.class file rmic
  SomeServerImpl
• If stub downloading is desired, copy the stub
  file to an appropriate directory on the HTTP
  host.
• Activate the RMIRegistry, if it has not already
  been activated.
• Set up a java.policy file.
• Activate the server, specifying (i) the codebase
  if stub downloading is desired, (ii) the server
  host name, and (iii) the security policy file.
•  

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Sample Code for Stub Downloading
public interface HelloInterface extends Remote
public String sayHello() throws
java.rmi.RemoteException // end of
HelloInterface interface public class HelloImpl
extends UnicastRemoteObject implements
HelloInterface public HelloImpl() throws
RemoteException super( )
public String sayHello() throws RemoteException
return "Hello, World!" // end
HelloImpl class
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Sample Code for Stub Downloading
public class HelloServer public static void
main(String args) try
// System.setSecurityManager( new
RMISecurityManager())
startRegistry(RMIPortNum) HelloImpl
exportedObj new HelloImpl()
registryURL "rmi//cs1.cs.gmu.edu" portNum
"/hello" Naming.rebind(registryURL,
exportedObj) System.out.println("Hello
Server ready.") // end try catch
(Exception re) System.out.println("Exce
ption in HelloServer.main " re) // end
main
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Sample Code for Stub Downloading
grant // Allows RMI clients to make socket
connections to the // public ports on any
host. // If you start the RMI registry on a
port in this range, you // will not incur a
resolve access violation. permission
java.net.SocketPermission "1024-65535",
"connect, accept, resolve" // Permits
socket access to port 80, the default HTTP port
- // needed by client to contact an HTTP
server for stub // downloading. permission
java.net.SocketPermission "80", "connect,
accept, resolve"
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Sample Code for Stub Downloading
build (JAVAC) HelloInterface.java
(JAVAC) HelloServer.java (JAVAC)
HelloImpl.java rmic (RMIC)
HelloImpl runs (JAVA) -D
java.security.policyjava.policy
-D java.rmi.server.codebasehttp//server_URL
HelloServer
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Algorithm for building an RMI Application

• Client side
• Open a directory for all the files to be
  generated for this application.
• Implement the client program or applet, and
  compile it to generate the client class.
• If stub downloading is not in effect, copy the
  server interface stub class file.
• Set up a java.policy file.
• Activate the client, specifying (i) the server
  host name, (ii) the security policy file, and
  (iii) the codebase if stub downloading is
  desired.

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Client Code for Stub Downloading - 1
public class HelloClient public static void
main(String args) try
System.setSecurityManager(new RMISecurityManager()
) String registryURL "rmi//ise.gmu.edu"
portNum "/hello" // find the remote
object and cast it to an interface object
HelloInterface h (HelloInterface)Naming.lookup(r
egistryURL) // invoke the remote method
String message h.sayHello() // end
try catch (Exception e)
System.out.println("Exception in HelloClient "
e) // end catch //end main //end class
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Client Code for Stub Downloading - 2
build (JAVAC) HelloClient.java
(JAVAC) HelloInterface.java runc
(JAVA) D java.rmi.server.useCodebaseOnlytrue
-D java.rmi.server.codebasehttp//URL_stub_
dir/ -D java.security.policyjava.policy
HelloClient
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RMI Callbacks
22
Introduction

• In the client server model, the server is
  passive the IPC is initiated by the client the
  server waits for the arrival of requests and
  provides responses.
• Some applications require the server to initiate
  communication upon certain events. Examples
  applications are
• monitoring
• games
• auctioning
• voting/polling
• chat-room
• message/bulletin board
• groupware

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Polling vs. Callback

• In the absence of callback, a client will have to
  poll a passive server repeatedly if it needs to
  be notified that an event has occurred at the
  server end.

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Two-way communications

• Some applications require that both sides may
  initiate IPC.
• Using sockets, duplex communication can be
  achieved by using two sockets on either side.
• With connection-oriented sockets, each side acts
  as both a client and a server.

Process 2
25
RMI Callbacks

• A callback client registers itself with an RMI
  server.
• The server makes a callback to each registered
  client upon the occurrence of a certain event.

26
Callback Client-Server Interactions
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Callback application files
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RMI Callback file placements
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The Hello Application with Callback

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RMI Callback Interface

• The server provides a remote method (in server
  interface), which allows a client to register
  itself for callbacks.
• A client remote interface for the callback is
  needed, in addition to the server-side interface.
• The client remote interface specifies a method
  for accepting a callback from the server.
• The client program is a subclass of RemoteObject,
  and implements the callback (client) remote
  interface, including the callback
  methodNotifyMe().
• The client registers itself for callback in its
  main method, by passing an object reference to
  the client remote interface.
• The server invokes the clients remote
  methodNotifyMe(), upon the occurrence of the
  anticipated event.

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Algorithm for building an RMI Callback Application

• Server side
• Open a directory for all the files to be
  generated for this application.
• Specify the remote-server interface, and compile
  it to generate the interface class file.
• Build the remote server class by implementing the
  interface, and compile it using javac.
• Use rmic to process the server class to generate
  a stub class file and a skeleton class file
  rmic ServerInterfaceImpl
• If stub downloading is desired, copy the stub
  file to an appropriate directory on the HTTP
  host.
• Activate the RMIRegistry, if it has not already
  been activated.
• Set up a java.policy file.
• Activate the server, specifying (i) the codebase
  if stub downloading is desired, (ii) the server
  host name, and (iii) the security policy file.
• Obtain the CallbackClientInterface and its stub
  file. Use rmic CallbackClientInterfaceImpl to
  generate the stub file for the callback.
•  

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Remote Interface for Server

• public interface CallbackServerInterface extends
  Remote
• // remote method
• public String sayHello() throws
  java.rmi.RemoteException
• // method to be invoked by a client to add
  itself to the callback list
• public void registerForCallback (
• CallbackClientInterface CallbackObject)
• throws java.rmi.RemoteException
• public void unregisterForCallback(
• CallbackClientInterface CallbackObject)
• throws java.rmi.RemoteException

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Client Remote Interface for Callback

• // a remote interface specifying a callback
  method
• public interface CallbackClientInterface extends
  java.rmi.Remote
• // callback method to be called by the server
• public void NotifyMe ( String message )
• throws java.rmi.RemoteException

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ServerInterfaceImpl with callback

• public class CallbackServerInterfaceImpl extends
  UnicastRemoteObject implements
  CallbackServerInterface
• public CallbackServerInterfaceImpl() throws
  RemoteException
• super( ) clientList new Vector()
• public String sayHello( ) throws
  java.rmi.RemoteException
• return("hello")
• public synchronized void registerForCallback(
  CallbackClientInterface callbackClientObject)
  throws java.rmi.RemoteException
• if (!(clientList.contains(callbackClientObject)))
  
• clientList.addElement(callbackClientObject)
• doCallbacks()
• private synchronized void doCallbacks( ) throws
  java.rmi.RemoteException
• for (int i 0 i lt clientList.size() i)
• CallbackClientInterface nextClient
  (CallbackClientInterface)clientList.elementAt(i)
• String returnMsg nextClient.notifyMe("Num of
  clients" clientList.size())

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ClientInterfaceImpl with callback
public class CallbackClientInterfaceImpl extends
UnicastRemoteObject implements CallbackClientInter
face public CallbackClientInterfaceImpl()
throws RemoteException super( )
public String notifyMe (String message)
String retMessage "Call back received "
message return retMessage
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Algorithm for building an RMI Callback Application

• Client side
• Open a directory for all the files to be
  generated for this application.
• Implement the client program or applet, and
  compile it to generate the client class.
• If stub downloading is not in effect, copy the
  server interface stub class file by hand.
• Implement the callback client interfaceclient
  interface impl class
• using rmic to generate a stub class and a
  skeleton class for it for both client callback
  interface and server interface.
• Set up a java.policy file.
• Activate the client, specifying (i) the server
  host name, (ii) the security policy file, and
  (iii) the codebase if stub downloading is desired.

37
CallbackClient

• public class CallbackClient
• public static void main(String args)
• try // stub downloading
• System.setSecurityManager(new
  RMISecurityManager())
• String registryURL "rmi//cs1.cs.gmu.edu
  " portNum "/callback"
• CallbackServerInterface h (CallbackServerInterfa
  ce)Naming.lookup(registryURL)
• CallbackClientInterface callbackObj new
  CallbackClientInterfaceImpl()
• // register for callback
• h.registerForCallback(callbackObj)
• System.out.println (Registered for
  callback.")
• h.unregisterForCallback(callbackObj)
• catch (Exception e)
• System.out.println ("Exception in
  CallbackClient " e) // end catch
• // end of main()
• //end class

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Summary-1

• Stub downloading allows a stub class to be
  downloaded to an object client at runtime,
  thereby allowing a remote objects implementation
  to be modified and its stub class regenerated
  without affecting the software on the client
  host.
• A security manager oversees access restrictions
  specified in a Java security policy file, which
  can be a system-wide policy file, or a policy
  file applied to an individual application only.
• For security protection, the use of security
  managers is recommended in all RMI applications,
  regardless of whether stub downloading is
  involved.

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Summary-2

• Client callback
• Client callback is useful for an application
  where the clients desire to be notified by the
  server of the occurrence of some event.
• Client callback allows an object server to make a
  remote method call to a client via a reference to
  a client remote interface.

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Summary-3

• Client callback
• To provide client callback, the client-side
• supplies a remote interface,
• instantiates a callback interface object
• passes a reference to the object to the server
  via a remote method call to the server.
• The object server
• collects these client references in a data
  structure.
• invokes a callback method, defined in the client
  remote interface, to pass data to the client,
  when the awaited event occurs.
• Two sets of stub-skeletons are needed one for
  the server remote interface, the other one for
  the client remote interface.