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					George Blank
University Lecturer
   Java RMI

Introduction to RMI
    Java and the Web
• Java’s popularity is due to its suitability for use
  on the World Wide Web (WWW).
• Several Web browsers have support for Java:
   – Netscape Navigator
   – Internet Explorer
   – HotJava
• Java incorporates audio, video, and animation
  directly on the web page.
Method Modifiers
•   Instance Methods
•   Class Methods
•   Abstract Methods
•   Native Methods
•   Final Methods
  Method Modifiers with Effect on
• public: Method can be accessed by any class
• private: methods accessed only by class
• friendly: methods accessed by other methods in
  the same package
• protected: methods accessed by subclass
• A Java interface is similar to a class, except
  there is no data associated with the interface.
• Example:
public interface MyInterface {
  methods-with no implementation details
  final variables
Properties of Interfaces
• The variables in an interface must be final.
• The methods are only declarations.
• A class that extends another class is
  guaranteed to support the contracts entered
  into by its superclass.
• Interfaces are used to approximate multiple
Java Native Interface (JNI)
• The Java Native Interface (JNI) is part of the
  core JDK and provides a framework for
  interfacing to native code. The native code is
  not easily portable across different hardware
  platforms. So using native code takes way one
  of the major advantages of java. JNI was
  developed to bridge the gap.
                  (JNI section by Rekha Telkar)
When to use JNI
• When the standard Java class library does not
  support the platform-dependent features
  needed by the application.
• You need to use a library written in another
  language, and wish to make it accessible to
  Java code through the JNI.
• You want to implement a small portion of
  time-critical code in a lower-level language
  such as assembly.
JNI Capabilities
• You can use native methods to:
  – Create, inspect, and update Java objects
    (including arrays and strings).
  – Call Java methods.
  – Catch and throw exceptions.
  – Load classes and obtain class information.
  – Perform runtime type checking.
Enabling Java code with JNI
• You can use the JNI with the Invocation
  API to enable an arbitrary native
  application to embed the Java VM.
  Programmers can make their existing
  applications Java-enabled without
  having to link with the VM source code.
     Remote Method Invocation
• RMI provides the means to invoke methods
• RMI allows for applications to communicate and
  execute across multiple systems on a network.
• RMI is supported by the java.rmi, java.rmi.server,
  and java.rmi.registry
• Enhanced security of Java 2 requires a security
  policy implementation.
    Parts in a RMI System
• Interface definitions for remote services
• Implementations of the remote services
• Stub and Skeleton files
• A server to host the remote services
• An RMI Naming service that allows clients to find
  the remote services
• A class file provider (an HTTP or FTP server)
                      RMI process

  Java Client                         Java Server

  Client Method                        Called Method
arguments         results

  Client Stub                         Server Skeleton
                                          Not needed
                                          In Java 2
  Network transport                   Network Transport

RMI Server, Client, and Registry
• The server process registers the remote
  object X with the registry using the
  Naming.bind() method.
• The client calls Naming.lookup(), which
  contacts the registry and obtains a stub object
  for X.
• The client then uses the stub as if it is a local
Stub Class
• A stub for a remote object is the client-
  side proxy for the remote object. Such
  a stub implements all the interfaces
  that are supported by the remote object
  implementation. The client-side stub
  responsibilities are shown on the next
    Stub Class Responsibilities
• Initiating a call to the remote object (by calling the
  remote reference layer).
• Marshaling arguments to a marshal stream
  (obtained from the remote reference layer).
• Informing the remote reference layer that the call
  should be invoked.
• Unmarshaling the return value or exception from a
  marshal stream.
• Informing the remote reference layer that the call is
      Skeleton Class
• A skeleton for a remote object is a server-side entity
  that contains a method which dispatches calls to the
  actual remote object implementation. The skeleton is
  responsible for:
  – Unmarshaling arguments from the marshal stream.
  – Making the up-call to the actual remote object
  – Marshaling the return value of the call or an exception
    (if one occurred) onto the marshal stream.
Remote Reference Layer
• The remote reference layer deals with the
  lower level transport interface and is
  responsible for carrying out a specific remote
  reference protocol which is independent of the
  client stubs and server skeletons. The remote
  reference layer has two cooperating
  components: the client-side and the server-
  side components.
Remote Reference Layer (2)
• The client-side component contains
  information specific to the remote server (or
  servers, if the remote reference is to a
  replicated object) and communicates via the
  transport to the server-side component.
  During each method invocation, the client and
  server-side components perform the specific
  remote reference semantics.
Remote Reference Layer (3)
• For example, if a remote object is part of a
  replicated object, the client-side component
  can forward the invocation to each replica
  rather than just a single remote object.
• In a corresponding manner, the server-side
  component implements the specific remote
  reference semantics prior to delivering a
  remote method invocation to the skeleton.
     Remote Reference Layer (4)
• For example, the server side could handle ensuring
  atomic multiple delivery by communicating with
  other servers in the replica group.The remote
  reference layer transmits data to the transport layer
  via the abstraction of a stream-oriented connection.
  The transport takes care of the implementation
  details of connections. Although connections present
  a streams-based interface, a connectionless
  transport may be implemented beneath the
RMI Registry
• The Registry tracks the addresses of the
  remote objects exported by applications
• It is the central management point for RMI
• Does not actually invoke remote methods
• Bind() links the object in the registry
• Rebind() replaces object with a new one
Parameter Passing
• When a remote procedure is executed, the
  java.rmi runtime encodes the arguments and
  sends them over the network to a server that
  decodes them.
• The server then invokes the method, encodes
  the results, and sends it back.
• Finally, the client-side java.rmi runtime
  decodes the result.
Parameter Marshalling
• RMI stubs are responsible for packaging
  parameters used in a remote method in a
  block of bytes using the big-endian byte
  order. This is called parameter marshalling.
  A receiver object on the server must
  unmarshall the parameters or report errors.
Building RMI Applications
• Define remote interfaces
• Create classes that implement the interfaces
• Create stub and skeleton classes for the
  implementation classes.
• Create Security Policy
A Distributed Hello World Program
Using RMI

   • It uses an applet to make a remote method
     call to the server from which it was
     downloaded to retrieve the message "Hello
   • When the applet runs, “Hello World!” is
     displayed on the client browser.
Steps Involved
• Write The HTML and Java Source Files.
• Compile and Deploy Class Files and HTML
• Start the Remote Object Registry, Server,
  and Applet
Source Files
• The Java remote interface. (
• The Java remote object (server) which implements
  the remote interface. (
• The Java applet that remotely invokes the remote
  method, sayHello(). (
• The HTML code for the web page that references the
  applet. (hello.html)
The Remote Interface
• Must be public.
• Extends the interface java.rmi.Remote.
• Each method must declare
  java.rmi.RemoteException in its throws
• A remote object passed as an argument or
  return value must be declared as the remote
Remote Interface
package examples.hello;

import java.rmi.Remote;
import java.rmi.RemoteException;

public interface Hello extends
  java.rmi.Remote {
  String sayHello() throws
The Implementation Class
• Specify the remote interface(s) being
• Define the constructor for the remote object.
• Provide implementations for the methods that
  can be invoked remotely.
• Create and install a security manager.
The Implementation Class
(Cont d)

   • Create one or more instances of a remote
   • Register at least one of the remote objects
     with the RMI remote object registry, for
     bootstrapping purposes.
Server Code (1)

package examples.hello;
import java.rmi.Naming;
import java.rmi.RemoteException;
import java.rmi.RMISecurityManager;
import java.rmi.server.UnicastRemoteObject;

public class HelloImpl extends UnicastRemoteObject
  implements Hello {
  public HelloImpl() throws RemoteException {
Server Code (2)

 // Implementation of remote method

 public String sayHello() {
      return "Hello World!";
    Server Code (3) main begins

public static void main(String args[]) {

// Create and install a security manager
if (System.getSecurityManager() == null)
System.setSecurityManager(new RMISecurityManager());
Server Code (4) main ends
try {

HelloImpl obj = new HelloImpl();
// Bind this object instance to the name "HelloServer"
Naming.rebind("//", obj);
System.out.println("HelloServer bound in registry");
catch (Exception e) {
System.out.println("HelloImpl err: " + e.getMessage());
Notes on Server Code
• By extending remote class UnicastRemoteObject, the HelloImpl
  class can be used to create a remote object that:
   – Uses RMI's default sockets-based transport for
   – Runs all the time (In Java 2 SDK, the object can be activated
      (created) when a client requests it, using Remote Object
      Activation, rather than running all the time – extend
• To bind to a different port, for example 2001, use:
  Naming.rebind("//", obj);
Notes on Server Code (contd)

   • Arguments to, or return values from, remote methods can be
     any data type for the Java platform, including objects, as long as
     those objects implement the interface
   • By default, local objects are passed by copy, which means that
     all data members (or fields) of an object are copied, except
     those marked as static or transient
   • Remote objects are passed by reference. A reference to a
     remote object is actually a reference to a stub, which is a client-
     side proxy for the remote object.
Notes on Server Code (contd)

   • Security manager guarantees that the classes that get loaded
     perform only allowed operations.
   • If no security manager is specified, no class loading, by RMI
     clients or servers, is allowed, aside from what can be found in
     the local CLASSPATH.
   • Client Applets use the security manager already installed in the
     client browser.
   • If the client were an application rather than an applet, Security
     manager would need to be installed.
   • A security manager is required in any JVM that needs to
     download code, and RMI clients need to download RMI stubs
     (as well as any other custom classes or interfaces needed to
     communicate with the RMI server).
  A Remote Service Applet
package examples.hello;
import java.applet.Applet;
import java.awt.Graphics;
import java.rmi.Naming;
import java.rmi.RemoteException;

public class HelloApplet extends Applet {
  String message = "";
  // "obj" is the identifier that we'll use
  // to refer to the remote object that
  // implements the "Hello" interface
     Hello obj = null;
   Remote Applet Code (2)
public void init() {
     try {
       Hello obj = (Hello)Naming.lookup("//"
         + getCodeBase().getHost()
         + "/HelloServer");
       message = obj.sayHello();
     } catch (Exception e) {
       System.out.println("Applet exception: "
             + e.getMessage());
    Remote Applet Code (3)
    public void paint(Graphics g) {
         g.drawString(message, 25, 50);

Notes on Client Applet
• The applet gets a reference to the remote object
  implementation (advertised as "HelloServer") from
  the server host's rmiregistry.
• The applet invokes the remote sayHello method on
  the server's remote object.
• The applet invokes the paint method, causing the
  string "Hello World!" to be displayed in the drawing
  area of the applet.
   The Web Page (hello.html)
<title>Hello World</title>
<center> <h1>Hello World</h1> </center>

<applet codebase="myclasses/ "
        width=500 height=120>
Compiling the Java Source Code

  • Create a directory myclasses in your
    public_html on AFS and compile the java
  • javac -d $HOME/public_html/myclasses
     Generate Stubs and Skeletons
rmic -d $HOME/public_html/myclasses examples.hello.HelloImpl

The following files are created in the directory:

      HelloImpl_Skel.class (before Java 2 only)
Java 2 SDK Stubs
• In the Java 2 SDK implementation of RMI, skeleton classes are
  obsolete. RMI uses reflection to make the connection to the
  remote service object.
• If you will never need support for 1.1 clients, rmic can be run
  with the -v1.2 option

• rmic –v1.2 -d $HOME/public_html/myclasses

• This creates only

    Deploying Applet (1)
• Move the HTML File (hello.html) to the Deployment

Sample Policy File for User
(.java.policy) in $HOME

   grant {
      permission "*:80",
      Replace * with name or IP of client to restrict
Security Policy
• In a Unix environment, you will probably need
  to create a Security Policy for the Client
  Application but not for an applet. The default
  policy might work in Windows.
• To create a security policy, see a Java text
  book. Just Java 2, pages 351-368, gives an
  explanation. Some students have expressed
  a preference for the explanation in Core Java
Start the Remote Object Registry

 • The RMI registry is a simple server-side bootstrap name server that
   allows remote clients to get a reference to a remote object.
 • Start registry on server - UNIX:
         rmiregistry &
 • Windows:
         start rmiregistry

 • To start on a port other than the default 1099, for example port

         rmiregistry 2001 &
   Starting the Server
java -
/ -$HOME/.java.policy examples.hello.HelloImpl

The output should look like this:

     HelloServer bound in registry
     Starting the Server
• When starting the server, the java.rmi.server.codebase
  property must be specified, so that the stub class can be
  dynamically downloaded to the registry and then to the
• Run the server, setting the codebase property to be the
  location of the implementation stubs. Because the
  codebase property in this example references a directory,
  make sure that any other classes that may need to be
  downloaded have also been installed in the directory
  referenced by java.rmi.server.codebase
Starting the Server
• Note: A stub class is dynamically downloaded to a
  client's virtual machine only when the class is not
  already available locally and the
  java.rmi.server.codebase property has been set
  properly to specify where the class files are located
  on the server.
• You can also specify the property
  for the server to set permissions.
Common Exception
• If you forget the trailing slash on the codebase
  property, or if the class files can't be located at the
  source (they aren't really being made available for
  download) or if you misspell the property name, you'll
  get thrown a java.lang.ClassNotFoundException.
• You will also need to use a VPN if you access the
  RMI server from off-campus because AFS security
  does not allow remote connections from unsigned
  applets. Otherwise you will get a
Running the Applet
   appletviewer &

Or use the Browser URL:

String Hello World should display.

Note:    The Applet sandbox allows them to connect only to
         their host URL. Therefore, the AFS hostname should be
   of the AFS machine on which the RMI          server is running.
Note: Use,
not afsxx as shown. That
will not work as Web
server access to AFS is
now disabled.

    Advantages of RMI
•   Object-Oriented
•   Safe and Secure
•   Easy to Write/Easy to Use
•   Connects to Existing/Legacy Systems (JNI)
•   Write Once, Run Anywhere
•   Distributed Garbage Collection
•   Parallel Computing
    Garbage Collection
• The Java Virtual Machine has an automatic
  garbage collector that will reclaim the memory
  from any object that has been discarded by the
  running program. Garbage collection is difficult if
  the server and client run on different machines.
  The RMI subsystem implements a reference
  counting-based distributed garbage collection
  (DGC) algorithm to provide automatic memory
  management facilities for remote server objects.
      How DGC works
• The remote server keeps track of all external client
  references. When a reference is made, the server
  increases the object reference count by one and
  marks the object as "dirty". When a client drops the
  reference, the DGC decreases its reference count by
  one and marks the object as "clean." When the
  reference count reaches zero, the remote object is
  free of any live client references. It is then placed
  on the weak reference list and subject to periodic
  garbage collection.
RMI Limitations
•   RMI is not full featured middleware
•   No mechanism for object description
•   No server events
•   Java only, not language independent
•   Firewalls are not supported
•   Naming service is not persistent
•   No load balancing
• Users fill in the fields of an expense report.
  Clients communicate with the server using
  RMI. The server stores the expense reports in
  a database using JDBC.
Remote Interface
import java.rmi.*;
public interface ExpenseServer extends Remote {
  Policy getPolicy() throws RemoteException;
  void submitReport(ExpenseReport report)
        throws RemoteException,
The Policy Interface
public interface Policy {
  void checkValid(ExpenseEntry entry)
        throws PolicyViolationException;
Client Code

Policy curPolicy = server.getPolicy();
           while (user keeps adding entries) {
            try {
              // throws exception if not OK
                add the entry to the expense report
            } catch (PolicyViolationException e) {
                show the error to the user
            } }
Server Code (1)

import java.rmi.*;
import java.rmi.server.*;
class ExpenseServerImpl extends UnicastRemoteObject
            implements ExpenseServer
Server Code (2)

    ExpenseServerImpl() throws RemoteException {
           // ...set up server state...
    public Policy getPolicy() {
           return new TodaysPolicy();
    public void submitReport(ExpenseReport report) {
             // ...write the report into the db...
    }            }
Implementing a Policy

public class TodaysPolicy implements Policy {
      public void checkValid(ExpenseEntry entry)
           throws PolicyViolationException
           if (entry.dollars() < 20)
                return;     // no receipt required
           throw new PolicyViolationException(
                                          "Need a receipt");
  New Policy Implementation
public class TomorrowsPolicy implements Policy {
      public void checkValid(ExpenseEntry entry)
               throws PolicyViolationException
              if (entry.isMeal() && entry.dollars() < 20)
                       return;      // no receipt required
             throw new PolicyViolationException(
                                        "Need a receipt");
• Java RMI is a distributed object model for the
  Java platform.
• RMI extends the Java object model beyond a
  single virtual machine address space.
• RMI uses object serialization to convert object
  graphs to byte streams for transport.
Defining Interfaces
• All methods that can be run remotely must be
  declared as part of an interface that extends
  Remote .

public interface RemOp extends Remote {
  public void call() throws RemoteException;
Creating Classes that Implement
the Interfaces
• Classes that implement the remote interfaces
  must be subclasses of the RemoteObject
• RemoteObject provides support for the
  hashCode(), equals(), and toString().
    Interface Class Example (1)

package rmi1;

import java.rmi.*;
import java.rmi.server.*;
     Interface Class Example (2)
public class RemImpl extends UnicastRemoteServer
implements RemoOp {
      public static void main(String[] args) {
                          new StubSecurityManager());
        try {
             Naming.rebind(“operator”, new RemImpl());
        } catch (Exception x) {
             x.printStackTrace(); return; } }
Interface Class Example (3)

    public RemImpl() throws RemoteException {

    public void call() throws RemoteException {
      System.out.println(“Location: “ +
    Interface Class Example (4)

          public String getName() {
          return “Remote operation: “ + hashCode();
Interface Class Example (5)

 package rmi1;

 import java.rmi.*;
 import java.rmi.server.*;
Interface Class Example (6)

public class OpTest {
 public static void main(String[] args) {
                          new StubSecurityManager()):
   try {
     RemOp ro = (RemOp) Naming.lookup(“operator”);;
   } catch (Exception x) { x.printStackTrace();
              System.exit(-1); } } }
Running the rmi1 Example
• java java.rmi.registry.RegistryImpl
• java -DLOCATION=server rmi1.RemImpl
• java -DLOCATION=client rmi1.OpTest

 Remote operation: 841549921
 Location: server
RMI Security

• One of the most common problems encountered with
  RMI is a failure due to security constraints.
• The security policy can be set by constructing a
  SecurityManager object and calling the
  setSecurityManager method of the System class.
The SecurityManager class has a large number of
  methods whose name begins with check. For
  example, checkConnect (String host, int port).
 Security Continued

• If the current security policy does not allow connection to
  this host and port, then the call throws an exception. This
  usually causes the program to terminate with a message
  such as: access denied
Java-RMI vs. CORBA and DCOM

 • Java-RMI is a Java-specific middleware spec that
   allows client Java programs to invoke server Java
   objects as if they were local.
 • Java-RMI is tightly coupled with the Java
Comparison Continued

• Since Java-RMI is tightly coupled with The Java
  Language, Java-RMI can work with true sub-classes.
• Because of this, parameters passed during method calls
  between machines can be true Java Objects. This is
  impossible in DCOM or CORBA at present.
Comparison Continued

• If a process in an RMI system receives an object of a
  class that it has never seen before, it can request that
  its class information be sent over the network.
• Over and above all this, Java-RMI supports
  Distributed Garbage Collection that ties into the local
  Garbage Collectors in each JVM.
 Trouble Shooting

• If a JVM that is exporting remote objects does not have the
  java.rmi.server.codebase property set correctly, you will get
  a class not found error.
• Default port for RMI RegistryImpl is 1099. If it is already in
  use, you will get a Address already in use.
• Do not try to use the loopback address,
  improperly. It does not work on a network.
• RMI is subject to Connection Exceptions that are
  idiosyncratic and difficult to overcome. This is one of the
  most common topics in RMI discussions.
 Problems Encountered

• A java.rmi.NotBoundException was overcome by
  bypassing the start command and starting the server
  directly. This is not recommended, as the program will not
  exit normally. NOTE: A probable cause is another version
  of Java on the system that is referenced by the start
• Several java.rmi.UnmarshallException were caused by
  classes not in the class path. Some students just had to
  add the current directory:
  set classpath=.’%classpath%
 Problems Encountered

• An Incompatible Type Exception error occurred when a
  group failed to cast a returned reference to the
  <ClassName>_stub type.
• A required creating
  a policy file and referencing it as
• Don’t overlook standard programming errors like syntax
  errors, missing try blocks, and naming and typing
• When you have a problem, you may have to kill all your
  processes and restart everything.
• You must start the registry (Windows command)
   start rmiregistrystart
• All remote interfaces must extend
  java.rmi.RemoteInterface, and all remote objects must
  extend java.rmi.UnicastRemoteObject.
• Security management and implementing class policies
  are complex and difficult in RMI. The client policy file
  permits connection to a specific referenced port number.
     Lessons Learned
• You can get a trace of the server activity using
  java -Djava.rmi.server.logCalls=true <YourServer>
• Properties beginning java.rmi.* are part of published RMI
  spec and docs, while sun.rmi.* indicates things that may
• You can find the host name of a caller of a remote
  method with
• On a server with multiple host names, you need to force
  a fully qualified host name with
     Environment Conflicts
• If you may have multiple versions of Java on your
  system, try java -version to see which version your
  environment defaults to. Remember that different
  versions seldom work together, and that your Windows
  autoexec.bat file may set some environment values that
  you might not think about.
Good RMI explanation
• The example from the Sun Web site is difficult
  to get working, because it includes a lot of
  material that goes beyond the basics. You
  may wish to find a simpler example. One
  excellent one was at
     RMI Versions
• Java 2 SDK, version 1.4 adds server-side stack trace
  retention, Service Provider Interface for RMIClassLoader and
  Dynamic Server Host Name.
• Java 2 SDK, version 1.3 adds enhancements to serialization
• Java 2 SDK, version 1.2 adds activation, the use of custom
  socket protocols and security policies.
• Java 1.1 added core RMI support, but usually requires
  running applets in the applet viewer.
• Java 1.0.2 was an interim release, and should not be used for


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