Remote Method Invocation: Agenda

1
1
2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
Web
core
programming
Remote Method
Invocation
Remote Method Invocation 2
www.corewebprogramming.com
Agenda
Steps to build and RMI application
Running and compiling an RMI program
Eample: Retrieving a String remotely
Example: Performing numerical integration
remotely
Enterprise RMI configuration
RMI Applets
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RMI: Remote Method Invocation
Idea
Distribute objects across different machines to take
advantage of hardware and dedicated software
Developer builds network service and installs it on
specified machine
User requests an instance of a class using URL syntax
User uses object as though it were a regular, local object
Network connections happen automatically behind
the scenes
Java serialization lets you pass complex data
structures over the network without writing code to
parse and reconstruct them
RMI Operations
Stub Operation
Package identifier of remote object
Package method identifier
Marshall parameters
Send package to server skeleton
Skeleton Operation
Unmarshall Parameters
Calls return value or exception
Marshall method return
Send package to client stub
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RMI Details
1. Starting: Build Four Required Classes
a. An interface for the remote object
Used by both the client and the server
b. The RMI client
This will look up the object on the remote server, cast it to the
type of the interface from Step 1, then use it like a local object.
Note that as long as there is a live reference to the remote
object, an open network connection is maintained. The
connection will be automatically closed when the remote
object is garbage collected on the client.
c. The object implementation
This object needs to implement the interface of Step a, and will
be used by the server
d. The RMI server
This will create an instance of the object from Step c and
register it with a particular URL
RMI Details, cont.
2. Compile and Run the System
a. Compile client and server.
Compiles the remote object interface and implementation
automatically
b. Generate the client stub and the server skeleton
Use the rmic compiler on the remote object implementation
for this.
The client system will need the client class, the interface
class, and the client stub class
If the client is an applet, these three classes must be
available from the applets home machine
The server system will need the server class, the remote
object interface and implementation, and the server
skeleton class
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RMI Details, cont.
2. Compile and Run the System, cont.
c. Start the RMI registry
This only needs to be done once, not for each remote object
The current version of RMI requires this registry to be running
on the same system as server
d. Start the server
This step must be on the same machine as the registry of
step c
e. Start the client
This step can be done on an arbitrary machine
A Very Simple RMI Example:
The Four Required Classes
1. The Interface for the Remote Object
The interface should extend java.rmi.Remote, and
all its methods should throw
java.rmi.RemoteException
import java.rmi.*;
/** The RMI client will use this interface directly.
* The RMI server will make a real remote object that
* implements this, then register an instance of it
* with some URL.
*/
public interface Rem extends Remote {
public String getMessage() throws RemoteException;
}
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Simple Example,
Required Classes, cont.
2. The RMI Client
Look up the object from the host using Naming.lookup, cast it
to the appropriate type, then use it like a local object
import java.rmi.*; // For Naming, RemoteException, etc.
import java.net.*; // For MalformedURLException
import java.io.*; // For Serializable interface
public class RemClient {
public static void main(String[] args) {
try {
String host = (args.length > 0) ? args[0] : "localhost";
Rem remObject = (Rem)Naming.lookup("rmi://" + host + "/Rem");
System.out.println(remObject.getMessage());
} catch(RemoteException re) {
System.out.println("RemoteException: " + re);
} catch(NotBoundException nbe) {
System.out.println("NotBoundException: " + nbe);
} catch(MalformedURLException mfe) {
System.out.println("MalformedURLException: " + mfe);
}
}
}
Simple Example,
3. The Remote Object Implementation
This class must extend UnicastRemoteObject and
implement the remote object interface defined earlier
The constructor should throw RemoteException
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
public class RemImpl extends UnicastRemoteObject
implements Rem {
public RemImpl() throws RemoteException {}
public String getMessage() throws RemoteException {
return("Here is a remote message.");
}
}
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Simple Example,
4. The RMI Server
The server builds an object and register it with a particular URL
Use Naming.rebind (replace any previous bindings) or
Naming.bind (throw AlreadyBoundException if a previous
binding exists)
import java.rmi.*;
import java.net.*;
public class RemServer {
try {
RemImpl localObject = new RemImpl();
Naming.rebind("rmi:///Rem", localObject);
}
}
}
Simple Example: Compiling and
Running the System
1. Compile the Client and the Server
Prompt> javac RemClient.java
This compiles the Rem interface automatically
Prompt> javac RemServer.java
This compiles the RemImpl object implementation automatically
2. Generate the Client Stub and Server Skeleton
Prompt> rmic RemImpl
This builds RemImpl_Stub.class and
RemImpl_Skeleton.class
The client machine needs Rem.class, RemClient.class,
and RemImpl_Stub.class
The server machine needs Rem.class, RemImpl.class,
RemServer.class, and RemImpl_Skeleton.class
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Simple Example: Compiling and
Running the System, cont.
3. Start the RMI Registry
Server> rmiregistry
On Unix systems you would probably add & to put the registry
process in the background
You can also specify a port number; if omitted, port 1099 is used
4. Start the Server
Server> java RemServer
Again, on Unix systems you would probably add & to put the
process in the background
5. Start the Client
Client> java RemClient hostname
Here is a remote message.
A Better RMI Example,
Numerical Integration
1. Simple Iterative Program
to Calculate Sums:
2. Use to Approximate
Numeric Integrals of the Form:
3. MidPoint Rule:
4. Motivation for RMI
Since smaller rectangles typically give better results, this can often
be very cpu-intensive
RMI can make it available on a fast floating-point box
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Numerical Integration,
Example, cont.
public class Integral {
/** Returns the sum of f(x) from x=start to x=stop, where the function f
* is defined by the evaluate method of the Evaluatable object.
*/
public static double sum(double start, double stop,
double stepSize,
Evaluatable evalObj) {
double sum = 0.0, current = start;
while (current <= stop) {
sum += evalObj.evaluate(current);
current += stepSize;
}
return(sum);
}
public static double integrate(double start, double stop,
int numSteps,
double stepSize = (stop - start) / (double)numSteps;
start = start + stepSize / 2.0;
return(stepSize * sum(start, stop, stepSize, evalObj));
}
}
Numerical Integration,
Example, cont.
/** An interface for evaluating functions y = f(x) at a specific
* value. Both x and y are double-precision floating-point
* numbers.
*/
public interface Evaluatable {
public double evaluate(double value);
}
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Integration Example:
Four Required Classes
1. The RemoteIntegral Interface
The interface shared by the client and server
import java.rmi.*;
public interface RemoteIntegral extends Remote {
public double sum(double start, double stop, double stepSize,
Evaluatable evalObj)
throws RemoteException;
public double integrate(double start, double stop,
int numSteps, Evaluatable evalObj)
throws RemoteException;
Four Required Classes, cont.
2. The Remote Integral Client
Sends the RemoteIntegral an Evaluatable to integrate
public class RemoteIntegralClient {
try {
RemoteIntegral remoteIntegral =
(RemoteIntegral)Naming.lookup("rmi://" + host + "/RemoteIntegral");
for(int steps=10; steps<=10000; steps*=10) {
System.out.println("Approximated with " + steps + " steps:" +
"\n Integral from 0 to pi of sin(x)=" +
remoteIntegral.integrate(0.0, Math.PI, steps, new Sin()));
}
System.out.println("'Correct' answer using Math library:" +
(-Math.cos(Math.PI) - -Math.cos(0.0)));
} catch(NotBoundException nbe) {
System.out.println("NotBoundException: " + nbe);
}
}
}
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2. The Remote Integral Client, cont.
Evaluatable Sin function
import java.io.Serializable;
class Sin implements Evaluatable, Serializable {
public double evaluate(double val) {
return(Math.sin(val));
}
public String toString() {
return("Sin");
}
}
3. The Remote Integral Implementation
Remote object that calculates the integral value
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
public class RemoteIntegralImpl extends UnicastRemoteObject
implements RemoteIntegral {
public RemoteIntegralImpl() throws RemoteException {}
public double sum(double start, double stop, double stepSize,
return(Integral.sum(start, stop, stepSize, evalObj));
}
public double integrate(double start, double stop, int numSteps,
return(Integral.integrate(start, stop, numSteps, evalObj));
}
}
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4. The Remote Integral Server
Creates the RemoteIntegral and registers it with
the rmi registry
import java.rmi.*;
import java.net.*;
public class RemoteIntegralServer {
try {
RemoteIntegralImpl integral = new RemoteIntegralImpl();
Naming.rebind("rmi:///RemoteIntegral", integral);
}
}
}
Integration Example: Compiling
and Running the System
Prompt> javac RemoteIntegralClient.java
Prompt> javac RemoteIntegralServer.java
Prompt> rmic v1.2 RemoteIntegralImpl
Client requires: RemoteIntegral.class,
RemoteIntegralClient.class and
RemoteIntegralImpl_Stub.class
Server requires: RemoteIntegral.class,
RemoteIntegralImpl.class, and
RemoteIntegralServer.class
If the server and client are both running JDK 1.1, use the -v1.1
switch to produce the RMI 1.1 skeleton stub,
RemoteIntegralImpl_Skeleton, required by the server
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Integral Example: Compiling
and Running the System, cont.
Prompt> rmiregistry
4. Start the Server
Prompt> java RemoteIntegralServer
5. Start the Client
Prompt> java RemoteIntegralClient
Approximated with 10 steps:
Integral from 0 to pi of sin(x)=2.0082484079079745
...
Enterprise RMI Configuration
Stub files need to be placed on a HTTP
server for downloading
In Java 2, the RMI 1.2 protocol does not require the
skeleton
Client must install an RMISecurityManager
to load the RMI classes remotely
System.setSecurityManager(new RMISecurityManager());
Client requires a policy file to connect to
registry and HTTP server
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Policy File for Client
grant {
// rmihost - RMI registry and the server
// webhost - HTTP server for stub classes
permission java.net.SocketPermission
"rmihost:1024-65535", "connect";
permission java.net.SocketPermission
"webhost:80", "connect";
};
Need to grant permission to ports 1024-65535 on the server
The server communicates with the rmiregistry (and client)
on a randomly selected source port
Alternatively, can set policies in java.policy located in
JAVA_HOME/lib/security/
Enterprise RMI,
Remote Integral, Example
public class RemoteIntegralClient2 {
try {
System.setSecurityManager(new RMISecurityManager());
RemoteIntegral remoteIntegral =
(RemoteIntegral)Naming.lookup("rmi://" + host +
"/RemoteIntegral");
for(int steps=10; steps<=10000; steps*=10) {
System.out.println
("Approximated with " + steps + " steps:" +
remoteIntegral.integrate(0.0, Math.PI,
steps, new Sin()));
}
...
}
...
}
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Enterprise Example: Compiling
and Running the System
Prompt> javac RemoteIntegralClient2.java
Prompt> javac RemeteIntegralServer.java
Prompt> rmic v1.2 RemoteIntegralImpl
3. Place the files on the correct machines
4. Start the HTTP Server
Place RemoteIntegral_Stub.class,
RemoteIntegeral.class, and
Evaluatable.class on an HTTP server
Verify that you can access the files through a browser
Server> /somedirectory/rmiregistry
Make sure that none of the class files are in the directory in
which you started the registry or available through the
classpath
6. Start the Server
Server> java -Djava.rmi.server.codebase=http://webhost/rmi/
RemoteIntegralServer
Server must be started on same host as rmiregistry
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7. Start the Client
Client> java -Djava.security.policy=rmiclient.policy
RemoteIntegralClient2 rmihost
...
The rmihost is where server in which the rmiregistry
was started
An RMI Applet
Applet does not require a RMI Security
Manager
Applet can only access server in which
class files were loaded
RMI Registry and remote object server must be the same
HTTP host in which the applet was loaded
RMI 1.1 stub protocol not properly
supported in IE
RMI 1.2 stub protocol require Java Plug-In
or Netscape 6
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RMI Applet, Example
...
import javax.swing.*;
public class RemoteIntegralApplet extends JApplet
implements ActionListener {
private Evaluatable[] shapes;
private RemoteIntegral remoteIntegral;
private JLabel result;
private JTextField startInput, stopInput, stepInput;
private JComboBox combo;
public void init() {
String host = getCodeBase().getHost();
try {
remoteIntegral =
(RemoteIntegral)Naming.lookup("rmi://" + host +
"/RemoteIntegral");
reportError("RemoteException: " + re);
}
...
RMI Applet, Example
...
public void actionPerformed(ActionEvent event) {
try {
int steps = Integer.parseInt(stepInput.getText());
double start = Double.parseDouble(startInput.getText());
double stop = Double.parseDouble(stopInput.getText());
showStatus("Calculating ...");
Evaluatable shape = (Evaluatable)combo.getSelectedItem();
double area = remoteIntegral.integrate(start, stop,
steps, shape);
result.setText(Double.toString(area));
showStatus("");
} catch(NumberFormatException nfe) {
reportError("Bad input: " + nfe);
reportError("RemoteException: " + re);
}
}
}
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RMI Applet, Result
Applet that communicates to a
remote object through RMI in Netscape 6
Summary
RMI is a pure Java-based protocol for
communicating with remote objects
Register (bind) and look-up remote objects in a
registry
Java 2 no longer requires the skeleton class
needed with the RMI 1.1 protocol
Enterprise RMI configuration requires a RMI
Security Manager and client policy file for
permissions
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2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
Web
core
programming
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