Lesson 8: Remote Method Invocation

The Java Remote Method Invocation (RMI) application programming interface (API) enables client and server communications over the net. Typically, client programs send requests to a server program, and the server program responds to those requests.

A common example is sharing a word processing program over a network. The word processor is installed on a server, and anyone who wants to use it starts it from his or her machine by double clicking an icon on the desktop or typing at the command line. The invocation sends a request to a server program for acess to the software, and the server program responds by making the software available to the requestor.

The RMI API lets you create a publicly accessible remote server object that enables client and server communications through simple method calls on the server object. Clients can easily communicate directly with the server object and indirectly with each other through the server object using Uniform Resource Locators (URLs) and HyperText Transfer Protocol (HTTP).

This lesson explains how to use the RMI API to establish client and server communications.

About the Example

This lesson converts the File Input and Output application from Lesson 6: File Access and Permissions to the RMI API.

Program Behavior

The RMIClient1 program presents a simple user interface and prompts for text input. When you click the Click Me button, the text is sent to the RMIClient2 program by way of the remote server object. When you click the Click Me button on the RMIClient2 program, the text sent from RMIClient1 appears.

First Instance of Client 1

If you start a second instance of RMIClient1 and type in some text, that text is sent to RMIClient2 when you click the Click Me button. To see the text received by RMIClient2, click its Click Me button.

Second Instance of Client 1

File Summary

The example program consists of the RMIClient1 program, remote object and interface, and the RMIClient2 program as illustrated in the diagram. The corresponding source code files for these executables are described in the bullet list below.

RMIClient1.java: Client program that calls the sendData method on the RemoteServer server object.
RMIClient2.java: Client program that calls the getData method on the RemoteServer server object.
RemoteServer.java: Remote server object that implements Send.java and the sendData and getData remote methods.
Send.java: Remote interface that declares the sendData and getData remote server methods.

In addition, the following java.policy security policy file grants the permissions needed to run the example.

grant {
  permission java.net.SocketPermission 
               "*:1024-65535", 
               "connect,accept,resolve";
  permission java.net.SocketPermission 
               "*:80", "connect";
  permission java.awt.AWTPermission 
               "accessEventQueue";
  permission java.awt.AWTPermission 
               "showWindowWithoutWarningBanner";
};

Compile the Example

These instructions assume development is in the zelda home directory. The server program is compiled in the home directory for user zelda, but copied to the public_html directory for user zelda where it runs.

Here is the command sequence for the Unix and Win32 platforms; an explanation follows.

                   
Unix:
cd /home/zelda/classes
javac Send.java
javac RemoteServer.java
javac RMIClient2.java
javac RMIClient1.java
rmic -d . RemoteServer
cp RemoteServer*.class /home/zelda/public_html/classes
cp Send.class /home/zelda/public_html/classes


                   
Win32:
cd \home\zelda\classes
javac Send.java
javac RemoteServer.java
javac RMIClient2.java
javac RMIClient1.java
rmic -d . RemoteServer
copy RemoteServer*.class \home\zelda\public_html\classes
copy Send.class \home\zelda\public_html\classes

The first two javac commands compile the RemoteServer and Send class and interface. The third javac command compiles the RMIClient2 class. The last javac command compiles the RMIClient1 class.

The next line runs the rmic command on the RemoteServer server class. This command produces output class files of the form ClassName_Stub.class and ClassName_Skel.class. These output classes let clients invoke methods on the RemoteServer server object.

The first copy command moves the RemoteServer class file with its associated skel and stub class files to a publicly accessible location in the /home/zelda/public_html/classes directory, which is on the server machine, so they can be publicly accessed and downloaded. They are placed in the public_html directory to be under the web server running on the server machine because these files are accessed by client programs using URLs.

The second copy command moves the Send class file to the same location for the same reason. The RMIClient1 and RMIClient2 class files are not made publicly accessible; they communicate from their client machines using URLs to access and download the remote object files in the public_html directory.

RMIClient1 is invoked from a client-side directory and uses the server-side web server and client-side Java VM to download the publicly accessible files.
RMIClient2 is invoked from a client-side directory and uses the server-side web server and client-side Java VM to download the publicly accessible files.

Start the RMI Registry

Before you start the client programs, you must start the RMI Registry, which is a server-side naming repository that allows remote clients to get a reference to the remote server object.

Before you start the RMI Registry, make sure the shell or window in which you run the rmiregistry command does not have a CLASSPATH environment variable that points to the remote object classes, including the stub and skel classes, anywhere on your system. If the RMI Registry finds these classes when it starts, it will not load them from the server-side Java VM, which will create problems when clients try to download the remote server classes.

The following commands unset the CLASSPATH and start the RMI Registry on the default 1099 port. You can specify a different port by adding the port number as follows: rmiregistry 4444 &. If you specify a different port number, you must specify the same port number in your server-side code as well.

                   
Unix:
cd /home/zelda/public_html/classes
unsetenv CLASSPATH
rmiregistry &


                   
Win32:
cd \home\zelda\public_html\classes
set CLASSPATH=
start rmiregistry

Note: You might want to set the CLASSPATH back to its original setting at this point.

Run the RemoteServer Server Object

To run the example programs, start RemoteServer first. If you start either RMIClient1 or RMIClient2 first, they will not be able to establish a connection because the remote server object is not running.

In this example, RemoteServer is started from the /home/zelda/public_html/classes directory.

The lines beginning at java should be all on one line with spaces where the lines break. The properties specified with the -D option to the java interpreter command are program attributes that manage the behavior of the program for this invocation.

                   
Unix:
cd /home/zelda/public_html/classes
java 
-Djava.rmi.server.codebase=http://kq6py/~zelda/classes
-Djava.rmi.server.hostname=kq6py.eng.sun.com
-Djava.security.policy=java.policy RemoteServer


                   
Win32:
cd \home\zelda\public_html\classes
java -Djava.rmi.server.codebase=file:
       c:\home\zelda\public_html\classes
-Djava.rmi.server.hostname=kq6py.eng.sun.com
-Djava.security.policy=java.policy RemoteServer

The java.rmi.server.codebase property specifies where the publicly accessible classes are located.
The java.rmi.server.hostname property is the complete host name of the server where the publicly accessible classes reside.
The java.rmi.security.policy property specifies the policy file with the permissions needed to run the remote server object and access the remote server classes for download.
The class to execute ( RemoteServer).

Run the RMIClient1 Program

Here is the command sequence for the Unix and Win32 platforms; an explanation follows.

In this example, RMIClient1 is started from the /home/zelda/classes directory.

The lines beginning at java should be all on one line with spaces where the lines break. Properties specified with the -D option to the java interpreter command are program attributes that manage the behavior of the program for this invocation.

                   
Unix:
cd /home/zelda/classes

java -Djava.rmi.server.codebase=
                         http://kq6py/~zelda/classes/
-Djava.security.policy=java.policy 
                  RMIClient1 kq6py.eng.sun.com


                   
Win32:
cd \home\zelda\classes

java -Djava.rmi.server.codebase=
                         file:c:\home\zelda\classes\
-Djava.security.policy=java.policy 
                         RMIClient1 kq6py.eng.sun.com

The java.rmi.server.codebase property specifies where the publicly accessible classes for downloading are located.
The java.security.policy property specifies the policy file with the permissions needed to run the client program and access the remote server classes.
The client program class to execute ( RMIClient1), and the host name of the server ( Kq6py) where the remote server classes are.

Run RMIClient2

Here is the command sequence for the Unix and Win32 platforms; an explanation follows.

In this example, RMIClient2 is started from the /home/zelda/classes directory.

                   
Unix:
cd /home/zelda/classes
java -Djava.rmi.server.codebase=
                         http://kq6py/~zelda/classes
-Djava.security.policy=java.policy 
                         RMIClient2 kq6py.eng.sun.com


                   
Win32:
cd \home\zelda\classes
java -Djava.rmi.server.codebase=
                file:c:\home\zelda\public_html\classes
-Djava.security.policy=java.policy 
                         RMIClient2 kq6py.eng.sun.com

The java.rmi.server.codebase property specifies where the publicly accessible classes are located.
The java.rmi.server.hostname property is the complete host name of the server where the publicly accessible classes reside.
The java.rmi.security.policy property specifies the policy file with the permissions needed to run the remote server object and access the remote server classes for download.
The class to execute ( RMIClient2).

RemoteServer Class

The RemoteServer class extends UnicastRemoteObject and implements the sendData and getData methods declared in the Send interface. These are the remotely accessible methods.

UnicastRemoteObject implements a number of java.lang.Object methods for remote objects and includes constructors and static methods to make a remote object available to receive method calls from client programs.

class RemoteServer extends UnicastRemoteObject
                 implements Send {

  String text;

  public RemoteServer() throws RemoteException {
    super();
  }

  public void sendData(String gotText){
    text = gotText;
  }

  public String getData(){
    return text;
  }

The main method installs the RMISecurityManager and opens a connection with a port on the machine where the server program runs. The security manager determines whether there is a policy file that lets downloaded code perform tasks that require permissions.

mainRemoteServerkq6pySend

By default the server name uses port 1099. If you want to use a different port number, you can add it with a colon as follows: kq6py:4444. If you change the port here, you must start the RMI Registry with the same port number.

The try block creates an instance of the RemoteServer class and binds the name to the remote object to the RMI Registry with the Naming.rebind(name, remoteServer); statement.

  public static void main(String[] args){
    if(System.getSecurityManager() == null) {
      System.setSecurityManager(new 
               RMISecurityManager());
    }
    String name = "//kq6py.eng.sun.com/Send";
    try {
      Send remoteServer = new RemoteServer();
      Naming.rebind(name, remoteServer);
      System.out.println("RemoteServer bound");
    } catch (java.rmi.RemoteException e) {
      System.out.println("Cannot create 
                   remote server object");
    } catch (java.net.MalformedURLException e) {
      System.out.println("Cannot look up 
                   server object");
    }
  }
}

Note: The remoteServer object is type Send (see instance declaration at top of class) because the interface available to clients is the Send interface and its methods; not the RemoteServer class and its methods.

Send Interface

The Send interface declares the methods implemented in the RemoteServer class. These are the remotely accessible methods.

public interface Send extends Remote {

  public void sendData(String text) 
                throws RemoteException;
  public String getData() throws RemoteException;
}

RMIClient1 Class

The RMIClient1 class establishes a connection to the remote server program and sends data to the remote server object. The code to do these things is in the actionPerformed and main methods.

actionPerformed Method

The actionPerformed method calls the RemoteServer.sendData method to send text to the remote server object.

public void actionPerformed(ActionEvent event){
   Object source = event.getSource();

   if(source == button){
//Send data over socket
      String text = textField.getText();
      try{
        send.sendData(text);
      } catch (java.rmi.RemoteException e) {
        System.out.println("Cannot send data to server");
      }
      textField.setText(new String(""));
   }
}

main Method

The main method installs the RMISecurityManager and creates a name to use to look up the RemoteServer server object. The client uses the Naming.lookup method to look up the RemoteServer object in the RMI Registry running on the server.

The security manager determines whether there is a policy file that lets downloaded code perform tasks that require permissions.

  RMIClient1 frame = new RMIClient1();

  if(System.getSecurityManager() == null) {
    System.setSecurityManager(new RMISecurityManager());
  }

  try {
//args[0] contains name of server where Send runs
    String name = "//" + args[0] + "/Send";
    send = ((Send) Naming.lookup(name));
  } catch (java.rmi.NotBoundException e) {
    System.out.println("Cannot look up 
                 remote server object");
  } catch(java.rmi.RemoteException e){
    System.out.println("Cannot look up 
                 remote server object");
  } catch(java.net.MalformedURLException e) {
    System.out.println("Cannot look up 
                 remote server object");
  }

RMIClient2 Class

The RMIClient2 class establishes a connection with the remote server program and gets the data from the remote server object and displays it. The code to do this is in the actionPerformed and main methods.

actionPerformed Method

The actionPerformed method calls the RemoteServer.getData method to retrieve the data sent by the client program. This data is appended to the TextArea object for display to the end user on the server side.

public void actionPerformed(ActionEvent event) {
   Object source = event.getSource();

   if(source == button){
      try{
        String text = send.getData();
        textArea.append(text);
      } catch (java.rmi.RemoteException e) {
        System.out.println("Cannot send data 
                     to server");
      }
      }
   }
}

main Method

The main method installs the RMISecurityManager and creates a name to use to look up the RemoteServer server object. The args[0] parameter provides the name of the server host. The client uses the Naming.lookup method to look up the RemoteServer object in the RMI Registry running on the server.

The security manager determines whether there is a policy file that lets downloaded code perform tasks that require permissions.

  RMIClient2 frame = new RMIClient2();

  if(System.getSecurityManager() == null) {
    System.setSecurityManager(new RMISecurityManager());
  }

  try {
    String name = "//" + args[0] + "/Send";
    send = ((Send) Naming.lookup(name));
  } catch (java.rmi.NotBoundException e) {
    System.out.println("Cannot look up remote 
                 server object");
  } catch(java.rmi.RemoteException e){
    System.out.println("Cannot look up remote 
                 server object");
  } catch(java.net.MalformedURLException e) {
    System.out.println("Cannot look up remote 
                 server object");
  }

More Information

You can find more information on the RMI API in the RMI trail of The Java Tutorial.

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