Remote Method Invocation (RMI)

The most fundamental means of inter-object communication in Java is method invocation. Mechanisms like the Java event model are built on simple method invocations between objects in the same virtual machine. Therefore, when we want to communicate between virtual machines on different hosts, it’s natural to want a mechanism with similar capabilities and semantics. Java’s Remote Method Invocation mechanism does just that. It lets us get a reference to an object on a remote host and use it as if it were in our own virtual machine. RMI lets us invoke methods on remote objects, passing real Java objects as arguments and getting real Java objects as returned values.

Remote invocation is nothing new. For many years C programmers have used remote procedure calls (RPC) to execute a C function on a remote host and return the results. The primary difference between RPC and RMI is that RPC, being an offshoot of the C language, is primarily concerned with data structures. It’s relatively easy to pack up data and ship it around, but for Java, that’s not enough. In Java we don’t just work with data structures; we work with objects, which contain both data and methods for operating on the data. Not only do we have to be able to ship the state of an object (the data) over the wire, but also the recipient has to be able to interact with the object (use its methods) after receiving it.

It should be no surprise that RMI uses object serialization, which allows us to send graphs of objects (objects and all of the connected objects that they reference). When necessary, RMI can also use dynamic class loading and the security manager to transport Java classes safely. Thus, the real breakthrough of RMI is that it’s possible to ship both data and behavior (code) around the Net.

Remote and Non-Remote Objects

Before an object can be used with RMI, it must be serializable. But that’s not sufficient. Remote objects in RMI are real distributed objects. As the name suggests, a remote object can be an object on a different machine; it can also be an object on the local host. The term remote means that the object is used through a special kind of object reference that can be passed over the network. Like normal Java objects, remote objects are passed by reference. Regardless of where the reference is used, the method invocation occurs at the original object, which still lives on its original host. If a remote host returns a reference to one of its objects to you, you can call the object’s methods; the actual method invocations will happen on the remote host, where the object resides.

Nonremote objects are simpler. They are just normal serializable objects. (You can pass these over the network as we did in Section 11.3.1 earlier.) The catch is that when you pass a nonremote object over the network it is simply copied. So references to the object on one host are not the same as those on the remote host. Nonremote objects are passed by copy (as opposed to by reference). This may be acceptable for many kinds of data-oriented objects in your application, especially those that are not being modified.

Stubs and skeletons

No, we’re not talking about a gruesome horror movie. Stubs and skeletons are used in the implementation of remote objects. When you invoke a method on a remote object (which could be on a different host), you are actually calling some local code that serves as a proxy for that object. This is the stub. (It is called a stub because it is something like a truncated placeholder for the object.) The skeleton is another proxy that lives with the real object on its original host. It receives remote method invocations from the stub and passes them to the object.

After you create stubs and skeletons you never have to work with them directly; they are hidden from you (in the closet, so to speak). Stubs and skeletons for your remote objects are created by running the rmic (RMI compiler) utility. After compiling your Java source files normally, you run rmic on the remote object classes as a second pass. It’s easy; we’ll show you how in the following examples.

Remote interfaces

Remote objects are objects that implement a special remote interface that specifies which of the object’s methods can be invoked remotely. The remote interface must extend the java.rmi.Remote interface. Your remote object will implement its remote interface; as will the stub object that is automatically generated for it. In the rest of your code, you should then refer to the remote object as an instance of the remote interface—not as an instance of its actual class. Because both the real object and stub implement the remote interface, they are equivalent as far as we are concerned (for method invocation); locally, we never have to worry about whether we have a reference to a stub or to an actual object. This “type equivalence” means that we can use normal language features, like casting with remote objects. Of course public fields (variables) of the remote object are not accessible through an interface, so you must make accessor methods if you want to manipulate the remote object’s fields.

All methods in the remote interface must declare that they can throw the exception java.rmi.RemoteException . This exception (actually, one of many subclasses to RemoteException) is thrown when any kind of networking error happens: for example, the server could crash, the network could fail, or you could be requesting an object that for some reason isn’t available.

Here’s a simple example of the remote interface that defines the behavior of RemoteObject; we’ll give it two methods that can be invoked remotely, both of which return some kind of Widget object:

import java.rmi.*;

public interface RemoteObject extends Remote { 
    public Widget doSomething( ) throws RemoteException; 
    public Widget doSomethingElse( ) throws RemoteException; 
}

The UnicastRemoteObject class

The actual implementation of a remote object (not the interface we discussed previously) will usually extend java.rmi.server.UnicastRemoteObject. This is the RMI equivalent to the familiar Object class. When a subclass of UnicastRemoteObject is constructed, the RMI runtime system automatically “exports” it to start listening for network connections from remote interfaces (stubs) for the object. Like java.lang.Object, this superclass also provides implementations of equals( ) , hashcode( ), and toString( ) that make sense for a remote object.

Here’s a remote object class that implements the RemoteObject interface; we haven’t supplied implementations for the two methods or the constructor:

public class MyRemoteObject implements RemoteObject 
        extends java.rmi.UnicastRemoteObject 
{ 
    public RemoteObjectImpl( ) throws RemoteException {...} 
    public Widget doSomething( ) throws RemoteException {...} 
    public Widget doSomethingElse( ) throws RemoteException {...}
    // other non-public methods 
    ... 
}

This class can have as many additional methods as it needs; presumably, most of them will be private, but that isn’t strictly necessary. We have to supply a constructor explicitly, even if the constructor does nothing, because the constructor (like any method) can throw a RemoteException; we therefore can’t use the default constructor.

What if we can’t or don’t want to make our remote object implementation a subclass of UnicastRemoteObject? Suppose, for example, that it has to be a subclass of BankAccount or some other special base type for our system. Well, we can simply export the object ourselves using the static method exportObject( ) of UnicastRemoteObject. The exportObject( ) method takes as an argument a Remote interface and accomplishes what the UnicastRemoteObject constructor normally does for us. It returns as a value the remote object’s stub. However, you will normally not do anything with this directly. In the next section, we’ll discuss how to get stubs to your client through the RMI registry.

Normally, exported objects listen on individual ephemeral (randomly assigned) port numbers by default. (This is implementation-dependent.) You can control the port number allocation explicitly by exporting your objects using another form of UnicastRemoteObject.exportObject( ), which takes both a Remote interface and a port number as arguments.

Finally, the name UnicastRemoteObject suggests the question, “what other kinds of remote objects are there?” Right now, none. It’s possible that Sun will develop remote objects using other protocols or multicast techniques in the future. They would take their place alongside UnicastRemoteObject.

The RMI registry

The registry is the RMI phone book. You use the registry to look up a reference to a registered remote object on another host. We’ve already described how remote references can be passed back and forth by remote method calls. But the registry is needed to bootstrap the process: the client needs some way of looking up some initial object.

The registry is implemented by a class called Naming and an application called rmiregistry . This application must be running on the local host before you start a Java program that uses the registry. You can then create instances of remote objects and bind them to particular names in the registry. (Remote objects that bind themselves to the registry sometimes provide a main( ) method for this purpose.) A registry name can be anything you choose; it takes the form of a slash-separated path. When a client object wants to find your object, it constructs a special URL with the rmi: protocol, the hostname, and the object name. On the client, the RMI Naming class then talks to the registry and returns the remote object reference.

Which objects need to register themselves with the registry? Well, initially any object that the client has no other way of finding. But a call to a remote method can return another remote object without using the registry. Likewise, a call to a remote method can have another remote object as its argument, without requiring the registry. So you could design your system such that only one object registers itself, and then serves as a factory for any other remote objects you need. In other words, it wouldn’t be hard to build a simple object request “bouncer” (we won’t say “broker”) that returns references to all of the remote objects that your application uses. Depending on how you structure your application, this may happen naturally anyway.

Why avoid using the registry for everything? The current RMI registry is not very sophisticated, and lookups tend to be slow. It is not intended to be a general-purpose directory service (like JNDI, the Java API for accessing directory/name services), but simply to bootstrap RMI communications. It wouldn’t be surprising if Sun releases a much improved registry in the future, but that’s not the one we have now. Besides, the factory design pattern is extremely flexible and useful.

An RMI Example

The first thing we’ll implement using RMI is a duplication of the simple serialized object protocol from the previous section. We’ll make a remote RMI object called MyServer on which we can invoke methods to get a Date object or execute a WorkRequest object. First, we’ll define our Remote interface:

//file: RmtServer.java
import java.rmi.*; 
import java.util.*; 
 
public interface RmtServer extends Remote { 
    Date getDate( ) throws RemoteException; 
    Object execute( WorkRequest work ) throws RemoteException; 
}

The RmtServer interface extends the java.rmi.Remote interface, which identifies objects that implement it as remote objects. We supply two methods that take the place of our old protocol: getDate( ) and execute( ).

Next, we’ll implement this interface in a class called MyServer that defines the bodies of these methods. (Note that a more common convention for naming the implementation of remote interfaces is to postfix the class name with "Impl“. Using that convention MyServer would instead be named something like ServerImpl.)

//file: MyServer.java
import java.rmi.*; 
import java.util.*; 

public class MyServer  
    extends java.rmi.server.UnicastRemoteObject
    implements RmtServer {    

    public MyServer( ) throws RemoteException { } 

    // implement the RmtServer interface 
    public Date getDate( ) throws RemoteException { 
        return new Date( ); 
    } 

    public Object execute( WorkRequest work )
      throws RemoteException {
        return work.execute( ); 
    } 
 
    public static void main(String args[]) { 
        try { 
            RmtServer server = new MyServer( ); 
            Naming.rebind("NiftyServer", server); 
        } catch (java.io.IOException e) { 
            // problem registering server 
        } 
    } 
}

MyServer extends java.rmi.UnicastRemoteObject, so when we create an instance of MyServer, it will automatically be exported and start listening to the network. We start by providing a constructor, which must throw RemoteException, accommodating errors that might occur in exporting an instance. (We can’t use the automatically generated default constructor, because it won’t throw the exception.) Next, MyServer implements the methods of the remote RmtServer interface. These methods are straightforward.

The last method in this class is main( ) . This method lets the object set itself up as a server. main( ) creates an instance of the MyServer object and then calls the static method Naming.rebind( ) to register the object with the registry. The arguments to rebind( ) are the name of the remote object in the registry (NiftyServer), which clients will use to look up the object, and a reference to the server object itself. We could have called bind( ) instead, but rebind( ) is less prone to problems: if there’s already a NiftyServer registered, rebind( ) replaces it.

We wouldn’t need the main( ) method or this Naming business if we weren’t expecting clients to use the registry to find the server. That is, we could omit main( ) and still use this object as a remote object. We would be limited to passing the object in method invocations or returning it from method invocations—but that could be part of a factory design, as we discussed before.

Now we need our client:

//file: MyClient.java
import java.rmi.*;
import java.util.*; 

public class MyClient { 
 
    public static void main(String [] args)
      throws RemoteException { 
        new MyClient( args[0] ); 
    } 
 
    public MyClient(String host) { 
        try { 
            RmtServer server = (RmtServer) 
                Naming.lookup("rmi://"+host+"/NiftyServer");
            System.out.println( server.getDate( ) ); 
            System.out.println(
              server.execute( new MyCalculation(2) ) );
        } catch (java.io.IOException e) {  
              // I/O Error or bad URL 
        } catch (NotBoundException e) {  
              // NiftyServer isn't registered 
        } 
    } 
}

When we run MyClient, we pass it the hostname of the server on which the registry is running. The main( ) method creates an instance of the MyClient object, passing the hostname from the command line as an argument to the constructor.

The constructor for MyClient uses the hostname to construct a URL for the object. The URL will look something like this: rmi://hostname/NiftyServer. (Remember, NiftyServer is the name under which we registered our RmtServer.) We pass the URL to the static Naming.lookup( ) method. If all goes well, we get back a reference to a RmtServer (the remote interface). The registry has no idea what kind of object it will return; lookup( ) therefore returns an Object, which we must cast to RmtServer.

Compile all of the code. Then run rmic , the RMI compiler, to make the stub and skeleton files for MyServer:

% rmic MyServer

Let’s run the code. For the first pass, we’ll assume that you have all of the class files, including the stubs and skeletons generated by rmic, available in the class path on both the client and server machines. (You can run this example on a single host to test it if you want.) Make sure your class path is correct and then start the registry; then start the server:

% rmiregistry &   
               (on Windows: 
               start rmiregistry
               )
% java MyServer

In each case, make sure the registry application has the class path including your server classes so that it can load the stub class. (Be warned, we’re going to tell you to do the opposite later as part of setting up the dynamic class loading!)

Finally, on the client machine, run MyClient, passing the hostname of the server:

% java MyClient 
               myhost

The client should print the date and the number 4, which the server graciously calculated. Hooray! With just a few lines of code you have created a powerful client/server application.

Dynamic class loading

Before running the example, we told you to distribute all the class files to both the client and server machines. However, RMI was designed to ship classes, in addition to data, around the network; you shouldn’t have to distribute all the classes in advance. Let’s go a step further, and have RMI load classes for us, as needed. This involves several steps.

First, we need to tell RMI where to find any other classes it needs. We can use the system property java.rmi.server.codebase to specify a URL on a web server (or FTP server) when we run our client or server. This URL specifies the location of a JAR file or a base directory in which RMI will begin its search for classes. When RMI sends a serialized object (i.e., an object’s data) to some client, it also sends this URL. If the recipient needs the class file in addition to the data, it fetches the file at the specified URL. In addition to stub classes, other classes referenced by remote objects in the application can be loaded dynamically. Therefore, we don’t have to distribute many class files to the client; we can let the client download them as necessary. In Figure 11.3, we see an example as MyClient is going to the registry to get a reference to the RmtServer object. Then MyClient dynamically downloads the stub class for RmtMyServer from a web server running on the server object’s host.

RMI applications and dynamic class loading

Figure 11-3. RMI applications and dynamic class loading

We can now split our class files between the server and client machines. For example, we could withhold the MyCalculation class from the server, since it really belongs to the client. Instead, we can make the MyCalculation class available via a web server on some machine (probably our client’s) and specify the URL when we run MyClient:

java -Djava.rmi.server.codebase="http://myserver/foo/" ...

In this case, we would expect that MyCalculation would be accessible at the URL http://myserver/foo/MyCalculation.class/. (Note that the trailing slash in the URL is important: it says that the location is a base directory that contains the class files.)

Next we have to set up security. Since we will be loading class files over the network and executing their methods, we must have a security manager in place to restrict the kinds of things those classes may do, at least in the case where they are not coming from a trusted code source. RMI will not load any classes dynamically unless a security manager is installed. One easy way to meet this condition is to install the RMISecurityManager as the system security manager for your application. It is an example security manager that works with the default system policy and imposes some basic restrictions on what downloaded classes can do. To install the RMISecurityManager, simply add the following line to the beginning of the main( ) method of both the client and server applications (yes, we’ll be sending code both ways in the next section):

main( ) {
    System.setSecurityManager( new RMISecurityManager( ) );
    ...

The RMISecurityManager will work with the system security policy file to enforce restrictions. So you’ll have to provide a policy file that allows the client and server to do basic operations like make network connections. Unfortunately allowing all of the operations needed to load classes dynamically would require us listing a lot of permission information and we don’t want to get into that here. So we’re going to resort to suggesting that for this example you simply grant the code all permissions. Here is an example policy file—call it mysecurity.policy:

grant {
   permission java.security.AllPermission ;
};

(It’s exceedingly lame to install a security manager and then tell it to enforce no real security, but we’re more interested in looking at the networking code at the moment.)

So, to run our MyServer application we would now do something like this:

java -Djava.rmi.server.codebase='http://myserver/foo/' 
    -Djava.security.policy=mysecurity.policy MyServer

Finally, there is one last magic incantation required to enable dynamic class loading. As of the current implementation, the rmiregistry must be run without the classes which are to be loaded being in its class path. If the classes are in the class path of rmiregistry, it will not annotate the serialized objects with the URLs of their class files and no classes will be dynamically loaded. This limitation is really annoying; all we can say is to heed the warning for now.

If you meet these conditions, you should be able to get the client to run starting with only the MyClient class and the RmtServer remote interface. All of the other classes will be loaded dynamically from a remote location.

Passing remote object references

So far, we haven’t done anything that we couldn’t have done with the simple object protocol. We only used one remote object, MyServer, and we got its reference from the RMI registry. Now we’ll extend our example to pass some remote references between the client and server (these will be prime candidates for dynamic class loading). We’ll add two methods to our remote RmtServer interface:

public interface RmtServer extends Remote { 
    ...
    StringIterator getList( ) throws RemoteException; 
    void asyncExecute( WorkRequest work, WorkListener listener )
        throws RemoteException; 
}

getList( ) retrieves a new kind of object from the server: a StringIterator. The StringIterator is a simple list of strings, with some methods for accessing the strings in order. We will make it a remote object, so that implementations of StringIterator stay on the server.

Next we’ll spice up our work request feature by adding an asyncExecute( ) method. asyncExecute( ) lets us hand off a WorkRequest object as before, but it does the calulation on its own time. The return type for asyncExecute( ) is void, because it doesn’t actually return a value; we get the result later. Along with the request, our client passes a reference to a WorkListener object that is to be notified when the WorkRequest is done. We’ll have our client implement WorkListener itself.

Because this is to be a remote object, our interface must extend Remote, and its methods must throw RemoteExceptions:

//file: StringIterator.java
import java.rmi.*;

public interface StringIterator extends Remote { 
    public boolean hasNext( ) throws RemoteException; 
    public String next( ) throws RemoteException; 
}

Next, we provide a simple implementation of StringIterator , called MyStringIterator:

//file: MyStringIterator.java
import java.rmi.*;

public class MyStringIterator  
  extends java.rmi.server.UnicastRemoteObject
  implements StringIterator { 
 
    String [] list; 
    int index = 0; 
  
    public MyStringIterator( String [] list )
      throws RemoteException {
        this.list = list; 
    } 
    public boolean hasNext( ) throws RemoteException { 
        return index < list.length; 
    } 
    public String next( ) throws RemoteException { 
        return list[index++]; 
    } 
}

MyStringIterator extends UnicastRemoteObject. Its methods are simple: it can give you the next string in the list, and it can tell you whether there are any strings that you haven’t seen yet.

Next, we’ll define the WorkListener remote interface. This is the interface that defines how an object should listen for a completed WorkRequest. It has one method, workCompleted( ), which the server that is executing a WorkRequest calls when the job is done:

//file: WorkListener.java
import java.rmi.*;

public interface WorkListener extends Remote { 
    public void workCompleted(WorkRequest request, Object result )
        throws RemoteException;
}

Next, let’s add the new features to MyServer. We need to add implementations of the getList( ) and asyncExecute( ) methods, which we just added to the RmtServer interface:

public class MyServer extends java.rmi.server.UnicastRemoteObject
                      implements RmtServer {
  ...
  public StringIterator getList( ) throws RemoteException { 
    return new MyStringIterator(  
        new String [] { "Foo", "Bar", "Gee" } ); 
  } 

  public void asyncExecute(
     WorkRequest request , WorkListener listener )
     throws java.rmi.RemoteException { 

     // should really do this in another thread 
     Object result = request.execute( ); 
     listener.workCompleted( request, result ); 
  }
}

getList( ) just returns a StringIterator with some stuff in it. asyncExecute( ) calls a WorkRequest’s execute( ) method and notifies the listener when it’s done. (Our implementation of asyncExecute( ) is a little cheesy. If we were forming a more complex calculation we would want to start a thread to do the calculation, and return immediately from asyncExecute( ), so the client won’t block. The thread would call workCompleted( ) at a later time, when the computation was done. In this simple example, it would probably take longer to start the thread than to perform the calculation.)

We have to modify MyClient to implement the remote WorkListener interface. This turns MyClient into a remote object, so we must make it a UnicastRemoteObject. We also add the workCompleted( ) method that the WorkListener interface requires:

public class MyClient
  extends java.rmi.server.UnicastRemoteObject
  implements WorkListener { 
    ... 
    public void workCompleted( WorkRequest request, Object result)
      throws RemoteException { 
        System.out.println("Async work result = " + result); 
    } 
}

Finally, we want MyClient to exercise the new features. Add these lines after the calls to getDate( ) and execute( ):

// MyClient constructor 
    ... 
StringIterator se = server.getList( ); 
while ( se.hasNext( ) ) 
    System.out.println( se.next( ) ); 
 
server.asyncExecute( new MyCalculation(100), this );

We use getList( ) to get the iterator from the server, then loop, printing the strings. We also call asyncExecute( ) to perform another calculation; this time, we square the number 100. The second argument to asyncExecute( ) is the WorkListener to notify when the data is ready; we pass a reference to ourself (this).

Now all we have to do is compile everything and run rmic to make the stubs for all our remote objects:

rmic MyClient MyServer MyStringIterator

Restart the RMI registry and MyServer on your server, and run the client somewhere. You should get the following:

Fri Jul 11 23:57:19 PDT 1999
4 
Foo 
Bar 
Gee 
Async work result = 10000

If you are experimenting with dynamic class loading, you should be able to have the client download all of the server’s auxiliary classes (the stubs and the StringIterator) from a web server. And, conversely, you should be able to have the MyServer download the Client stub and WorkRequest related classes when it needs them.

We hope that this introduction has given you a feel for the tremendous power that RMI offers through object serialization and dynamic class loading. Java is one of the first programming languages to offer this kind of powerful framework for distributed applications.

RMI Object Activation

One of the newest features of RMI is the ability to create remote objects that are persistent. They can save their state and be reactivated when a request from a client arrives. This is an important feature for large systems with remote objects that must remain accessible across long periods of time. RMI activation effectively allows a remote object to be stored away—in a database, for example—and automatically be reincarnated when it is needed. RMI activation is not particularly easy to use and would not have benefited us in any of our simple examples; we won’t delve into it here. Much of the functionality of activatable objects can be achieved by using factories of shorter-lived objects that know how to retrieve some state from a database (or other location). The primary users of RMI activation may be systems like Enterprise JavaBeans, which need a generalized mechanism to save remotely accessible objects and revive them at later times.

RMI and CORBA

Java supports an important alternative to RMI, called CORBA (Common Object Request Broker Architecture). We won’t say much about CORBA here, but you should know that it exists. CORBA is a distributed object standard developed by the Object Management Group (OMG), of which Sun Microsystems is one of the founding members. Its major advantage is that it works across languages: a Java program can use CORBA to talk to objects written in other languages, like C or C++. This is may be a considerable advantage if you want to build a Java front end for an older program that you can’t afford to re-implement. CORBA also provides other services similar to those in the Java Enterprise APIs. CORBA’s major disadvantages are that it’s complex, inelegant, and somewhat arcane.

Sun and OMG have been making efforts to bridge RMI and CORBA. There is an implementation of RMI that can use IIOP (the Internet Inter-Object Protocol) to allow some RMI-to-CORBA interoperability. However, CORBA currently does not have many of the semantics necessary to support true RMI style distributed objects. So this solution is somewhat limited at this time.

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