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2.4 Overview of RMI Interfaces and Classes

The interfaces and classes that are responsible for specifying the remote behavior of the RMI system are defined in the java.rmi package hierarchy. The following figure shows the relationship between several of these interfaces and classes:

2.4.1 The java.rmi.Remote Interface

In RMI, a remote interface is an interface that declares a set of methods that may be invoked from a remote Java virtual machine. A remote interface must satisfy the following requirements:

• A remote interface must at least extend, either directly or indirectly, the interface java.rmi.Remote.

• Each method declaration in a remote interface or its super-interfaces must satisfy the requirements of a remote method declaration as follows:
• A remote method declaration must include the exception java.rmi.RemoteException (or one of its superclasses such as java.io.IOException or java.lang.Exception) in its throws clause, in addition to any application-specific exceptions (note that application specific exceptions do not have to extend java.rmi.RemoteException).
• In a remote method declaration, a remote object declared as a parameter or return value (either declared directly in the parameter list or embedded within a non-remote object in a parameter) must be declared as the remote interface, not the implementation class of that interface.

The interface java.rmi.Remote is a marker interface that defines no methods:

public interface Remote {}

A remote interface must at least extend the interface java.rmi.Remote (or another remote interface that extends java.rmi.Remote). However, a remote interface may extend a non-remote interface under the following condition:

• A remote interface may also extend another non-remote interface, as long as all of the methods (if any) of the extended interface satisfy the requirements of a remote method declaration.

For example, the following interface BankAccount defines a remote interface for accessing a bank account. It contains remote methods to deposit to the account, to get the account balance, and to withdraw from the account:

public interface BankAccount extends java.rmi.Remote {
	public void deposit(float amount)
		throws java.rmi.RemoteException;
	public void withdraw(float amount)
		throws OverdrawnException, java.rmi.RemoteException;
	public float getBalance()
		throws java.rmi.RemoteException;
}

The next example shows a valid remote interface Beta that extends a non-remote interface Alpha, which has remote methods, and the interface java.rmi.Remote:

public interface Alpha {
	public final String okay = "constants are okay too";
	public Object foo(Object obj)
		throws java.rmi.RemoteException;
	public void bar() throws java.io.IOException;
	public int baz() throws java.lang.Exception;
}


public interface Beta extends Alpha, java.rmi.Remote {
	public void ping() throws java.rmi.RemoteException;
}

2.4.2 The RemoteException Class

The java.rmi.RemoteException class is the superclass of exceptions thrown by the RMI runtime during a remote method invocation. To ensure the robustness of applications using the RMI system, each remote method declared in a remote interface must specify java.rmi.RemoteException (or one of its superclasses such as java.io.IOException or java.lang.Exception) in its throws clause.

The exception java.rmi.RemoteException is thrown when a remote method invocation fails for some reason. Some reasons for remote method invocation failure include:

• Communication failure (the remote server is unreachable or is refusing connections; the connection is closed by the server, etc.)

• Failure during parameter or return value marshalling or unmarshalling
• Protocol errors

The class RemoteException is a checked exception (one that must be handled by the caller of a remote method and is checked by the compiler), not a RuntimeException.

2.4.3 The RemoteObject Class and its Subclasses

RMI server functions are provided by java.rmi.server.RemoteObject and its subclasses, java.rmi.server.RemoteServer and java.rmi.server.UnicastRemoteObject and java.rmi.activation.Activatable.

• The class java.rmi.server.RemoteObject provides implementations for the java.lang.Object methods, hashCode, equals, and toString that are sensible for remote objects.

• The methods needed to create remote objects and export them (make them available to remote clients) are provided by the classes UnicastRemoteObject and Activatable. The subclass identifies the semantics of the remote reference, for example whether the server is a simple remote object or is an activatable remote object (one that executes when invoked).
• The java.rmi.server.UnicastRemoteObject class defines a singleton (unicast) remote object whose references are valid only while the server process is alive.

• The class java.rmi.activation.Activatable is an abstract class that defines an activatable remote object that starts executing when its remote methods are invoked and can shut itself down when necessary.