Remote Method Invocation (RMI) Interview Questions And Answers
Java Remote Method Invocation (Java RMI) enables the programmer to create distributed Java technology-based to Java technology-based applications, in which the methods of remote Java objects can be invoked from other Java virtual machines, possibly on different hosts. RMI uses object serialization to marshal and unmarshal parameters and does not truncate types, supporting true object-oriented polymorphism.
11. How do RMI clients contact remote RMI servers?
For an RMI client to contact a remote RMI server, the client must first hold a reference to the server. The Naming.lookup method call is the most common mechanism by which clients initially obtain references to remote servers. Remote references may be obtained by other means, for example: all remote method calls can return remote references. This is what Naming.lookup does; it uses a well-known stub to make a remote method call to the rmiregistry, which sends back the remote reference to the object requested by the lookup method.
Every remote reference contains a server hostname and port number that allow clients to locate the VM that is serving a particular remote object. Once an RMI client has a remote reference, the client will use the hostname and port provided in the reference to open a socket connection to the remote server.
Please note that with RMI the terms client and server can refer to the same program. A Java program that acts as an RMI server contains an exported remote object. An RMI client is a program that invokes one or more methods on a remote object in another virtual machine. If a VM performs both of these functions, it may be referred to as an RMI client and an RMI server.
12. How many types of protocol implementations does RMI have?
RMI has at least three protocol implementations: Java Remote Method Protocol(JRMP), Internet Inter ORB Protocol(IIOP), and Jini Extensible Remote Invocation(JERI). These are alternatives, not part of the same thing, All three are indeed layer 6 protocols for those who are still speaking OSI reference model.
13. What are the services in RMI ?
An RMI "service" could well be any Java method that can be invoked remotely. The other service is the JRMP RMI naming service which is a lookup service.
14. What is difference RMI registry and OSAgent ?
RMI Registry
The rmiregistry command creates and starts a remote object registry on the specified port on the current host. If port is omitted, the registry is started on port 1099. The rmiregistry command produces no output and is typically run in the background. For example:
start rmiregistry
A remote object registry is a bootstrap naming service that is used by RMI servers on the same host to bind remote objects to names. Clients on local and remote hosts can then look up remote objects and make remote method invocations.
The registry is typically used to locate the first remote object on which an application needs to invoke methods. That object in turn will provide application-specific support for finding other objects.
The methods of the java.rmi.registry.LocateRegistry class are used to get a registry operating on the local host or local host and port.
The URL-based methods of the java.rmi.Naming class operate on a registry and can be used to look up a remote object on any host, and on the local host: bind a simple (string) name to a remote object, rebind a new name to a remote object (overriding the old binding), unbind a remote object, and list the URLs bound in the registry.
OSAgent
The osagent is a process that allows CORBA servers to register their objects and assists client applications in the location of objects. To avoid a single point of failure, the osagent is designed to be operated on more than one node. Osagent processes (and the clients and servers that access them) follow a set of rules to discover each other and cooperate in the location of registered objects.
15. What is the difference between RMI & Corba ?
The most significant difference between RMI and CORBA is that CORBA was made specifically for interoperability across programming languages. That is CORBA fosters the notion that programs can be built to interact in multiple languages. The server could be written in C++, the business logic in Python, and the front-end written in COBOL in theory. RMI, on the other hand is a total Java solution, the interfaces, the implementations and the clients--all are written in Java.
RMI allows dynamic loading of classes at runtime. In a multi-language CORBA environment, dynamic class loading is not possible. The important advantage to dynamic class loading is that it allows arguments to be passed in remote invocations that are subtypes of the declared types. In CORBA, all types have to be known in advance. RMI (as well as RMI/IIOP) provides support for polymorphic parameter passing, whereas strict CORBA does not. CORBA does have support for multiple languages which is good for some applications, but RMI has the advantage of being dynamic, which is good for other applications.
16. What is the main functionality of the Remote Reference Layer ?
RRL exists in both the RMI client and server. It is used by the stub or skeleton protocol layer and uses the transport layer. RRL is reponsible for transport-independent functioning of RMI, such as connection management or unicast/multicast object invocation.
17. Why do I get the exception "java.net.SocketException: Address already in use" when I try to run the registry?
This exception means that the port that the RegistryImpl uses (by default 1099) is already in use. You may have another registry running on your machine and will need to stop it.
18. Why does the RMI implementation create so many sockets when my application uses custom socket factories; or why are stubs (using a custom socket factory) that refer to the same remote object not equal; or why does the RMI implementation not reuse server-s
The RMI implementation attempts to reuse open sockets where possible for remote invocations. When a remote method is invoked on a stub that uses a custom socket factory, the RMI implementation will reuse an open connection (if any) as long as that socket was created by an equivalent socket factory. Since client socket factories are serialized to clients, a single client may have several distinct copies of the same logical socket factory. To ensure that the RMI implementation will reuse sockets created by custom socket factories, make sure your custom client socket factory classes implement the hashCode and equals methods appropriately. If the client socket factory does not implement these methods correctly, another ramification is that stubs (using the client socket factory) that refer to the same remote object will not be equal.
The RMI implementation attempts to reuse server-side ports as well. It will only do so if there is an existing server socket for the port created by an equivalent socket factory. Make sure the server socket factory class implements the hashCode and equals methods too.
If your socket factory has no instance state, a trivial implementation of the hashCode and equals methods are the following:
public int hashCode() { return 57; }
public boolean equals(Object o) { return this.getClass() == o.getClass() }
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