Java RMI, or Remote Method Invocation, is a Java API that allows you to create distributed applications in which objects and methods can be invoked remotely, typically between different Java Virtual Machines (JVMs) running on separate devices over a network. RMI enables communication between Java objects on different machines as if they were local.

Here are the key concepts and components of Java RMI:

1. Remote Interface: In RMI, you define a remote interface that extends the java.rmi.Remote interface. This interface declares the methods that can be invoked remotely. Each method in the remote interface should throw a java.rmi.RemoteException.

2. Remote Object: A remote object is an instance of a class that implements the remote interface. These objects are responsible for providing the actual implementation of the methods declared in the remote interface. Remote objects are typically registered on an RMI registry.

3. RMI Registry: The RMI registry is a central repository that maintains references to remote objects. Clients can look up these objects in the registry to obtain references and invoke remote methods. The rmiregistry tool is used to start an RMI registry.

4. Stub and Skeleton: In earlier versions of RMI, stubs and skeletons were used for communication between the client and server JVMs. However, starting with Java 5, stubs and skeletons are generated automatically by the Java compiler and are not explicitly needed in your code.

5. Client: The client is the program that invokes methods on remote objects. To do this, it looks up the remote object in the RMI registry, obtains a reference to it, and then calls methods on the reference as if they were local.

6. Server: The server is the program that provides the remote objects and registers them with the RMI registry. It listens for incoming remote method calls from clients and delegates those calls to the appropriate remote objects.

Here’s a high-level overview of how RMI works:

1. The server creates and publishes one or more remote objects to the RMI registry.

2. The client looks up the remote object(s) it needs in the RMI registry.

3. The client invokes methods on the remote object(s) as if they were local objects.

4. Behind the scenes, RMI handles the network communication and marshalling/unmarshalling of method arguments and return values.

Java RMI simplifies the development of distributed Java applications by abstracting many of the complexities of network communication. It is commonly used for building distributed systems, client-server applications, and remote service invocations in Java.