3.5 The Transport Layer

In general, the transport layer of the RMI system is responsible for:

• Setting up connections to remote address spaces.
• Managing connections.
• Monitoring connection "liveness."
• Listening for incoming calls.
• Maintaining a table of remote objects that reside in the address space.
• Setting up a connection for an incoming call.
• Locating the dispatcher for the target of the remote call and passing the connection to this dispatcher.

The concrete representation of a remote object reference consists of an endpoint and an object identifier. This representation is called a live reference. Given a live reference for a remote object, a transport can use the endpoint to set up a connection to the address space in which the remote object resides. On the server side, the transport uses the object identifier to look up the target of the remote call.

The transport for the RMI system consists of four basic abstractions:

• An endpoint is the abstraction used to denote an address space or Java virtual machine. In the implementation, an endpoint can be mapped to its transport. That is, given an endpoint, a specific transport instance can be obtained.
• A channel is the abstraction for a conduit between two address spaces. As such, it is responsible for managing connections between the local address space and the remote address space for which it is a channel.
• A connection is the abstraction for transferring data (performing input/output).
• The transport abstraction manages channels. Each channel is a virtual connection between two address spaces. Within a transport, only one channel exists per pair of address spaces (the local address space and a remote address space). Given an endpoint to a remote address space, a transport sets up a channel to that address space. The transport abstraction is also responsible for accepting calls on incoming connections to the address space, setting up a connection object for the call, and dispatching to higher layers in the system.

A transport defines what the concrete representation of an endpoint is, so multiple transport implementations may exist. The design and implementation also supports multiple transports per address space, so both TCP and UDP can be supported in the same virtual machine. Note that the RMI transport interfaces are only available to the virtual machine implementation and are not available directly to the application.