After the connection is made, you don’t know what order to read or write in.
Use Java Communication Events to notify you when data becomes available.
While lock-step mode is acceptable for dialing a modem, it breaks down when you have two independent agents communicating over a port. Either end may be a person, as in a remote login session, or a program, either a server or a client program. A client program, in turn, may be driven by a person (as is a web browser) or may be self-driven (such as an FTP client transferring many files at one request). You cannot predict, then, who will need to read and who will need to write. Consider the simplest case: the programs at both end try to read at the same time! Using the lock-step model, each end will wait forever for the other end to write something. This error condition is known as a deadlock , since both ends are locked up, dead, until a person intervenes, or the communication line drops, or the world ends, or the universe ends, or somebody making tea blows a fuse and causes one of the machines to halt.
There are two general approaches to this problem: event-driven activity, wherein the Communications API notifies you when the port is ready to be read or written, and threads-based activity, wherein each “direction” (from the user to the remote, and from the remote to the user) has its own little flow of control, causing only the reads in that direction to wait. We’ll discuss each ...