Java Threads, 3rd Edition

Chapter 12. Threads and I/O

If you’re not interested in parallel processing, the area where you’re most likely to encounter threads in Java is in dealing with I/O—and particularly in dealing with network I/O. That’s the topic we explore in this chapter.

In early versions of Java, all I/O was blocking. If your program attempted to read data from a socket and no data was present, the read() method would block until at least some data was available. That situation is also true of reading a file. For the most part, delays in reading files aren’t noticeable; you may have to wait a few cycles for the disk to rotate to the correct location and the operating system to transfer data from the disk. In most programs, blocking for that amount of time makes little difference, but in those programs where it does make a difference, the concepts that apply to network I/O are just as relevant to file I/O.

For network I/O, the delay can be quite significant. Networks are subject to delays at various points (particularly if the network involves long distances or slow links). Even if there’s no physical delay on the network lines, network I/O is done in the context of a conversation between two peers, and a peer may not be ready to furnish its output when its partner wants it. A database server reads commands from a user, but the user may take a few minutes to type in the SQL to be executed. Once the SQL has been sent to the database, the user is ready to read back the response, but it may take the database ...