Let’s look at a complete example. One of the primary uses for threads within a Java program is to read data asynchronously. In this section, we’ll develop a class to read a network socket asynchronously.
Why is threading important for I/O? Whether you are reading from or writing to a file or network socket, a common problem exists, namely, that the action of reading or writing depends on other resources. These resources may be other programs; they may be hardware, like the disk or the network; they may be the operating system or browser. These resources may become temporarily unavailable for a variety of reasons: reading from a network socket may involve waiting until the data is available, writing large amounts of data to a file may take a long period of time to complete if the disk is busy with other requests, and so on. Unfortunately, the mechanism to check whether these resources are available does not exist in the Java API. This is particularly a problem for network sockets, where data is likely to take a long time to be transmitted over the network; it is possible for a read from a network socket to wait forever.