Learning Java, 4th Edition

The NIO Package

We are now going to complete our introduction to core Java I/O facilities by returning to the java.nio package. The name NIO stands for “New I/O” and, as we saw earlier in this chapter in our discussion of java.nio.file, one aspect of NIO is simply to update and enhance features of the legacy java.io package. Much of the general NIO functionality does indeed overlap with existing APIs. However, NIO was first introduced to address specific issues of scalability for large systems, especially in networked applications. The following section outlines the basic elements of NIO, which center on working with buffers and channels.

Asynchronous I/O

Most of the need for the NIO package was driven by the desire to add nonblocking and selectable I/O to Java. Prior to NIO, most read and write operations in Java were bound to threads and were forced to block for unpredictable amounts of time. Although certain APIs such as Sockets (which we’ll see in Chapter 13) provided specific means to limit how long an I/O call could take, this was a workaround to compensate for the lack of a more general mechanism. In many languages, even those without threading, I/O could still be done efficiently by setting I/O streams to a nonblocking mode and testing them for their readiness to send or receive data. In a nonblocking mode, a read or write does only as much work as can be done immediately—filling or emptying a buffer and then returning. Combined with the ability to test for readiness, this ...