Chapter 6. Under the Hood

Some of the subtleties of network programming are difficult to grasp without some understanding of the data structures associated with the socket implementation and certain details of how the underlying protocols work. This is especially true of TCP sockets (i.e., instances of Socket). This chapter describes some of what goes on under the hood when you create and use an instance of Socket or ServerSocket. (The initial discussion and Section 6.5 apply as well to DatagramSocket and MulticastSocket. Also, since each SocketChannel has an underlying Socket (and similarly for the other flavors of channels), the discussion applies to them as well. However, most of this chapter focuses on TCP sockets, that is, Socket and Server-Socket.) Please note that this description covers only the normal sequence of events and glosses over many details. Nevertheless, we believe that even this basic level of understanding is helpful. Readers who want the full story are referred to the TCP specification [15] or to one of the more comprehensive treatises on the subject [5,18].

Figure 6.1 is a simplified view of some of the information associated with a Socket instance. The classes are supported by an underlying implementation that is provided by the JVM and/or the platform on which it is running (i.e., the “socket layer” of the host’s OS). Operations on the Java objects are translated into manipulations of this underlying abstraction. In this chapter, "Socket" refers generically to one of the classes in Figure 6.1, while “socket” refers to the underlying abstraction, whether it is provided by an underlying OS or the JVM implementation itself (e.g., in an embedded system). It is important to note that other (possibly non-Java) programs running on the same host may be using the network via the underlying socket abstraction, and thus competing with Java Socket instances for resources such as ports.