Direct Stream Manipulation Subclassing Socket Is a Better Solution

data. Ive got the buffer space for it now. This behavior, which results in a horrible waste of bandwidth, is called the silly-window problem. TCP usually avoids the silly window problem by grouping information before sending it. That is, rather than sending small amounts of information repeatedly, TCP usually waits until a large amount of information is available and sends it together. The setTCPNoDelay method enables you to turn this behavior off. An argument of true will force the sockets layer to send information as soon as it becomes available.

2.5 Special-Purpose Sockets

Socket and ServerSocket are object-oriented wrappers that encapsulate the TCPIP communication protocol. They are designed to simply pass data along the wire, without transforming the data or changing it in any way. This can be either an advantage or a drawback, depending on the particular application. Because data is simply passed along the network, the default implementation of Socket is fast and efficient. Moreover, sockets are easy to use and highly compatible with existing applications. For example, consider the WebBrowser application discussed earlier in the chapter. We wrote a Java program that accepted connections from an already existing application in our case, Netscape Navigator that was written in C++. There are, however, two important potential downsides to simply passing along the data: • The data isnt very secure. • Communications may use excessive bandwidth. Security is an issue because many applications run over large-scale networks, such as the Internet. If data is not encrypted before being sent, it can easily be intercepted by third parties who are not supposed to have access to the information. Bandwidth is also an issue because data being sent is often highly redundant. Consider, for example, a typical web page. My web browser has 145 HTML files stored in its cache. The CompressFile application from Chapt er 1 , on average, compresses these files to less than half their original size. If HMTL pages are compressed before being sent, they can be sent much faster. Of course, HTML is a notoriously verbose data format, and this measurement is therefore somewhat tainted. But, even so, its fairly impressive. Simply using compression can cut bandwidth costs in half, even though it adds additional processing time on both the client and server. Moreover, many data formats are as verbose as HTML. Two examples are XML-based communication and protocols such as RMIs JRMP, which rely on object serialization well discuss serialization in detail in Chapt er 10 .

2.5.1 Direct Stream Manipulation

As with most problems, security and bandwidth issues have a simple, and almost correct, solution. Namely: If your application doesnt have security or bandwidth issues, or must use ordinary sockets to connect with pre-existing code, use ordinary sockets. Otherwise, use ordinary sockets, but layer additional streams to encrypt or compress the data. This solution is nice for a number of reasons. First and foremost, its a straightforward use of the Java streams library that does exactly what the streams library was intended to do. Consider the following code from the CompressFile application: OutputStream destination = _destinationFileTextField.getFileOutputStream ; BufferedOutputStream bufferedDestination = new BufferedOutputStreamdestination; GZIPOutputStream zippedDestination = new GZIPOutputStreambufferedDestination; Rewriting the first line yields the exact code needed to implement compression over a socket: OutputStream destination = _socket.getOutputStream ; BufferedOutputStream bufferedDestination = new BufferedOutputStreamdestination; GZIPOutputStream zippedDestination = new GZIPOutputStreambufferedDestination;

2.5.2 Subclassing Socket Is a Better Solution

There is, however, a related solution that has identical performance characteristics and yields much more reliable code: create a subclass of Socket that implements the layering internally and returns the specialized stream. This is a better approach for three reasons: • It lowers the chances of socket incompatibilities. Consider the previous example™any part of the application that opens a socket must also implement the correct stream layering. If an application opens sockets in multiple locations in the code, theres a good chance that it will be done differently in different places e.g., during an update a developer will forget to update one of the places in the code where a socket is opened. [ 8] This is especially true if the application has a long lifecycle. [ 8] This is a particular instance of a more general principle known as Once and Only Once. Namely: if information is written down two ways, one of the versions will soon be out of date. See ht t p: w w w .c2.com cgi w iki?OnceAndOnlyOnce for a detailed discussion of this idea. • This sort of error is particularly bad because it isnt caught by the compiler. Instead, incorrectly encoded data will be sent over the wire, and the recipient will either then throw an exception the good case or perform computations with incorrect data the bad case. • It isolates code that is likely to change. If most of the application simply creates instances of a subclass of Socket or, better yet, calls a method named something like getSocket on a factory object, and uses only the basic methods defined in Socket , then the application can quickly and easily be modified to use a different subclass of Socket . This not only allows an application to seamlessly add things such as an encryption layer, but it can be very useful when trying to debug or monitor a distributed application see the LoggingSocket class from the sample code provided with this book as an example of this. • Custom sockets can be used with RMI. RMI is an object-oriented layer for distributed programming, built on top of the sockets library. Though it doesnt give application programmers direct access to the socket input and output streams, it does allow programmers to specify what type of sockets to use when making a connection between a client and a server via the RMIClientSocketFactory and RMIServerSocketFactory interfaces; see Chapt er 18 for more details.

2.5.3 A Special-Purpose Socket