Java - Networking snippets
A network is just another possible source of input data, and another place where data can be output. That does oversimplify things, because networks are not as easy to work with as files are. But in Java, you can do network communication using input streams and output streams, just as you can use such streams to communicate with the user or to work with files. Nevertheless, opening a network connection between two computers is a bit tricky, since there are two computers involved and they have to somehow agree to open a connection. And when each computer can send data to the other, synchronizing communication can be a problem.
One of the standard Java packages is called java.net. This package includes several classes that can be used for networking. Two different styles of network I/O are supported. One of these, which is fairly high-level, is based on the World-Wide Web, and provides the sort of network communication capability that is used by a Web browser when it downloads pages for you to view. The main classes for this style of networking are java.net.URL and java.net.URLConnection. An object of type URL is an abstract representation of a Universal Resource Locator, which is an address for an HTML document or other resource on the Web. A URLConnection represents a network connection to such a resource.
The second style of I/O, which is more general and much more important, views the network at a lower level. It is based on the idea of a socket. A socket is used by a program to establish a connection with another program on a network. Communication over a network involves two sockets, one on each of the computers involved in the communication. Java uses a class called java.net.Socket to represent sockets that are used for network communication. The term “socket” presumably comes from an image of physically plugging a wire into a computer to establish a connection to a network, but it is important to understand that a socket, as the term is used here, is simply an object belonging to the class Socket. In particular, a program can have several sockets at the same time, each connecting it to another program running on some other computer on the network. All these connections use the same physical network connection.
URLs and URLConnections
Once you have a URL object, you can use it to open a URLConnection to the resource at that address. A url is ordinarily specified as a string. There are also relative url’s. A relative url specifies the location of a resource relative to the location of another url, which is called the base or context for the relative url.
These constructors have prototypes
public URL(String urlName) throws MalformedURLException
and
public URL(URL context, String relativeName) throws MalformedURLException
Note that these constructors will throw an exception of type MalformedURLException if the specified strings don’t represent legal url’s. The MalformedURLException class is a subclass of IOException, and it requires mandatory exception handling. That is, you must call the constructor inside a try..catch statement that handles the exception or in a subroutine that is declared to throw the exception.
The second constructor is especially convenient when writing applets. In an applet, two methods are available that provide useful URL contexts. The method getDocumentBase(), defined in the Applet and JApplet classes, returns an object of type URL. This URL represents the location from which the HTML page that contains the applet was downloaded. This allows the applet to go back and retrieve other files that are stored in the same location as that document. For example,
URL url = new URL(getDocumentBase(), “data.txt”);constructs a URL that refers to a file named data.txt on the same computer and in the same directory as the source file for the web page on which the applet is running. Another method, getCodeBase(), returns a URL that gives the location of the applet class file (which is not necessarily the same as the location of the document).
Once you have a valid URL object, you can call its openConnection() method to set up a connection. This method returns a URLConnection. The URLConnection object can, in turn, be used to create an InputStream for reading data from the resource represented by the URL. This is done by calling its getInputStream() method. For example:
URL url = new URL(urlAddressString);URLConnection connection = url.openConnection();InputStream in = connection.getInputStream();
The openConnection() and getInputStream() methods can both throw exceptions of type IOException. Once the InputStream has been created, you can read from it in the usual way, including wrapping it in another input steam type, such as TextReader, or using a Scanner. Reading from the stream can, of course, generate exceptions.
One of the other useful instance methods in the URLConnection class is getContentType(), which returns a String that describes the type of information available from the URL. The return value can be null if the type of information is not yet known or if it is not possible to determine the type. The type might not be available until after the input stream has been created, so you should generally call getContentType() after getInputStream().
Let’s look at a short example that uses all this to read the data from a URL. This subroutine opens a connection to a specified URL, checks that the type of data at the URL is text, and then copies the text onto the screen. Many of the operations in this subroutine can throw exceptions. They are handled by declaring that the subroutine “throws IOException” and leaving it up to the main program to decide what to do when an error occurs.
static void readTextFromURL( String urlString ) throws IOException { /* Open a connection to the URL, and get an input stream for reading data from the URL. */URL url = new URL(urlString);
URLConnection connection = url.openConnection();
InputStream urlData = connection.getInputStream();
/* Check that the content is some type of text. */
String contentType = connection.getContentType();
if (contentType == null || contentType.startsWith("text") == false)
throw new IOException("URL does not seem to refer to a text file.");
/* Copy lines of text from the input stream to the screen, until
end-of-file is encountered (or an error occurs). */
BufferedReader in; // For reading from the connection's input stream.
in = new BufferedReader( new InputStreamReader(urlData) );
while (true) {
String line = in.readLine();
if (line == null)
break;
System.out.println(line);
}
} // end readTextFromURL()
TCP/IP and Client/Server
Communication over the Internet is based on a pair of protocols called the Transmission Control Protocol and the Internet Protocol, which are collectively referred to as TCP/IP. (In fact, there is a more basic communication protocol called UDP that can be used instead of TCP in certain applications. UDP is supported in Java, but for this discussion, I’ll stick to the full TCP/IP, which provides reliable two-way communication between networked computers.)
For two programs to communicate using TCP/IP, each program must create a socket, as discussed earlier in this section, and those sockets must be connected. Once such a connection is made, communication takes place using input streams and output streams. Each program has its own input stream and its own output stream. Data written by one program to its output stream is transmitted to the other computer. There, it enters the input stream of the program at the other end of the network connection. When that program reads data from its input stream, it is receiving the data that was transmitted to it over the network.
A program that connects to a server is called a client, and the socket that it uses to make a connection is called a client socket. The idea is that the server is out there somewhere on the network, waiting for a connection request from some client. The server can be thought of as offering some kind of service, and the client gets access to that service by connecting to the server. This is called the client/server model of network communication. In many actual applications, a server program can provide connections to several clients at the same time.
A client program has to have some way to specify which computer, among all those on the network, it wants to communicate with. Every computer on the Internet has an IP address which identifies it uniquely among all the computers on the net.
Traditional (or IPv4) IP addresses are 32-bit integers. They are usually written in the so-called “dotted decimal” form, such as 69.9.161.200, where each of the four numbers in the address represents an 8-bit integer in the range 0 through 255. A new version of the Internet Protocol, IPv6, is currently being introduced. IPv6 addresses are 128-bit integers and are usually written in hexadecimal form (with some colons and maybe some extra information thrown in). In actual use, IPv6 addresses are still farily rare.
Now, a single computer might have several programs doing network communication at the same time, or one program communicating with several other computers. To allow for this possibility, a network connection is actually identified by a port number in combination with an IP address. A port number is just a 16-bit integer. A server does not simply listen for connections — it listens for connections on a particular port. A potential client must know both the Internet address (or domain name) of the computer on which the server is running and the port number on which the server is listening. A Web server, for example, generally listens for connections on port 80; other standard Internet services also have standard port numbers. (The standard port numbers are all less than 1024, and are reserved for particular services. If you create your own server programs, you should use port numbers greater than 1024.)
Sockets
When you construct a ServerSocket object, you have to specify the port number on which the server will listen. The specification for the constructor is
public ServerSocket(int port) throws IOException
The port number must be in the range 0 through 65535, and should generally be greater than 1024. (A value of 0 tells the server socket to listen on any available port.) The constructor might throw a SecurityException if a smaller port number is specified. An IOException can occur if, for example, the specified port number is already in use.
As soon as a ServerSocket is created, it starts listening for connection requests. The accept() method in the ServerSocket class accepts such a request, establishes a connection with the client, and returns a Socket that can be used for communication with the client. The accept() method has the form
public Socket accept() throws IOException
When you call the accept() method, it will not return until a connection request is received (or until some error occurs). The method is said to block while waiting for the connection. You can call accept() repeatedly to accept multiple connection requests. The ServerSocket will continue listening for connections until it is closed, using its close() method, or until some error occurs, or until the program is terminated in some way.
Suppose that you want a server to listen on port 1728, and suppose that you’ve written a method provideService(Socket) to handle the communication with one client. Then the basic form of the server program would be:
try {
ServerSocket server = new ServerSocket(1728);
while (true) {
Socket connection = server.accept();
provideService(connection);
}
}
catch (IOException e) {
System.out.println("Server shut down with error: " + e);
}
On the client side, a client socket is created using a constructor in the Socket class. To connect to a server on a known computer and port, you would use the constructor
public Socket(String computer, int port) throws IOException
The first parameter can be either an IP number or a domain name. This constructor will block until the connection is established or until an error occurs.
Once you have a connected socket, no matter how it was created, you can use the Socket methods getInputStream() and getOutputStream() to obtain streams that can be used for communication over the connection. These methods return objects of type InputStream and OutputStream, respectively. Here is the outline of a method for working with a client connection:
/**
* Open a client connection to a specified server computer and
* port number on the server, and then do communication through
* the connection.
*/
void doClientConnection(String computerName, int serverPort) {
Socket connection;
InputStream in;
OutputStream out;
try {
connection = new Socket(computerName,serverPort);
in = connection.getInputStream();
out = connection.getOutputStream();
}
catch (IOException e) {
System.out.println(
"Attempt to create connection failed with error: " + e);
return;
}
.
. // Use the streams, in and out, to communicate with server.
.
try {
connection.close();
// (Alternatively, you might depend on the server
// to close the connection.)
}
catch (IOException e) {
}
} // end doClientConnection()
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