JAVA NETWORKING basics for Projects

In datagram-based communication such as UDP, the datagram packet contains the port number of its destination and UDP routes the packet to the appropriate application, as illustrated in this figure:

Port numbers range from 0 to 65,535 because ports are represented by 16-bit numbers. The port numbers ranging from 0 - 1023 are restricted; they are reserved for use by well-known services such as HTTP and FTP and other system services. These ports are called well-known ports. Your applications should not attempt to bind to them. ​

Through the classes in java.net, Java programs can use TCP or UDP to communicate over the Internet. The URL, URLConnection, Socket, and ServerSocket classes all use TCP to communicate over the network. The DatagramPacket, DatagramSocket, and MulticastSocket classes are for use with UDP.

If you've been surfing the Web, you have undoubtedly heard the term URL and have used URLs to access HTML pages from the Web. It's often easiest, although not entirely accurate, to think of a URL as the name of a file on the World Wide Web because most URLs refer to a file on some machine on the network. However, remember that URLs also can point to other resources on the network, such as database queries and command output.



The following is an example of a URL which addresses the Java Web site hosted by Sun Microsystems:
As in the previous diagram, a URL has two main components:

• Protocol identifier
• Resource name

Note that the protocol identifier and the resource name are separated by a colon and two forward slashes. The protocol identifier indicates the name of the protocol to be used to fetch the resource. The example uses the Hypertext Transfer Protocol (HTTP), which is typically used to serve up hypertext documents. HTTP is just one of many different protocols used to access different types of resources on the net. Other protocols include File Transfer Protocol (FTP), Gopher, File, and News. The resource name is the complete address to the resource. The format of the resource name depends entirely on the protocol used, but for many protocols, including HTTP, the resource name contains one or more of the components listed in the following table:
Host Name The name of the machine on which the resource lives. Filename The pathname to the file on the machine. Port Number The port number to which to connect (typically optional). Reference A reference to a named anchor within a resource that usually identifies a specific location within a file (typically optional).
For many protocols, the host name and the filename are required, while the port number and reference are optional. For example, the resource name for an HTTP URL must specify a server on the network (Host Name) and the path to the document on that machine (Filename); it also can specify a port number and a reference. In the URL for the Java Web site java.sun.com is the host name and an empty path or the trailing slash is shorthand for the file named /index.html.
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The easiest way to create a URL object is from a String that represents the human-readable form of the URL address. This is typically the form that another person will use for a URL. For example, the URL for the Gamelan site, which is a directory of Java resources, takes the following form: In your Java program, you can use a String containing this text to create a URL object: The URL object created above represents an absolute URL. An absolute URL contains all of the information necessary to reach the resource in question. You can also create URL objects from a relative URL address. ​

A relative URL contains only enough information to reach the resource relative to (or in the context of) another URL. Relative URL specifications are often used within HTML files. For example, suppose you write an HTML file called JoesHomePage.html. Within this page, are links to other pages, PicturesOfMe.html and MyKids.html, that are on the same machine and in the same directory as JoesHomePage.html. The links to PicturesOfMe.html and MyKids.html from JoesHomePage.html could be specified just as filenames, like this:
These URL addresses are relative URLs. That is, the URLs are specified relative to the file in which they are contained--JoesHomePage.html. In your Java programs, you can create a URL object from a relative URL specification. For example, suppose you know two URLs at the Gamelan site:
You can create URL objects for these pages relative to their common base URL: https://www.gamelan.com/pages/ like this: This code snippet uses the URL constructor that lets you create a URL object from another URL object (the base) and a relative URL specification. The general form of this constructor is: The first argument is a URL object that specifies the base of the new URL. The second argument is a String that specifies the rest of the resource name relative to the base. If baseURL is null, then this constructor treats relativeURL like an absolute URL specification. Conversely, if relativeURL is an absolute URL specification, then the constructor ignores baseURL. This constructor is also useful for creating URL objects for named anchors (also called references) within a file. For example, suppose the Gamelan.network.html file has a named anchor called BOTTOM at the bottom of the file. You can use the relative URL constructor to create a URL object for it like this:

The URL class provides two additional constructors for creating a URL object. These constructors are useful when you are working with URLs, such as HTTP URLs, that have host name, filename, port number, and reference components in the resource name portion of the URL. These two constructors are useful when you do not have a String containing the complete URL specification, but you do know various components of the URL. For example, suppose you design a network browsing panel similar to a file browsing panel that allows users to choose the protocol, host name, port number, and filename. You can construct a URL from the panel's components. The first constructor creates a URL object from a protocol, host name, and filename. The following code snippet creates a URL to the Gamelan.net.html file at the Gamelan site:
This is equivalent to The first argument is the protocol, the second is the host name, and the last is the pathname of the file. Note that the filename contains a forward slash at the beginning. This indicates that the filename is specified from the root of the host. The final URL constructor adds the port number to the list of arguments used in the previous constructor:
This creates a URL object for the following URL: If you construct a URL object using one of these constructors, you can get a String containing the complete URL address by using the URL object's toString method or the equivalent toExternalForm method. ​

Some URL addresses contain special characters, for example the space character. Like this: To make theses characters legal they need to encoded before passing them to the URL constructor. Encoding the special character(s) in this example is easy as there is only one character that needs encoding, but for URL addresses that have several of these characters or if you are unsure when writing your code what URL addresses you will need to access, you can use the multi-argument constructors of the java.net.URI class to automatically take care of the encoding for you. And then convert the URI to a URL.

try {
URL myURL = new URL(. . .)
} catch (MalformedURLException e) {
. . .
// exception handler code here
. . .
}
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The URL class provides several methods that let you query URL objects. You can get the protocol, authority, host name, port number, path, query, filename, and reference from a URL using these accessor methods: getProtocol Returns the protocol identifier component of the URL. getAuthority Returns the authority component of the URL. getHost Returns the host name component of the URL. getPort Returns the port number component of the URL. The getPort method returns an integer that is the port number. If the port is not set, getPort returns -1. getPath Returns the path component of this URL. getQuery Returns the query component of this URL. getFile Returns the filename component of the URL. The getFile method returns the same as getPath, plus the concatenation of the value of getQuery, if any. getRef Returns the reference component of the URL.
You can use these getXXX methods to get information about the URL regardless of the constructor that you used to create the URL object.
The URL class, along with these accessor methods, frees you from ever having to parse URLs again! Given any string specification of a URL, just create a new URL object and call any of the accessor methods for the information you need. This small example program creates a URL from a string specification and then uses the URL object's accessor methods to parse the URL:
Here's the output displayed by the program:

After you've successfully created a URL, you can call the URL's openStream() method to get a stream from which you can read the contents of the URL. The openStream() method returns a java.io.InputStream object, so reading from a URL is as easy as reading from an input stream. The following small Java program uses openStream() to get an input stream on the URL Yahoo!. It then opens a BufferedReader on the input stream and reads from the BufferedReader thereby reading from the URL. Everything read is copied to the standard output stream:
When you run the program, you should see, scrolling by in your command window, the HTML commands and textual content from the HTML file located at Yahoo!. Alternatively, the program might hang or you might see an exception stack trace. If either of the latter two events occurs, you may have to set the proxy host so that the program can find the Yahoo server. ​

After you've successfully created a URL object, you can call the URL object's openConnection method to get a URLConnection object, or one of its protocol specific subclasses, e.g. java.net.HttpURLConnection You can use this URLConnection object to setup parameters and general request properties that you may need before connecting. Connection to the remote object represented by the URL is only initiated when the URLConnection.connect method is called. When you do this you are initializing a communication link between your Java program and the URL over the network. For example, you can open a connection to the Yahoo site with the following code:
A new URLConnection object is created every time by calling the openConnection method of the protocol handler for this URL. You are not always required to explicitly call the connect method to initiate the connection. Operations that depend on being connected, like getInputStream, getOutputStream, etc, will implicitly perform the connection, if necessary.
Now that you've successfully connected to your URL, you can use the URLConnection object to perform actions such as reading from or writing to the connection. The next section shows you how.
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The URLConnection class contains many methods that let you communicate with the URL over the network. URLConnection is an HTTP-centric class; that is, many of its methods are useful only when you are working with HTTP URLs. However, most URL protocols allow you to read from and write to the connection. This section describes both functions. ​

The following program performs the same function as the URLReader program shown in Reading Directly from a URL. However, rather than getting an input stream directly from the URL, this program explicitly retrieves a URLConnection object and gets an input stream from the connection. The connection is opened implicitly by calling getInputStream. Then, like URLReader, this program creates a BufferedReader on the input stream and reads from it. The bold statements highlight the differences between this example and the previous
The output from this program is identical to the output from the program that opens a stream directly from the URL. You can use either way to read from a URL. However, reading from a URLConnection instead of reading directly from a URL might be more useful. This is because you can use the URLConnection object for other tasks (like writing to the URL) at the same time. Again, if the program hangs or you see an error message, you may have to set the proxy host so that the program can find the Yahoo server.
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Many HTML pages contain forms-- text fields and other GUI objects that let you enter data to send to the server. After you type in the required information and initiate the query by clicking a button, your Web browser writes the data to the URL over the network. At the other end the server receives the data, processes it, and then sends you a response, usually in the form of a new HTML page. Many of these HTML forms use the HTTP POST METHOD to send data to the server. Thus writing to a URL is often called posting to a URL. The server recognizes the POST request and reads the data sent from the client.
For a Java program to interact with a server-side process it simply must be able to write to a URL, thus providing data to the server. It can do this by following these steps:

1. Create a URL.
2. Retrieve the URLConnection object.
3. Set output capability on the URLConnection.
4. Open a connection to the resource.
5. Get an output stream from the connection.
6. Write to the output stream.
7. Close the output stream.

Here is a small servlet named ReverseServlet ( or if you prefer a cgi-bin script ). You can use this servlet to test the following example program.
The servlet running in a container reads from its InputStream, reverses the string, and writes it to its OutputStream. The servlet requires input of the form string=string_to_reverse, where string_to_reverse is the string whose characters you want displayed in reverse order.
Here's an example program that runs the ReverseServlet over the network through a URLConnection:
Let's examine the program and see how it works. First, the program processes its command-line arguments: These statements ensure that the user provides two and only two command-line arguments to the program. The command-line arguments are the location of the ReverseServlet and the string that will be reversed. It may contain spaces or other non-alphanumeric characters. These characters must be encoded because the string is processed on its way to the server. The URLEncoder class methods encode the characters. Next, the program creates the URL object, and sets the connection so that it can write to it:
The program then creates an output stream on the connection and opens an OutputStreamWriter on it: If the URL does not support output, getOutputStream method throws an UnknownServiceException. If the URL does support output, then this method returns an output stream that is connected to the input stream of the URL on the server side--the client's output is the server's input. Next, the program writes the required information to the output stream and closes the stream:
This code writes to the output stream using the write method. So you can see that writing data to a URL is as easy as writing data to a stream. The data written to the output stream on the client side is the input for the servlet on the server side. The Reverse program constructs the input in the form required by the script by prepending string= to the encoded string to be reversed. The serlvet reads the information you write, performs a reverse operation on the string value, and then sends this back to you. You now need to read the string the server has sent back. The Reverse program does it like this:
If your ReverseServlet is located at https://foobar.com/servlet/ReverseServlet, then when you run the Reverse program using as the argument (including the double quote marks), you should see this output:

A socket is one end-point of a two-way communication link between two programs running on the network. Socket classes are used to represent the connection between a client program and a server program. The java.net package provides two classes--Socket and ServerSocket--that implement the client side of the connection and the server side of the connection, respectively. ​

This page contains a small example that illustrates how a client program can read from and write to a socket. ​

The previous page showed an example of how to write a client program that interacts with an existing server via a Socket object. This page shows you how to write a program that implements the other side of the connection--a server program. ​