19.1 Java4620: general information  (Page 6/8)

• (External Link)
• (External Link)

In addition to specifying the name of the file of interest, it is also sometimes possible to specify an anchor or reference that has been established inside the file. The anchor is identified by the text following the# character in the URL.

For example, as of this writing, the following URL will take you to a specific location inside one of my modules on Java servlets.

(External Link)

(Note the #Images at the end of the URL. That is commonly known as the anchor .)

The general syntax of a URL is as follows:

protocol://hostname[:port]/path/filename#ref

The port is optional, and is not normally required if you are accessing a server that provides the required service on a standard port. The browser (or other software being used to connect) should know which port supports the specified protocol and should connect to that port by default.

You could fill in the optional port number and use the following URL to access the same location in my servlets module, but that would simply requireyou to do some extra typing.

(External Link)

However, if you were to change the 80 to a 25, you would not be able to connect and successfully communicate with the server because the server does notsupport the HTTP protocol on port 25. In fact, here is the text of the Firefox error message that resulted from me doing that:

"This address is restricted This address uses a network port which is normally used for purposes other than Web browsing. Firefox has canceled the request for your protection."

The Google Chrome browser simply provided the following error message when I attempted to do that.

"This webpage is not available."

When I clicked a "More" button on the Chrome error message, this is what I got:

"The webpage at http://cnx.org:25/content/m48518/latest/#Images might be temporarily down or it may have moved permanently to a new web address. Error code: ERR_UNSAFE_PORT"

Note, however, that we will be properly addressing port 8080 on a particular server in some future modules. Since 8080 is not a standard port, it will be necessary forus to enter the port number following a colon in order to access the material on the server.

Socket Classes and the URL Class

Java provides at least two different approaches for doing network programming (and possibly more) , insofar as the web is concerned. The two approaches are associated with

• The Socket , DatagramSocket , and ServerSocket classes
• The URL, URLEncoder, and URLConnection classes.

Socket Programming

Socket programming primarily makes use of two socket classes named Socket and DatagramSocket along with the ServerSocket class. The first two socket classes represent TCP and UDP communications respectively.

Generally, the two socket classes are used to implement both clients and servers , while the ServerSocket class is only used to implement servers . We will see several examples of socket programming in this series of modules.

Socket programming provides a low-level approach by which you can connect two computers for the exchange of data. One of those is generally considered to bethe client while the other is considered to be the server .