Java IO.pdf - Nguyen Dang Binh

Java I/O
In particular, URL connections provide not just input streams by which the client can read
data from the server, but also output streams to send data from the client to the server. This is
essential for protocols like mailto.
The java.net.URLConnection class is an abstract class that handles communication with
different kinds of servers, like FTP servers and web servers. Protocol-specific subclasses of
URLConnection, hidden inside the sun classes, handle different kinds of servers.
5.2.1 Reading Data from URL Connections
URL connections take place in five steps:
1. The URL object is constructed.
2. The openConnection() method of the URL object creates the URLConnection object.
3. The parameters for the connection and the request properties that the client sends to
the server are set up.
4. The connect() method makes the connection to the server, perhaps using a socket for
a network connection or a file input stream for a local connection. The response
header information is read from the server.
5. Data is read from the connection by using the input stream returned by
getInputStream() or through a content handler with getContent(). Data can be
sent to the server using the output stream provided by getOutputStream().
This scheme is very much based on the HTTP/1.0 protocol. It does not fit other schemes that
have a more interactive "request, response, request, response, request, response" pattern
instead of HTTP/1.0's "single request, single response, close connection" pattern. In
particular, FTP and even HTTP/1.1 aren't well suited to this pattern. I wouldn't be surprised to
see this replaced with something more general in a future version of Java.
URLConnection objects are not constructed directly in your own programs. Instead, you
create a URL for the particular resource, and call that URL's openConnection() method. This
gives you a URLConnection. Then the getInputStream() method returns an input stream
that reads data from the URL. (The openStream() method of the URL class is just a thin
veneer over the getInputStream() method of the URLConnection class.) For example:
try {
URL u = new URL("http://www.digitalthink.com/");
URLConnection uc = u.openConnection();
uc.connect();
InputStream in = uc.getInputStream();
//...
}
catch (IOException e) { //...
If the connection cannot be opened (possibly because the remote host is unreachable), an
IOException is thrown. Example 5.2 reads a URL from the command line, opens a
connection to that URL, then prints the data returned by the server at that URL. This is similar
to Example 5.1; WebCat differs primarily in that it uses a URLConnection instead of a URL.
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