Java IO.pdf - Nguyen Dang Binh

Java I/O
Finally, many HTML files contain relative URLs. The fourth constructor in the previous code
creates URLs relative to a given URL and is particularly useful when parsing HTML. For
example, the following code creates a URL pointing to the file 08.html, taking the rest of the
URL from u1:
URL u1, u2;
try {
u1 = new URL("http://metalab.unc.edu/javafaq/course/week12/07.html");
u2 = new URL(u1, "08.html");
}
catch (MalformedURLException e) { }
Once a URL object has been constructed, there are two ways to retrieve its data. The
openStream() method returns a raw stream of bytes from the source. The getContent()
method returns a Java object that represents the data. When you call getContent(), Java
looks for a content handler that matches the MIME type of the data. It is the openStream()
method that is of concern in this book.
The openStream() method makes a socket connection to the server and port specified in the
URL. It returns an input stream from which you can read the data at that URL, allowing you
to download data from the server. Any headers that come before the actual data or file
requested are stripped off before the stream is opened. You get only raw data.
public final InputStream openStream() throws IOException
You read from the input stream using an input stream or reader class. For example:
try {
URL u = new URL("http://www.amnesty.org/");
InputStream in = u.openStream();
int b;
while ((b = in.read()) != -1) {
System.out.write(b);
}
}
catch (MalformedURLException e) {System.err.println(e);}
catch (IOException e) {System.err.println(e);}
Applets running under the control of a security manager—that is untrusted applets that run
inside a web browser—are normally only allowed to connect back to the host they were
downloaded from. This host can be determined from the URL returned by the
getCodeBase() method of the Applet class. Attempts to connect to other hosts throw
security exceptions. You can create URLs that point to other hosts, but you may not download
data from them using openStream() or any other method. (This security restriction for
applets applies to any network connection, regardless of how you get it.)
Example 5.1 reads a series of URLs from the command line. The program connects to the
specified URLs, downloads the data, and copies it to System.out.
Example 5.1. The URLTyper Program
import java.net.*;
import java.io.*;
import com.macfaq.io.*;
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