لطفا منتظر باشید ...
5.1 Downloading the Contents of a URL
Example 5-1 shows how you can download the network
resource referred to by a URL using the URL class.
This class serves mainly to represent and parse URLs but also has
several important methods for downloading URLs. The most high-level
of these methods is getContent( ), which downloads
the content of a URL, parses it, and returns the parsed object. This
method relies on special content handlers having been installed to
perform the parsing. By default, the Java SDK has content handlers
for plain text and for several common image formats. When you call
the getContent( ) method of a
URL object that refers to a plain text or GIF or
JPEG image file, the method returns a String or
Image object. More commonly, when
getContent( ) doesn't know how to
handle the data type, it simply returns an
InputStream so that you can read and parse the
data yourself.Example 5-1 doesn't use the
getContent( ) method. Instead, it calls
openStream( ) to return an
InputStream from which the contents of the URL can
be downloaded. This InputStream is connected,
through the network, to the remote resource named by the URL, but the
URL class hides all the details of setting up this
connection. (In fact, the connection is set up by a protocol handler
class; the Java SDK has default handlers for the most common network
protocols, including http:,
ftp:, mailto: and
file:.)Example 5-1 is a simple standalone program that
downloads the contents of a specified URL and saves it in a file or
writes it to the console. You'll note that most of
this program looks like it belongs in Chapter 3.
In fact, as we'll see in this and other examples in
this chapter, almost all networking involves the use of the
stream-based I/O techniques we learned about in that chapter.
Example 5-1. GetURL.java
package je3.net;
import java.io.*;
import java.net.*;
/**
*This simple program uses the URL class and its openStream( )method to
*download the contents of a URL and copy them
to a file or to the console.
**/
public class GetURL {
public static void main(String[ ] args) {
InputStream in = null;
OutputStream out = null;
try {
// Check the arguments
if ((args.length != 1)&& (args.length != 2))
throw new IllegalArgumentException("Wrong number of args");
// Set up the streams
URL url = new URL(args[0]); // Create the URL
in = url.openStream( ); // Open a stream to it
if (args.length == 2) // Get an appropriate output stream
out = new FileOutputStream(args[1]);
else out = System.out;
// Now copy bytes from the URL to the output stream
byte[ ] buffer = new byte[4096];
int bytes_read;
while((bytes_read = in.read(buffer)) != -1)
out.write(buffer, 0, bytes_read);
}
// On exceptions, print error message and usage message.
catch (Exception e) {
System.err.println(e);
System.err.println("Usage: java GetURL <URL> [<filename>]");
}
finally { // Always close the streams, no matter what.
try { in.close( ); out.close( ); } catch (Exception e) { }
}
}
}
