Java Reference
In-Depth Information
URLConnection
does not have the best-designed API in the Java class
library. One of several problems is that the
URLConnection
class is too
closely tied to the HTTP protocol. For instance, it assumes that each
file transferred is preceded by a MIME header or something very much
like one. However, most classic protocols such as FTP and SMTP don't
use MIME headers.
Opening URLConnections
A program that uses the
URLConnection
class directly follows this basic sequence of
steps:
1. Construct a
URL
object.
2. Invoke the
URL
object's
openConnection()
method to retrieve a
URLConnection
object for that URL.
3. Configure the
URLConnection
.
4. Read the header fields.
5. Get an input stream and read data.
6. Get an output stream and write data.
7. Close the connection.
You don't always perform all these steps. For instance, if the default setup for a particular
kind of URL is acceptable, you can skip step 3. If you only want the data from the server
and don't care about any metainformation, or if the protocol doesn't provide any met‐
ainformation, you can skip step 4. If you only want to receive data from the server but
not send data to the server, you'll skip step 6. Depending on the protocol, steps 5 and 6
may be reversed or interlaced.
The single constructor for the
URLConnection
class is protected:
protected
URLConnection
(
URL
url
)
Consequently, unless you're subclassing
URLConnection
to handle a new kind of URL
(i.e., writing a protocol handler), you create one of these objects by invoking the
open
Connection()
method of the
URL
class. For example:
try
{
URL
u
=
new
URL
(
"http://www.overcomingbias.com/"
);
URLConnection
uc
=
u
.
openConnection
();
// read from the URL...
}
catch
(
MalformedURLException
ex
)
{
System
.
err
.
println
(
ex
);
}
catch
(
IOException
ex
)
{
