Java Reference
In-Depth Information
The
ContentConnection
class inherits from
StreamConnection
and provides three new
methods:
getEncoding
,
getLength
, and
getType
. This example uses the
getLength
method to
determine the number of bytes being returned by the remote server; unfortunately,
there's no guarantee that the server will actually report this information. Consequently,
this example tests the returned content length; if it's undefined, the code will read the
data in 512-byte chunks. Either way, the code creates a new image with the returned data
and closes both the input stream and
ContentConnection
. This code is encapsulated in a
runnable method,
run
, because in practice you're likely to use threads for all of your I/O.
(I show you a concrete example of this in the “Putting HTTP to Work” section, which
details a full application that uses HTTP.)
The
ContentConnection
class is a high-level abstraction of the kind of metadata
that HTTP provides, and it lets you perform only a fetch of remote content. Using the
ContentConnection
, you specify a remote resource when you create the connection
using the
Connector
class. Under the hood, the platform makes the desired request
using the HTTP method
GET
and the connection you specify when you request the
DataInputStream
from which the remote resource will be read, giving you no control
over the semantics of the HTTP request itself. Once the platform has completed the
request, you can learn only three things about the response: how the response was
encoded (using the
getEncoding
method), the number of bytes returned (using the
getLength
method), and the Multipurpose Internet Mail Extensions (MIME) type of the
method (using the
getType
method).
■
Note
The content type and other information about the response are only available if the server provides
them; your application must be prepared to deal with the possibility that a server won't return this informa-
tion (unless you have control over the server implementation as well, and even then you would need to
handle any errors that arise in your application from a lack of this metadata).
Because
ContentConnection
is such a simple interface, it's ideal when your applica-
tion needs only to obtain the contents of a remote resource. It is, however, woefully
inadequate for more sophisticated uses of HTTP, because it does not provide access to
most of the metadata that accompanies HTTP requests and responses, nor does it let
you specify the method HTTP invokes when making the request. For finer-grained
access to the HTTP protocol, you need to use instances of the
HttpConnection
class—
a class that inherits from
ContentConnection
. (In fact, when you invoke
Connector.open
on a URL with the
http
scheme, the instance returned is the
HttpConnection
, not the
ContentConnection
the code in Listing 12-8 suggests.)
Reflecting the underlying connection with a remote HTTP server, an
HttpConnection
instance can be in one of three states:
