Download Files in Applets or Applications

Downloading a File in an Applet or Application

Write a java application or applet that downloads a file from across the network. Due to security considerations, applets may only download files on the server from which they were downloaded. To download a file, you must construct the URL location as an instance of the URL class. With the URL class in hand, you can invoke the openConnection() method to retrieve an instance of the URLConnection class. From here there are two options:

Get an input stream from the connection using the getInputStream() method of URLConnection and use it to retrieve information over the connection. Note: it is also possible to send information over an output stream with the counterpart method getOutputStream().
Write a custom ContentHandler that manipulates the data for a file of a certain type. To do this you must first implement the ContentHandlerFactory interface. The createContentHandler(String mimetype) method returns a subclass of ContentHandler that handles the data for a file of type mimetype. For example for content type "text/plain" the call to your ContentHandlerFactory would return your special ContentHandler, PlainTextHandler. The next step, then, is to implement PlainTextHandler by defining the abstract method getContent(URLConnection). The PlainTextHandler will then get an InputStream from the URLConnection, manipulate the data and return an Object representing the data. In the demo, the ContentHandler sandwiches the data between a header and trailer and returns the new String. To make use of the ContentHandler, do the following:
```
	url_connection.setContentHandlerFactory(new your_conyentHandlerFactory());
	Object data url_connection.getContent();
```
In getContent() the url_connection asks the factory for a ContentHandler based on the content type. If a something other than null returns, then the URLConnection inokes the getContent() method of the ContentHandler. Otherwise, the URLConnection uses the default handler for the content type. Note: This doesn't work in applets, a SecurityException is invariably thrown.

Added Sophistication:

Write a viewer for the file you intend to download and launch the viewer when the file is downloaded. The file shouldn't be an image, because image downloading is already implemented.
For an application, write a directory browser that allows the user to select from appropriate files in a directory. For an applet, write an interface to select from file parameterized in the html applet tag. When a connection is made to a URL that is a directory, a directory listing in html is returned.

Script to run the Download demo

References

Flanagan, David; Java in a Nutshell, Section 7 on Networking
The Java tutorial is a useful online resource, especially the chapters on networking overview and working with URLs.
Look at the Javasoft Java API Documentation on the java.net package.