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Creating Content and Protocol Handlers in Java
by Anghel Leonard
Certainly, you've noticed that when you "ask" for an Internet file, your browser has different ways to access it, depending on the file type. For example, if the file is a simple HTML or a common image type (.gif, .jpg), the browser becomes a viewer for it. However, if the file is a .pdf, a movie, or some other complex file type, the browser calls for external help. The browser identifies the file types, knows how to process the "simple" ones, and knows it needs special viewers or some plug-ins to handle complex file types.
Browsers also know how to "talk" with different kinds of Internet servers—HTTP servers, FTP servers, and any other kind of familiar server, depending on the kind of URL that's supplied.
Did you know that it's possible to simulate this behavior in Java? This article will show you how to create the Internet client, the Internet server (a minimal HTTP server), and a dedicated viewer for a new image type—also known as a content handler. You'll also learn how to use an existing content handler.
Working with an Existing Content Handler
When you "ask" for an Internet file, the browser usually inspects the MIME (Multipurpose Internet Mail Extensions) type of that file to determine how to process it. MIME types provide the browser with information about the file type—for example, if the file is HTML, then the MIME type will be text/html. If the file is a .jpg, then the MIME type will be image/jpeg, if the file type is a video MPEG, then the MIME type will be video/mpeg, etc.) Click here for a detailed description of existing MIME types.
Java allows you to create a network application that uses a URL to access a Web resource through the java.net.URLConnection class. Similar to a Web browser, this class inspects the MIME type of the received file and determines how to process it. It then calls a specialized class that understands the file format and puts that format into a "readable" form. These specialized classes are known as "content handlers" and they are subclasses of the java.net.ContentHandler abstract class. The practical purpose of a content handler is to extract an object specific to a MIME type from an URLConnection. You can extend this abstract class for new viewers, new MIME types, or for existing types that are not, by default, supported by Java.
Java offers a default set of content handlers that cover the most used MIME types. The content handlers' class names-along with the names of the packages where you can find them—reflects the MIME type name. For instance, the MIME type image/png's content handler is in the sun.net.www.content.image package and its name is png.class. So, the sun.net.www.content path is fixed and the rest of it depends on the MIME type.
Note: You can usually find the sun.net.www.content... path in the <JDK_HOME>/jre/lib/rt.jar.
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MIME TYPE
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Class
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package
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audio/aiff
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aiff.class
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sun.net.www.content.audio
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audio/basic
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basic.class
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sun.net.www.content.audio
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audio/wav
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wav.class
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sun.net.www.content.audio
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audio/x_aiff
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x_aiff.class
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sun.net.www.content.audio
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image/gif
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gif.class
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sun.net.www.content.image
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image/jpeg
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jpeg.class
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sun.net.www.content.image
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image/png
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png.class
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sun.net.www.content.image
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text/plain
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plain.class
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sun.net.www.content.text
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Table 1: Java correspondence between MIME types and their content handlers.
When you access an Internet resource through URLConnection, Java joins the proper content handler with the resource MIME type. Java determines the proper content handler by using the MIME type to find the class it implements.
To create a content handler, you must first create a subclass of the ContentHandler class and override the getContent method. This method gets a URLConnection argument and returns an Object object that fits with the MIME type. The most effective way to use the object returned by this method is to make a cast conversion to the right object. Here's the syntax for the getContent method:
public abstract Object getContent(URLConnection URLcon) throws IOException
To join a content handler with a MIME type, you must use the java.net.Content HandlerFactory interface. Implementing this interface forces you to implement a behavior to the ContentHandlerFactory.createContentHandler method. This method is gets a string argument representing the MIME type and uses this string for returning the right content handler:
ContentHandler createContentHandler(String MIME_type)
To install a ContentHandlerFactory as your default type, you must call the URLConnection.setContentHandlerFactory method:
public static void setContentHandlerFactory(ContentHandlerFactory CHF)
To create a viewer for a MIME type for which a default content handler already exists, choose an ordinary .jpg image who's type matches a default content handler. Request the .jpg file from an HTTP server using the proper URL. Listing 1 shows the use of a minimal HTTP server (HTTPServer) that runs on the local host on port 80. It's using the URL http://localhost/image.jpg (though you can use any other .jpg image as long as you put it into the C:\JEditor\handlers directory—this is the default directory that is used by the HTTPServer for locating the requested resource). Of course, you can make minimal changes to the application for access any .jpg image on the Internet. All you have to do in this case, is to change the URL and eventually configure your application to pass through a proxy server.
To test this application, make sure you have in the classes for HTTPServer and JPGViewer in C:\JEditor\handlers. You also need in the same directory a .jpg file named image that can be any JPG image.
As shown in Figure 1, IE displays the same result as the JPGViewer application. This is because the image/jpeg MIME type is a "member" of the Internet MIME types standard.
Listing 2 coresponds to the MIME type, text/plain and can be used for viewing the content of any text file. The Java content handler for this MIME type is plain.class and is located in the sun.net.www.content.text package. This example uses the same HTTPServer and the URL http://localhost/test.txt. The text file is named test.txt and contains the text "This is a text file...". Of course, you can use any other text file—just remember to copy the text file into the C:\\JEditor\handlers directory, so that HTTPServer can find it.
Figure 2 shows how the test.txt file content is viewed by IE and in the ListTextFiles application.
ListTextFiles
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