Java Network Programming, Second Edition

Content Handlers

The URLConnection class is intimately tied to Java’s protocol and content handler mechanism. The protocol handler is responsible for making connections, exchanging headers, requesting particular documents, and so forth. It handles all the overhead of the protocol for requesting files. The content handler deals only with the actual data. It takes the raw input after all headers and so forth are stripped and converts it to the right kind of object for Java to deal with; for instance, an InputStream or an ImageProducer.

Getting Content

The getContent( ) methods of URLConnection use a content handler to turn the raw data of a connection into a Java object.

public Object getContent( ) throws IOException

This method is virtually identical to the getContent( ) method of the URL class. In fact, that method just calls this method. getContent( ) downloads the object selected by the URL of this URLConnection. For getContent( ) to work, the environment needs to recognize and understand the content type. The only content types that are supported in the JDK are text/plain, image/gif, and image/jpeg. Other VMs and applications may support additional types. For instance, HotJava 3.0 includes a PDF content handler. Furthermore, you can install additional content handlers that understand other content types.

getContent( ) works only for protocols like HTTP that have a clear understanding of MIME content types. If the content type is unknown, or the protocol doesn’t understand content ...