URLs and URIs - Java Network Programming - page 128

The java.io.File class has a toURL() method that returns a file URL matching the
given file. The exact format of the URL returned by this method is platform dependent.
For example, on Windows it may return something like file:/D:/JAVA/JNP4/05/
ToURLTest.java . On Linux and other Unixes, you're likely to see file:/home/elharo/
books/JNP4/05/ToURLTest.java . In practice, file URLs are heavily platform and pro‐
gram dependent. Java file URLs often cannot be interchanged with the URLs used by
web browsers and other programs, or even with Java programs running on different
platforms.
Class loaders are used not only to load classes but also to load resources such as images
and audio files. The static ClassLoader.getSystemResource(String name) method
returns a URL from which a single resource can be read. The ClassLoader.getSystem
Resources(String name) method returns an Enumeration containing a list of URL s
from which the named resource can be read. And finally, the instance method getRe
source(String name) searches the path used by the referenced class loader for a URL
to the named resource. The URLs returned by these methods may be file URLs, HTTP
URLs, or some other scheme. The full path of the resource is a package qualified Java
name with slashes instead of periods such as /com/macfaq/sounds/swale.au or com/
macfaq/images/headshot.jpg . The Java virtual machine will attempt to find the requested
resource in the classpath, potentially inside a JAR archive.
There are a few other methods that return URL objects here and there throughout the
class library, but most are simple getter methods that return a URL you probably already
know because you used it to construct the object in the first place; for instance, the
getPage() method of javax.swing.JEditorPane and the getURL() method of
java.net.URLConnection .
Retrieving Data from a URL
Naked URLs aren't very exciting. What's interesting is the data contained in the docu‐
ments they point to. The URL class has several methods that retrieve data from a URL:
public InputStream openStream () throws IOException
public URLConnection openConnection () throws IOException
public URLConnection openConnection ( Proxy proxy ) throws IOException
public Object getContent () throws IOException
public Object getContent ( Class [] classes ) throws IOException
The most basic and most commonly used of these methods is openStream() , which
returns an InputStream from which you can read the data. If you need more control
over the download process, call openConnection() instead, which gives you a URLCon
nection which you can configure, and then get an InputStream from it. We'll take this
up in Chapter 7 . Finally, you can ask the URL for its content with getContent() which
may give you a more complete object such as String or an Image . Then again, it may
just give you an InputStream anyway.