The java.net package can be roughly divided in two sections:
+A Low Level API, which deals with the following abstractions:
Addresses, which are networking identifiers, like IP addresses.
Sockets, which are basic bidirectional data communication mechanisms.
Interfaces, which describe network interfaces.
A High Level API, which deals with the following abstractions:
URIs, which represent Universal Resource Identifiers.
URLs, which represent Universal Resource Locators.
Connections, which represents connections to the resource pointed to by URLs.
Addresses are used throughout the java.net APIs as either host identifiers, or socket endpoint identifiers.
+The {@link java.net.InetAddress} class is the abstraction representing an IP (Internet Protocol) address. It has two subclasses: +
But, in most cases, there is no need to deal directly with the subclasses, as the InetAddress abstraction should cover most of the needed functionality.
+Not all systems have support for the IPv6 protocol, and while the Java networking stack will attempt to detect it and use it transparently when available, it is also possible to disable its use with a system property. In the case where IPv6 is not available, or explicitly disabled, Inet6Address are not valid arguments for most networking operations any more. While methods like {@link java.net.InetAddress#getByName} are guaranteed not to return an Inet6Address when looking up host names, it is possible, by passing literals, to create such an object. In which case, most methods, when called with an Inet6Address will throw an Exception.
+Sockets are means to establish a communication link between machines over the network. The java.net package provides 4 kinds of Sockets:
+Sending and receiving with TCP sockets is done through InputStreams and OutputStreams which can be obtained via the {@link java.net.Socket#getInputStream} and {@link java.net.Socket#getOutputStream} methods.
+The {@link java.net.NetworkInterface} class provides APIs to browse and query all the networking interfaces (e.g. ethernet connection or PPP endpoint) of the local machine. It is through that class that you can check if any of the local interfaces is configured to support IPv6.
+ +A number of classes in the java.net package do provide for a much higher level of abstraction and allow for easy access to resources on the network. The classes are: +
The recommended usage is to use {@link java.net.URI} to identify resources, then convert it into a {@link java.net.URL} when it is time to access the resource. From that URL, you can either get the {@link java.net.URLConnection} for fine control, or get directly the InputStream.
+
Here is an example:
+
+URI uri = new URI("http://java.sun.com/");
+URL url = uri.toURL();
+InputStream in = url.openStream();
+
myproto://myhost.mydomain/resource/), a similar URL will try to instantiate the handler for the specified protocol; if it doesn't exist an exception will be thrown.
+By default the protocol handlers are loaded dynamically from the default location. It is, however, possible to add to the search path by setting the java.protocol.handler.pkgs system property. For instance if it is set to myapp.protocols, then the URL code will try, in the case of http, first to load myapp.protocols.http.Handler, then, if this fails, http.Handler from the default location.
+
Note that the Handler class has to be a subclass of the abstract class {@link java.net.URLStreamHandler}.
+