S7- IWP notes - package java.net

Package java.net

Provides the classes for implementing networking applications.

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

Addresses are used throughout the java.net APIs as either host identifiers, or socket endpoint identifier.

The InetAddress class is the abstraction representing an IP (Internet Protocol) address, it has two subclasses:

• Inet4Address for IPv4 addresses.
• Inet6Address for IPv6 addresses.

But, in most cases, there is no need to deal directly with the subclasses, as the InetAddress abstraction should cover most of needed functionalities.

About IPv6

Not all systems do 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 java.net.InetAddress.getByName is 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

Sockets are means to establish a communication link between machines over the network. The java.net package provides 4 kinds of Sockets:

• Socket is a TCP client API, and will typically be used to connect (java.net.Socket.connect(SocketAddress)) to a remote host.
• ServerSocket is a TCP server API, and will typically accept (java.net.ServerSocket.accept) connections from client sockets.
• DatagramSocket is a UDP endpoint API and is used to send, and receive, java.net.DatagramPackets.
• MulticastSocket is a subclass of the DatagramSocket used when dealing with multicast groups.

Sending and receiving with TCP sockets is done through InputStreams and OutputStreams which can be obtained via the java.net.Socket.getInputStream and java.net.Socket.getOutputStream methods.

Interfaces

The 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.

High level API

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:

• URI is the class representing a Universal Resource Identifier, as specified in RFC 2396. As the name indicates, this is just an Identifier and doesn't provide directly the means to access the resource.
• URL is the class representing a Universal Resource Locator, which is both an older concept for URIs and a mean to access the resources.
• URLConnection is created from a URL and is the communication link used to access the resource pointed by the URL. This abstract class will delegate most of the work to the underlying protocol handlers like http or ftp.
• HttpURLConnection is a subclass of URLConnection and provides some additional functionalities specific to the HTTP protocol.

The recommended usage is to use URI to identify resources, then convert it into a URL when it is time to access the resource. From that URL, you can either get the 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();

Protocol Handlers

As mentioned, URL and URLConnection rely on protocol handlers which must be present, otherwise an Exception is thrown. This is the major difference with URIs which only identify resources, and therefore don't need to have access to the protocol handler. So, while it is possible to create an URI with any kind of protocol scheme (e.g. 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 the the Handler class has to be a subclass of the abstract class URLStreamHandler.