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.
InetAddress class is the abstraction representing an IP (Internet Protocol) address. It has two subclasses:
Inet4Addressfor IPv4 addresses.
Inet6Addressfor IPv6 addresses.
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
InetAddress.getByName(java.lang.String) 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:
Socketis a TCP client API, and will typically be used to connect to a remote host.
ServerSocketis a TCP server API, and will typically accept connections from client sockets.
DatagramSocketis a UDP endpoint API and is used to send and receive datagram packets.
MulticastSocketis a subclass of
DatagramSocketused when dealing with multicast groups.
Sending and receiving with TCP sockets is done through InputStreams and OutputStreams which can be obtained via 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.
Note, all conforming implementations must support at least one
NetworkInterface object, which must either be connected to a network, or be a "loopback" interface that can only communicate with entities on the same machine.
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:
URIis 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.
URLis the class representing a Universal Resource Locator, which is both an older concept for URIs and a means to access the resources.
URLConnectionis 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 https.
HttpURLConnectionis a subclass of URLConnection and provides some additional functionalities specific to the HTTP protocol. This API has been superceded by the newer HTTP client API described in the previous section.
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();
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 deploy additional protocols handlers as
services. Service providers of type URLStreamHandlerProvider are located at runtime, as specified in the URL constructor.
This class loader is used to load classes and resources from a search path of URLs referring to both JAR files and directories.
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