The Java Networking API (java.net) provides the interfaces/classes
for the following functions:
The goal of this document is to provide a high-level overview of the
facilities the java.net package provides. For details about interfaces,
classes, or factories, see the networking
API. For basic networking concepts, see Trail:
Custom Networking in the Java Tutorial.
java.net provides the following addressing-related classes:
For IP addressing, three classes are provided: InetAddress, Inet4Address,
and Inet6Address. InetAddress represents an IP address,
which is either a 32- or 128-bit unsigned number used by IP, the lower-level
protocol on which protocols like TCP and UDP are built. To represent 32-bit
IPv4 address, Inet4Address is provided. (An IPv4 address has the
familiar form nnn.nnn.nnn.nnn, where n is an integer;
e.g., 129.250.35.250). It is a subclass of InetAddress.
To represent 128-bit IPv6 addresses, Inet6Address is provided.
It is also a subclass of InetAddress.
For socket addressing, two classes are provided: SocketAddress
and InetSocketAddress. SocketAddress is an abstract
socket address, independent of a specific protocol. It is intended for subclassing
for a specific protocol. InetSocketAddress below is an example.
InetSocketAddress is a subclass of SocketAddress;
it represents an IP socket address. It can include an IP address (e.g., 129.250.35.250)
and port (e.g., 80); a hostname (e.g., coastnews.com)
and port (e.g., 1000); or port only (e.g., 1010).
In the latter case, a wildcard IP address is assumed.
These classes are related to making normal TCP connections:
ServerSocket SocketFor simple connections between a client and a server, ServerSocket
and Socket are all that you will probably need.
ServerSocket represents the socket on a server that waits and
listens for requests for service from a client. Socket represents
the endpoints for communication between a server and a client. When a server
gets a request for service, it creates a Socket for communication
with the client and continues to listen for other requests on the ServerSocket.
The client also creates a Socket for communication with the server.
The sequence is shown below:
Once the connection is established, getInputStream()
and getOutputSteam() may be used in communication between the sockets
The following are related to sending and receiving datagram packets via UDP.
DatagramPacketDatagramSocketDatagramPacket represents a datagram packet. Datagram packets
are used for connectionless delivery and normally include destination address
and port information. DatagramSocket is a socket used for sending
and receiving datagram packets over a network via UDP. A DatagramPacket
is sent from a DatagramSocket by calling the send(...)
method of DatagramSocket with DatagramPacket as the
argument: send(DatagramPacket dp). receive(DatagramPacket
dp) is use for receiving a DatagramPacket. (The MulticastSocket
class may be used for sending/receiving a DatagramPacket to a mulitcast
group. It is a subclass of DatagramSocket that adds functionality
for multicasting.)
These classes are related to locating or identifying network resources:
URI URL URLClassLoader URLConnection URLStreamHandlerHttpURLConnectionJarURLConnectionThe most commonly used classes are URI, URL, URLConnection,
and HttpURLConnection.
URI represents a Uniform Resource Identifier for a resource; it
is an identifier for a resource but not necessarily a locator for that resource.
URL represents a Uniform Resource Locator for a resource. URLs are a
subset of URIs, though the class URL is not a subclass of the URI
class. In short, a URL tells how to access the resource, while a URI may or
may not. The Uniform Resource Name (URN) is another subset of URI.
No Java class exists for it.
URLConnection is the abstract superclass of all classes that represent
a connection between an application and a network resource identified by a URL.
Given a URL and hence a protocol, URL.openConnection()
returns an instance of the appropriate implementation of URLConnection
for the protocol. (The protocol is known from the URL.) The instance
provides the means—URLConnection.connect()—to actually
open the connection and access the URL.
HttpURLConnection is the most commonly used implementation of
URLConnection. It is for http protocol, the protocol
used for accessing content on web servers. In the above diagram, if the access
protocol for the URL were http, then an instance of
HttpURLConnection would be returned by the openConnection()
method.
Security includes authentication- and permissions-related classes. Authentication
relates to user authentication and involves username and password checking.
Authentication of a user may be required in a number of situations, such as
when a user tries to access a URL. Permissions relate to what actions may be
performed; e.g., unless the NetPermission object "setDefaultAuthenticator"
exists, then invoking the method Authenticator.setDefault(Authenticator
a) will cause a security exception.
AuthenticatorPasswordAuthenticationThe abstract class Authenticator has methods for
authenticating a network connection. It is typically subclassed and an instance
of the subclass is registered with the system by calling setDefault(Authenticator
a). (Note that if there is a security manager, it checks to see that
the security policy permits the NetPermission "setDefaultAthenticator".
) Then, when the system requires authentication, it will call a method such
as getPasswordAuthentication().
PasswordAuthentication is simply a data holder for a user name
and a password.
SocketPermissionNetPermissionA SocketPermission consists of a host, with optional port range,
and a set of actions that may be performed on that host—connect,
accept, listen and/or resolve. It includes
methods to determine if one SocketPermission is equal to another
or implies another Permission. A SocketPermission
may be included in a PermissionCollection for easy checking if
a permission exists.
NetPermission is a class for various named network permissions.
Currently there are three: setDefaultAuthenticator, as mentioned
above; requestPasswordAuthentication; and specifyStreamHandler.
A NetPermission may be included in a PermissionCollection
for easy checking if a permission exists.
For more information about permissions, see http://java.sun.com/j2se/1.4/docs/guide/security/permissions.html.