Java Networking Overview

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.

Addressing

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.

Flow chart showing the logic flows from InetAddress to Inet4Address and Inet6Address

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.

Diagram showing the logic flows between SocketAddress and InetSocketAddress

Making TCP Connections

These classes are related to making normal TCP connections:

For 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:

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Once the connection is established, getInputStream() and getOutputSteam() may be used in communication between the sockets

Sending/Receiving Datagram Packets via UDP

The following are related to sending and receiving datagram packets via UDP.

DatagramPacket 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.)

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Locating/Identifying Network Resources

These classes are related to locating or identifying network resources:

The 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.

Diagram showing URL and URN as subclasses of URI.

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.

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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

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.

Authentication

Some proxies and origin servers require authentication information, using authentication schemes such as BASIC and DIGEST. For instance, when connecting with http via a proxy and the proxy requires authentication, we call the Authenticator class to obtain usernames, passwords, and other items needed to authenticate. The following classes relate to authentication:

In addition to methods for user authentication, the abstract class Authenticator also has methods for querying about the authentication being requested (see getRequestingXXX()). 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 requestPasswordAuthentication().

PasswordAuthentication is simply a data holder for a user name and a password.

Permissions

A 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.