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 identifiers.
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 the needed functionality.
About IPv6
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 getByName(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
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 {@linkplain java.net.Socket#connect(SocketAddress) connect} to a remote host.ServerSocket
is a TCP server API, and will typically {@linkplain java.net.ServerSocket#accept accept} connections from client sockets.DatagramSocket
is a UDP endpoint API and is used to {@linkplain java.net.DatagramSocket#send send} and {@linkplain java.net.DatagramSocket#receive receive} {@linkplain java.net.DatagramPacket datagram packets}.MulticastSocket
is a subclass ofDatagramSocket
used when dealing with multicast groups.
Sending and receiving with TCP sockets is done through InputStreams and
OutputStreams which can be obtained via the
getInputStream()
and
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.
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.
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 means 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 https.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 that the Handler class has to be a subclass of the abstract
class URLStreamHandler
.
Additional Specification
Interfaces
ContentHandlerFactory | This interface defines a factory for content handlers. |
CookiePolicy | CookiePolicy implementations decide which cookies should be accepted and which should be rejected. |
CookieStore | A CookieStore object represents a storage for cookie. |
DatagramSocketImplFactory | This interface defines a factory for datagram socket implementations. |
FileNameMap | A simple interface which provides a mechanism to map between a file name and a MIME type string. |
ProtocolFamily | Represents a family of communication protocols. |
SocketImplFactory | This interface defines a factory for socket implementations. |
SocketOption<T> | A socket option associated with a socket. |
SocketOptions | Interface of methods to get/set socket options. |
URLStreamHandlerFactory | This interface defines a factory for URL stream
protocol handlers. |
Classes
Authenticator | The class Authenticator represents an object that knows how to obtain authentication for a network connection. |
CacheRequest | Represents channels for storing resources in the ResponseCache. |
CacheResponse | Represent channels for retrieving resources from the ResponseCache. |
ContentHandler | The abstract class ContentHandler is the superclass
of all classes that read an Object from a
URLConnection . |
CookieHandler | A CookieHandler object provides a callback mechanism to hook up a HTTP state management policy implementation into the HTTP protocol handler. |
CookieManager | CookieManager provides a concrete implementation of CookieHandler ,
which separates the storage of cookies from the policy surrounding accepting
and rejecting cookies. |
DatagramPacket | This class represents a datagram packet. |
DatagramSocket | This class represents a socket for sending and receiving datagram packets. |
DatagramSocketImpl | Abstract datagram and multicast socket implementation base class. |
HttpCookie | An HttpCookie object represents an HTTP cookie, which carries state information between server and user agent. |
HttpURLConnection | A URLConnection with support for HTTP-specific features. |
IDN | Converts internationalized domain names between Unicode and the ASCII Compatible Encoding (ACE) representation. |
Inet4Address | This class represents an Internet Protocol version 4 (IPv4) address. |
Inet6Address | This class represents an Internet Protocol version 6 (IPv6) address. |
InetAddress | This class represents an Internet Protocol (IP) address. |
InetSocketAddress | This class implements an IP Socket Address (IP address + port number) It can also be a pair (hostname + port number), in which case an attempt will be made to resolve the hostname. |
InterfaceAddress | This class represents a Network Interface address. |
JarURLConnection | A URL Connection to a Java ARchive (JAR) file or an entry in a JAR file. |
MulticastSocket | The multicast datagram socket class is useful for sending and receiving IP multicast packets. |
NetPermission | Legacy security code; do not use. |
NetworkInterface | This class represents a Network Interface made up of a name, and a list of IP addresses assigned to this interface. |
PasswordAuthentication | The class PasswordAuthentication is a data holder that is used by Authenticator. |
Proxy | This class represents a proxy setting, typically a type (http, socks) and a socket address. |
ProxySelector | Selects the proxy server to use, if any, when connecting to the network resource referenced by a URL. |
ResponseCache | Represents implementations of URLConnection caches. |
SecureCacheResponse | Represents a cache response originally retrieved through secure means, such as TLS. |
ServerSocket | This class implements server sockets. |
Socket | This class implements client sockets (also called just "sockets"). |
SocketAddress | This class represents a Socket Address with no protocol attachment. |
SocketImpl | The abstract class SocketImpl is a common superclass
of all classes that actually implement sockets. |
SocketPermission | Legacy security code; do not use. |
StandardSocketOptions | Defines the standard socket options. |
URI | Represents a Uniform Resource Identifier (URI) reference. |
URL | Class URL represents a Uniform Resource
Locator, a pointer to a "resource" on the World
Wide Web. |
URLClassLoader | This class loader is responsible for loading classes and resources from a list of URLs which can refer to either directories or JAR files. |
URLConnection | The abstract class URLConnection is the superclass
of all classes that represent a communications link between the
application and a URL. |
URLDecoder | Utility class for HTML form decoding. |
URLEncoder | Utility class for HTML form encoding. |
URLImpl | J2ObjC-specific delegate of URL , to support the separation
of jre_net dependencies from jre_core. |
URLStreamHandler | The abstract class URLStreamHandler is the common
superclass for all stream protocol handlers. |
Enums
Authenticator.RequestorType | The type of the entity requesting authentication. |
Proxy.Type | Represents the proxy type. |
StandardProtocolFamily | Defines the standard families of communication protocols. |
Exceptions
BindException | Signals that an error occurred while attempting to bind a socket to a local address and port. |
ConnectException | Signals that an error occurred while attempting to connect a socket to a remote address and port. |
HttpRetryException | Thrown to indicate that a HTTP request needs to be retried but cannot be retried automatically, due to streaming mode being enabled. |
MalformedURLException | Thrown to indicate that a malformed URL has occurred. |
NoRouteToHostException | Signals that an error occurred while attempting to connect a socket to a remote address and port. |
PortUnreachableException | Signals that an ICMP Port Unreachable message has been received on a connected datagram. |
ProtocolException | Thrown to indicate that there is an error in the underlying protocol, such as a TCP error. |
SocketException | Thrown to indicate that there is an error creating or accessing a Socket. |
SocketTimeoutException | Signals that a timeout has occurred on a socket read or accept. |
UnknownHostException | Thrown to indicate that the IP address of a host could not be determined. |
UnknownServiceException | Thrown to indicate that an unknown service exception has occurred. |
URISyntaxException | Checked exception thrown to indicate that a string could not be parsed as a URI reference. |