Java Network Programming (3rd ed) [Electronic resources] نسخه متنی

7.4 Proxies

Many
systems access the Web and sometimes other non-HTTP parts of the
Internet through proxy
servers. A proxy server receives a request for
a remote server from a local client. The proxy server makes the
request to the remote server and forwards the result back to the
local client. Sometimes this is done for security reasons, such as to
prevent remote hosts from learning private details about the local
network configuration. Other times it's done to
prevent users from accessing forbidden sites by filtering outgoing
requests and limiting which sites can be viewed. For instance, an
elementary school might want to block access to http://www.playboy.com. And still other times
it's done purely for performance, to allow multiple
users to retrieve the same popular documents from a local cache
rather than making repeated downloads from the remote server.

Java programs based on the URL class can work
through most common proxy servers and protocols. Indeed, this is one
reason you might want to choose to use the URL
class rather than rolling your own HTTP or other client on top of raw
sockets.

7.4.1 System Properties

For basic operations, all you have
to do is set a few system properties to point to the addresses of
your local proxy servers. If you are using a pure HTTP proxy, set
http.proxyHost to the domain name or the IP
address of your proxy server and http.proxyPort to
the port of the proxy server (the default is 80). There are several
ways to do this, including calling System.setProperty() from within your Java code or using the -D options when
launching the program. This example sets the proxy server to
192.168.254.254 and the port to 9000:

% java -Dhttp.proxyHost=192.168.254.254  -Dhttp.proxyPort=9000  
com.domain.Program

If you want to exclude a host from being proxied and connect directly
instead, set the http.nonProxyHosts system
property to its hostname or IP address. To exclude multiple hosts,
separate their names by vertical bars. For example, this code
fragment proxies everything except
java.oreilly.com and
xml.oreilly.com:

System.setProperty("http.proxyHost", "192.168.254.254");
System.setProperty("http.proxyPort", "9000");
System.setProperty("http.nonProxyHosts", "java.oreilly.com|xml.oreilly.com");

You can also use an asterisk as a wildcard to indicate that all the
hosts within a particular domain or subdomain should not be proxied.
For example, to proxy everything except hosts in the
oreilly.com domain:

% java -Dhttp.proxyHost=192.168.254.254  -Dhttp.nonProxyHosts=*.oreilly.com  
com.domain.Program

If you are using an FTP proxy server, set the
ftp.proxyHost, ftp.proxyPort,
and ftp.nonProxyHosts properties in the same way.

Java does not support any other application layer proxies, but if
you're using a transport layer SOCKS proxy for all
TCP connections, you can identify it with the
socksProxyHost and
socksProxyPort system properties. Java does not
provide an option for nonproxying with SOCKS. It's
an all-or-nothing decision.

7.4.2 The Proxy Class

Java 1.5 allows more fine-grained
control of proxy servers from within a Java program. Specifically,
this allows you to choose different proxy servers for different
remote hosts. The proxies themselves are represented by instances of
the java.net.Proxy class. There are still only
three kinds of proxies, HTTP, SOCKS, and direct
connections (no proxy at all), represented by three constants in the
Proxy.Type enum:

Proxy.Type.DIRECT

Proxy.Type.HTTP

Proxy.Type.SOCKS

Besides its type, the other important piece of information about a
proxy is its address and port, given as a
SocketAddress object. For example, this code
fragment creates a Proxy object representing an
HTTP proxy server on port 80 of
proxy.example.com:

SocketAddress address = new InetSocketAddress("proxy.example.com", 80);
Proxy proxy = new Proxy(Proxy.Type.HTTP, address);

Although there are only three kinds of proxy objects, there can be
many proxies of the same type for different proxy servers on
different hosts.

7.4.3 The ProxySelector Class

Each running Java 1.5 virtual machine
has a single java.net.ProxySelector object it uses
to locate the proxy server for different connections. The default
ProxySelector merely inspects the various system
properties and the URL's protocol to decide how to
connect to different hosts. However, you can install your own
subclass of ProxySelector in place of the default
selector and use it to choose different proxies based on protocol,
host, path, time of day, or other criteria.

The key to this class is the abstract select( )
method:

public abstract List<Proxy> select(URI uri)

Java passes this method a URI object (not a
URL object) representing the host to which a
connection is needed. For a connection made with the URL class, this
object typically has the form
http://www.example.com/ or
ftp://ftp.example.com/pub//image/library/english/10151_, or some such.
For a pure TCP connection made with the Socket class, this URI will
have the form socket://host:port:, for instance,
socket://www.example.com:80. The
ProxySelector object then chooses the right
proxies for this type of object and returns them in a
List<Proxy>.

The second abstract method in this class you must implement is
connectFailed( ):

public void connectFailed(URI uri, SocketAddress address, IOException ex)

This is a callback method used to warn a program that the proxy
server isn't actually making the connection. Example 7-11 demonstrates with a
ProxySelector that attempts to use the proxy
server at proxy.example.com for all HTTP
connections unless the proxy server has previously failed to resolve
a connection to a particular URL. In that case, it suggests a direct
connection instead.

Example 7-11. A ProxySelector that remembers what it can connect to

import java.net.*;
import java.util.*;
import java.io.*;
public class LocalProxySelector extends ProxySelector {
private List failed = new ArrayList( );
public List<Proxy> select(URI uri) {
List<Proxy> result = new ArrayList<Proxy>( );
if (failed.contains(uri) 
|| "http".equalsIgnoreCase(uri.getScheme( ))) {
result.add(Proxy.NO_PROXY);
}
else {
SocketAddress proxyAddress 
= new InetSocketAddress( "proxy.example.com", 8000);
Proxy proxy = new Proxy(Proxy.Type.HTTP, proxyAddress);
result.add(proxy);
}
return result;
}
public void connectFailed(URI uri, SocketAddress address, IOException ex) {
failed.add(uri);
}
}

As I already said, each running virtual machine has exactly one
ProxySelector. To change the
ProxySelector, pass the new selector to the static
ProxySelector.setDefault( ) method, like so:

ProxySelector selector = new LocalProxySelector( ):
ProxySelector.setDefault(selector);

From this point forward, all connections opened by that virtual
machine will ask the ProxySelector for the right
proxy to use. You normally shouldn't use this in
code running in a shared environment. For instance, you
wouldn't change the ProxySelector
in a servlet because that would change the
ProxySelector for all servlets running in the same
container.