Download URLS - Cafe au Lait Java

URLs, InetAddresses, and
URLConnections
High Level Network Programming
Elliotte Rusty Harold
[email protected]
http://sunsite.unc.edu/javafaq/URLS.PPT
© 1997 Elliotte Rusty Harold
5/12/2017
We will learn how Java handles
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Internet Addresses
URLs
CGI
URLConnection
Content and Protocol handlers
© 1997 Elliotte Rusty Harold
5/12/2017
I assume you
• Understand basic Java syntax and I/O
• Have a user’s view of the Internet
• No prior network programming experience
© 1997 Elliotte Rusty Harold
5/12/2017
Applet Network Security
Restrictions
• Applets may:
– send data to the code base
– receive data from the code base
• Applets may not:
– send data to hosts other than the code base
– receive data from hosts other than the code base
© 1997 Elliotte Rusty Harold
5/12/2017
Some Background
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Hosts
Internet Addresses
Ports
Protocols
© 1997 Elliotte Rusty Harold
5/12/2017
Hosts
• Devices connected to the Internet are called
hosts
• Most hosts are computers, but hosts also
include routers, printers, fax machines, soda
machines, bat houses, etc.
© 1997 Elliotte Rusty Harold
5/12/2017
Internet addresses
• Every host on the Internet is identified by a
unique, four-byte Internet Protocol (IP)
address.
• This is written in dotted quad format like
199.1.32.90 where each byte is an unsigned
integer between 0 and 255.
• There are about four billion unique IP
addresses, but they aren’t very efficiently
allocated
© 1997 Elliotte Rusty Harold
5/12/2017
Domain Name System (DNS)
• Numeric addresses are mapped to names
like "www.blackstar.com" or
"star.blackstar.com" by DNS.
• Each site runs domain name server software
that translates names to IP addresses and
vice versa
• DNS is a distributed system
© 1997 Elliotte Rusty Harold
5/12/2017
The InetAddress Class
• The java.net.InetAddress class
represents an IP address.
• It converts numeric addresses to host names
and host names to numeric addresses.
• It is used by other network classes like
Socket and ServerSocket to identify
hosts
© 1997 Elliotte Rusty Harold
5/12/2017
Creating InetAddresses
• There are no public InetAddress()
constructors. Arbitrary addresses may not
be created.
• All addresses that are created must be
checked with DNS
© 1997 Elliotte Rusty Harold
5/12/2017
The getByName() factory
method
public static InetAddress getByName(String
host) throws UnknownHostException
InetAddress utopia, duke;
try {
utopia = InetAddress.getByName("utopia.poly.edu");
duke = InetAddress.getByName("128.238.2.92");
}
catch (UnknownHostException e) {
System.err.println(e);
}
© 1997 Elliotte Rusty Harold
5/12/2017
Other ways to create
InetAddress objects
public static InetAddress[] getAllByName(String host)
throws UnknownHostException
public static InetAddress getLocalHost() throws
UnknownHostException
© 1997 Elliotte Rusty Harold
5/12/2017
Getter Methods
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public
public
public
public
© 1997 Elliotte Rusty Harold
boolean isMulticastAddress()
String getHostName()
byte[] getAddress()
String getHostAddress()
5/12/2017
Utility Methods
• public int hashCode()
• public boolean equals(Object o)
• public String toString()
© 1997 Elliotte Rusty Harold
5/12/2017
Ports
• In general a host has only one Internet
address
• This address is subdivided into 65,536 ports
• Ports are logical abstractions that allow one
host to communicate simultaneously with
many other hosts
• Many services run on well-known ports. For
example, http tends to run on port 80
© 1997 Elliotte Rusty Harold
5/12/2017
Protocols
• A protocol defines how two hosts talk to
each other.
• The daytime protocol, RFC 867, specifies
an ASCII representation for the time that's
legible to humans.
• The time protocol, RFC 868, specifies a
binary representation, for the time that's
legible to computers.
• There are thousands of protocols, standard
and non-standard
© 1997 Elliotte Rusty Harold
5/12/2017
IETF RFCs
• Requests For Comment
• Document how much of the Internet works
• Various status levels from obsolete to
required to informational
• TCP/IP, telnet, SMTP, MIME, HTTP, and
more
• http://ds.internic.net/rfc/
© 1997 Elliotte Rusty Harold
5/12/2017
W3C Standards
• IETF is based on “rough consensus and
running code”
• W3C tries to run ahead of implementation
• IETF is an informal organization open to
participation by anyone
• W3C is a vendor consortium open only to
companies
© 1997 Elliotte Rusty Harold
5/12/2017
URLs
• A URL, short for "Uniform Resource
Locator", is a way to unambiguously
identify the location of a resource on the
Internet.
© 1997 Elliotte Rusty Harold
5/12/2017
Example URLs
• http://www.javasoft.com/
• file:///Macintosh%20HD/Java/Docs/JDK%201.1.1%20
docs/api/java.net.InetAddress.html#_top_
• http://www.macintouch.com:80/newsrecent.shtml
• ftp://ftp.info.apple.com/pub/
• mailto:[email protected]
• telnet://utopia.poly.edu
© 1997 Elliotte Rusty Harold
5/12/2017
The Pieces of a URL
• Most URLs can be broken into about five
pieces, not all of which are necessarily
present in any given URL. These are:
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–
–
the protocol
the host
the port
the file
the ref, section, or anchor
© 1997 Elliotte Rusty Harold
5/12/2017
The java.net.URL class
• A URL object represents a URL.
• The URL class contains methods to
– create new URLs
– parse the different parts of a URL
– get an input stream from a URL so you can read
data from a server
– get content from the server as a Java object
© 1997 Elliotte Rusty Harold
5/12/2017
Content and Protocol Handlers
• Content and protocol handlers separate the
data being downloaded from the the
protocol used to download it.
• The protocol handler negotiates with the
server and parses any headers. It gives the
content handler only the actual data of the
requested resource.
• The content handler translates those bytes
into a Java object like an InputStream
or ImageProducer.
© 1997 Elliotte Rusty Harold
5/12/2017
Finding Protocol Handlers
• When you construct a URL object, the
virtual machine looks for a protocol handler
that understands the protocol part of the
URL such as "http" or "mailto".
• If no such handler is found, the constructor
throws a MalformedURLException.
© 1997 Elliotte Rusty Harold
5/12/2017
Supported Protocols
• The exact protocols that Java supports vary
from implementation to implementation
though http and file are supported pretty
much everywhere. Sun's JDK 1.1
understands ten:
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file
ftp
gopher
http
mailto
© 1997 Elliotte Rusty Harold
–appletresource
–doc
–netdoc
–systemresource
–verbatim
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URL Constructors
• There are four constructors in the
java.net.URL class. All can throw
MalformedURLExceptions.
public URL(String u) throws
MalformedURLException
public URL(String protocol, String host,
String file) throws MalformedURLException
public URL(String protocol, String host, int
port, String file) throws
MalformedURLException
public URL(URL context, String u) throws
MalformedURLException
© 1997 Elliotte Rusty Harold
5/12/2017
Constructing URL Objects
• Construct a URL object for a complete, absolute
URL like
http://www.poly.edu/fall97/grad.html#cs like this:
try {
URL u = new
URL("http://www.poly.edu/fall97/grad.html#cs
” );
}
catch (MalformedURLException e) {}
© 1997 Elliotte Rusty Harold
5/12/2017
Constructing URL Objects in
Pieces
• You can also construct the URL by passing
its pieces to the constructor, like this:
URL u = null;
try {
u = new URL("http", "www.poly.edu",
"/schedule/fall97/bgrad.html#cs");
}
catch (MalformedURLException e) {}
© 1997 Elliotte Rusty Harold
5/12/2017
Including the Port
URL u = null;
try {
u = new URL("http", "www.poly.edu", 8000,
"/fall97/grad.html#cs");
}
catch (MalformedURLException e) {}
© 1997 Elliotte Rusty Harold
5/12/2017
Relative URLs
• Many HTML files contain relative URLs.
• Consider the page
http://sunsite.unc.edu/javafaq/index.html
• On this page a link to “books.html" refers to
http://sunsite.unc.edu/javafaq/books.html.
© 1997 Elliotte Rusty Harold
5/12/2017
Constructing Relative URLs
• The fourth constructor creates URLs
relative to a given URL. For example,
try {
URL u1 = new
URL("http://sunsite.unc.edu/index.htm
l");
URL u2 = new URL(u1, ”books.html");
}
catch (MalformedURLException e) {}
• This is particularly useful when parsing
HTML.
© 1997 Elliotte Rusty Harold
5/12/2017
Parsing URLs
• The java.net.URL class has five
methods to spilt a URL into its component
parts. These are:
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–
public
public
public
public
public
© 1997 Elliotte Rusty Harold
String getProtocol()
String getHost()
int getPort()
String getFile()
String getRef()
5/12/2017
For example,
try {
URL u = new
URL("http://www.poly.edu/fall97/grad.html#
cs ");
System.out.println("The protocol is " +
u.getProtocol());
System.out.println("The host is " +
u.getHost());
System.out.println("The port is " +
u.getPort());
System.out.println("The file is " +
u.getFile());
System.out.println("The anchor is " +
u.getRef());
}
catch (MalformedURLException e) { }
© 1997 Elliotte Rusty Harold
5/12/2017
Missing Pieces
• If a port is not explicitly specified in the
URL it's set to -1. This means the default
port is to be used.
• If the ref doesn't exist, it's just null, so watch
out for NullPointerExceptions.
Better yet, test to see that it's non-null
before using it.
• If the file is left off completely, e.g.
http://www.javasoft.com, then it's set to "/".
© 1997 Elliotte Rusty Harold
5/12/2017
Reading Data from a URL
• The openStream() method connects to the
server specified in the URL and returns an
InputStream object fed by the data from that
connection.
public final InputStream openStream() throws
IOException
• Any headers that precede the actual data are stripped
off before the stream is opened.
• Network connections are less reliable and slower
than files. Buffer with a BufferedInputStream
or a BufferedReader.
© 1997 Elliotte Rusty Harold
5/12/2017
import java.net.*;
import java.io.*;
public class Webcat {
public static void main(String[] args) {
for (int i = 0; i < args.length; i++) {
try {
URL u = new URL(args[i]);
InputStream in = u.openStream();
InputStreamReader isr = new InputStreamReader(in);
BufferedReader br = new BufferedReader(isr);
String theLine;
while ((theLine = br.readLine()) != null) {
System.out.println(theLine);
}
}
catch (MalformedURLException e) {
System.err.println(e);}
catch (IOException e) { System.err.println(e);}
}
}
}
© 1997 Elliotte Rusty Harold
5/12/2017
CGI
• Common Gateway Interface
• A lot is written about writing server side
CGI. I’m going to show you client side
CGI.
• We’ll need to explore HTTP a little deeper
to do this
© 1997 Elliotte Rusty Harold
5/12/2017
Normal web surfing uses these
two steps:
– The browser request a page
– The server sends the page
• Data flows primarily from the server to the
client.
© 1997 Elliotte Rusty Harold
5/12/2017
Forms
• There are times when the server needs to
get data from the client rather than the other
way around. The common way to do this is
with a form like this one:
© 1997 Elliotte Rusty Harold
5/12/2017
CGI
• The user types the requested data into the
form and hits the submit button.
• The client browser then sends the data to
the server using the Common Gateway
Interface, CGI for short.
• CGI uses the HTTP protocol to transmit the
data, either as part of the query string or as
separate data following the MIME header.
© 1997 Elliotte Rusty Harold
5/12/2017
GET and POST
• When the data is sent as a query string
included with the file request, this is called
CGI GET.
• When the data is sent as data attached to the
request following the MIME header, this is
called CGI POST
© 1997 Elliotte Rusty Harold
5/12/2017
HTTP
• Web browsers communicate with web
servers through a standard protocol known
as HTTP, an acronym for HyperText
Transfer Protocol.
• This protocol defines
– how a browser requests a file from a web server
– how a browser sends additional data along with
the request (e.g. the data formats it can accept),
– how the server sends data back to the client
– response codes
© 1997 Elliotte Rusty Harold
5/12/2017
A Typical HTTP Connection
– Client opens a socket to port 80 on the server.
– Client sends a GET request including the name
and path of the file it wants and the version of
the HTTP protocol it supports.
– The client sends a MIME header.
– The client sends a blank line.
– The server sends a MIME header
– The server sends the data in the file.
– The server closes the connection.
© 1997 Elliotte Rusty Harold
5/12/2017
MIME
• MIME is an acronym for "Multipurpose
Internet Mail Extensions".
• an Internet standard defined in RFCs 2045
through 2049
• originally intended for use with email
messages, but has been been adopted for
use in HTTP.
© 1997 Elliotte Rusty Harold
5/12/2017
Browser Request MIME Header
• When the browser sends a request to a web
server, it also sends a MIME header. MIME
headers contain name-value pairs,
essentially a name followed by a colon and
a space, followed by a value.
Connection: Keep-Alive
User-Agent: Mozilla/3.01 (Macintosh; I; PPC)
Host: www.digitalthink.com:80
Accept: image/gif, image/x-xbitmap,
image/jpeg, image/pjpeg, */*
© 1997 Elliotte Rusty Harold
5/12/2017
Server Response MIME Header
• When a web server responds to a web
browser it sends a MIME header along with
the response that looks something like this:
Server: Netscape-Enterprise/2.01
Date: Sat, 02 Aug 1997 07:52:46 GMT
Accept-ranges: bytes
Last-modified: Tue, 29 Jul 1997 15:06:46
GMT
Content-length: 2810
Content-type: text/html
© 1997 Elliotte Rusty Harold
5/12/2017
Query Strings
• CGI GET data is sent in URL encoded
query strings
• a query string is a set of name=value pairs
separated by ampersands
Author=Sadie, Julie&Title=Women Composers
• separated from rest of URL by a question
mark
© 1997 Elliotte Rusty Harold
5/12/2017
URL Encoding
• Alphanumeric ASCII characters (a-z, A-Z,
and 0-9) and the $-_.!*'(), punctuation
symbols are left unchanged.
• The space character is converted into a plus
sign (+).
• Other characters (e.g. &, =, ^, #, %, ^, {,
and so on) are translated into a percent sign
followed by the two hexadecimal digits
corresponding to their numeric value.
© 1997 Elliotte Rusty Harold
5/12/2017
For example,
• The comma is ASCII character 44 (decimal)
or 2C (hex). Therefore if the comma
appears as part of a URL it is encoded as
%2C.
• The query string "Author=Sadie,
Julie&Title=Women Composers" is
encoded as:
– Author=Sadie%2C+Julie&Title=Women+Composers
© 1997 Elliotte Rusty Harold
5/12/2017
The URLEncoder class
• The java.net.URLEncoder class
contains a single static method which
encodes strings in x-www-form-urlencoded format
URLEncoder.encode(String s)
© 1997 Elliotte Rusty Harold
5/12/2017
For example,
String qs = "Author=Sadie,
Julie&Title=Women Composers";
String eqs = URLEncoder.encode(qs);
System.out.println(eqs);
• This prints:
Author%3dSadie%2c+Julie%26Title%3dWomen+Com
posers
© 1997 Elliotte Rusty Harold
5/12/2017
String eqs = "Author=" +
URLEncoder.encode("Sadie, Julie");
eqs += "&";
eqs += "Title=";
eqs += URLEncoder.encode("Women
Composers");
• This prints the properly encoded query
string:
Author=Sadie%2c+Julie&Title=Women+Composers
© 1997 Elliotte Rusty Harold
5/12/2017
GET URLs
String eqs = "Author=" + URLEncoder.encode("Sadie,
Julie");
eqs += "&";
eqs += "Title=";
eqs += URLEncoder.encode("Women Composers");
try {
URL u = new
URL("http://www.superbooks.com/search.cgi?" +
eqs);
InputStream in = u.openStream();
//...
}
catch (IOException e) { //...
© 1997 Elliotte Rusty Harold
5/12/2017
URLConnections
• The java.net.URLConnection class
is an abstract class that handles
communication with different kinds of
servers like ftp servers and web servers.
• Protocol specific subclasses of
URLConnection handle different kinds
of servers.
• By default, connections to HTTP URLs use
the GET method.
© 1997 Elliotte Rusty Harold
5/12/2017
URLConnections vs. URLs
• Can send output as well as read input
• Can post data to CGIs
• Can read headers from a connection
© 1997 Elliotte Rusty Harold
5/12/2017
URLConnection five steps:
1. The URL is constructed.
2. The URL’s openConnection() method creates
the URLConnection object.
3. The parameters for the connection and the request
properties that the client sends to the server are set
up.
4. The connect() method makes the connection to
the server.
5. The response header information is read using
getHeaderField().
© 1997 Elliotte Rusty Harold
5/12/2017
I/O Across a URLConnection
• Data may be read from the connection in
one of two ways
– raw by using the input stream returned by
getInputStream()
– through a content handler with
getContent().
• Data can be sent to the server using the
output stream provided by
getOutputStream().
© 1997 Elliotte Rusty Harold
5/12/2017
For example,
try {
URL u = new URL("http://www.sd98.com/");
URLConnection uc = u.openConnection();
uc.connect();
InputStream in = uc.getInputStream();
// read the data...
}
catch (IOException e) { //...
© 1997 Elliotte Rusty Harold
5/12/2017
Reading Header Data
• The getHeaderField(String
name) method returns the string value of a
named header field.
• Names are case-insensitive.
• If the requested field is not present, null is
returned.
String lm = uc.getHeaderField("Lastmodified");
© 1997 Elliotte Rusty Harold
5/12/2017
getHeaderFieldKey()
• The keys of the header fields are returned by the
getHeaderFieldKey(int n) method.
• The first field is 1.
• If a numbered key is not found, null is
returned.
• You can use this in combination with
getHeaderField() to loop through the
complete header
© 1997 Elliotte Rusty Harold
5/12/2017
For example
String key = null;
for (int i=1; (key =
uc.getHeaderFieldKey(i))!=null); i++) {
System.out.println(key + ": " +
uc.getHeaderField(key));
}
© 1997 Elliotte Rusty Harold
5/12/2017
getHeaderFieldInt() and
getHeaderFieldDate()
• These are utility methods that read a named
header and convert its value into an int
and a long respectively.
public int getHeaderFieldInt(String name,
int default)
public long getHeaderFieldDate(String name,
long default)
© 1997 Elliotte Rusty Harold
5/12/2017
• The long returned by
getHeaderFieldDate() can be
converted into a Date object using a
Date() constructor like this:
String s = uc.getHeaderFieldDate("Lastmodified", 0);
Date lm = new Date(s);
© 1997 Elliotte Rusty Harold
5/12/2017
Six Convenience Methods
• These return the values of six particularly
common header fields:
public
public
public
public
public
public
© 1997 Elliotte Rusty Harold
int getContentLength()
String getContentType()
String getContentEncoding()
long getExpiration()
long getDate()
long getLastModified()
5/12/2017
try {
URL u = new URL(“http://www.sd98.com/”);
URLConnection uc = u.openConnection();
uc.connect();
String key=null;
for (int n = 1; (key =
uc.getHeaderFieldKey(n)) != null; n++) {
System.out.println(key + ": " +
uc.getHeaderField(key));
}
}
catch (IOException e) {
System.err.println(e);
© 1997 Elliotte Rusty Harold
5/12/2017
Writing data to a URLConnection
• Similar to reading data from a URLConnection.
• First inform the URLConnection that you plan to
use it for output
• Before getting the connection's input stream, get
the connection's output stream and write to it.
• Commonly used to talk to CGIs that use the
POST method
© 1997 Elliotte Rusty Harold
5/12/2017
Nine Steps:
• Construct the URL.
• Call the URL’s openConnection() method to
create the URLConnection object.
• Pass true to the URLConnection’s
setDoOutput() method
• Invoke setDoInput(true) to indicate that this
URLConnection will also be used for input.
• Create the data you want to send, preferably
as a byte array.
© 1997 Elliotte Rusty Harold
5/12/2017
• Call getOutputStream() to get an output
stream object.
• Write the byte array calculated in step 5
onto the stream.
• Close the output stream.
• Call getInputStream() to get an input stream
object. Read and write it as usual.
© 1997 Elliotte Rusty Harold
5/12/2017
POST CGIs
• A typical POST request to a CGI looks like
this:
POST /cgi-bin/booksearch.pl HTTP/1.0
Referer:
http://www.macfaq.com/sampleform.html
User-Agent: Mozilla/3.01 (Macintosh; I;
PPC)
Content-length: 60
Content-type: text/x-www-form-urlencoded
Host: utopia.poly.edu:56435
username=Sadie%2C+Julie&realname=Women+Comp
osers
© 1997 Elliotte Rusty Harold
5/12/2017
A POST request includes
• the POST line
• a MIME header which must include
– content type
– content length
• a blank line that signals the end of the
MIME header
• the actual data of the form, encoded in xwww-form-urlencoded format.
© 1997 Elliotte Rusty Harold
5/12/2017
• A URLConnection for an http URL will set
up the request line and the MIME header for
you as long as you set its doOutput field to
true by invoking setDoOutput(true).
• If you also want to read from the
connection, you should set doInput to true
with setDoInput(true) too.
© 1997 Elliotte Rusty Harold
5/12/2017
For example,
URLConnection uc =
u.openConnection();
uc.setDoOutput(true);
uc.setDoInput(true);
© 1997 Elliotte Rusty Harold
5/12/2017
• The request line and MIME header are sent
as soon as the URLConnection connects.
Then use getOutputStream() to get an
output stream on which you'll write the xwww-form-urlencoded name-value pairs.
© 1997 Elliotte Rusty Harold
5/12/2017
HttpURLConnection
• java.net.HttpURLConnection is an abstract
subclass of URLConnection that provides
some additional methods specific to the
HTTP protocol.
• URL connection objects that are returned by
an http URL will be instances of
java.net.HttpURLConnection.
© 1997 Elliotte Rusty Harold
5/12/2017
Recall
• a typical HTTP response from a web server
begins like this:
HTTP/1.0 200 OK
Server: Netscape-Enterprise/2.01
Date: Sat, 02 Aug 1997 07:52:46 GMT
Accept-ranges: bytes
Last-modified: Tue, 29 Jul 1997 15:06:46
GMT
Content-length: 2810
Content-type: text/html
© 1997 Elliotte Rusty Harold
5/12/2017
Response Codes
• The getHeaderField() and
getHeaderFieldKey() don't return the HTTP
response code
• After you've connected, you can retrieve the
numeric response code--200 in the above
example--with the getResponseCode()
method and the message associated with it-OK in the above example--with the
getResponseMessage() method.
© 1997 Elliotte Rusty Harold
5/12/2017
• Java 1.0 only supported GET and POST
requests to HTTP servers, but Java 1.1
allows the much broader range of requests
specified in the HTTP/1.1 specification
including GET, POST, HEAD, OPTIONS,
PUT, DELETE, and TRACE.
• These are set with the void
setRequestMethod(String method) method.
• This method throws a
java.net.ProtocolException, a subclass of
IOException, if an unknown protocol is
specified.
© 1997 Elliotte Rusty Harold
5/12/2017
getRequestMethod()
• The getRequestMethod() method returns the
string form of the request method currently
set for the URLConnection. GET is the
default method.
© 1997 Elliotte Rusty Harold
5/12/2017
disconnect()
• The void disconnect() method of the
HttpURLConnection class allows you to
close the connection to the web server.
• Needed for HTTP/1.1 Keep-alive
© 1997 Elliotte Rusty Harold
5/12/2017
For example,
try {
URL u = new
URL("http://www.amnesty.org/");
HttpURLConnection huc =
(HttpURLConnection) u.openConnection();
huc.setRequestMethod("PUT");
OutputStream os = huc.getOutputStream();
int code = huc.getResponseCode();
if (code >= 200 && < 300) {
// put the data...
}
huc.disconnect();
}
catch (IOException e) { //...
© 1997 Elliotte Rusty Harold
5/12/2017
usingProxy
• The boolean usingProxy() method returns
true if web connections are being funneled
through a proxy server, false if they're not.
© 1997 Elliotte Rusty Harold
5/12/2017
• The HttpURLConnection class also has two
static methods that affect how all
URLConnection objects interact with web
servers. With a true argument, the
HttpURLConnection.setFollowRedirects(bo
olean followRedirects) method says that
connections will follow redirect instructions
from the web server. Untrusted applets are
not allowed to set this. The boolean method
HttpURLConnection.getFollowRedirects()
returns true if redirect requests are honored,
false if they're not.
© 1997 Elliotte Rusty Harold
5/12/2017
Redirect Instructions
• Most web servers can be configured to
automatically redirect browsers to the new
location of a page that's moved.
• To redirect browsers, a server sends a 300
level response and a Location header that
specifies the new location of the requested
page.
© 1997 Elliotte Rusty Harold
5/12/2017
GET /~elharo/macfaq/index.html HTTP/1.0
HTTP/1.1 302 Moved Temporarily
Date: Mon, 04 Aug 1997 14:21:27 GMT
Server: Apache/1.2b7
Location:
http://www.macfaq.com/macfaq/index.html
Connection: close
Content-type: text/html
<HTML><HEAD>
<TITLE>302 Moved Temporarily</TITLE>
</HEAD><BODY>
<H1>Moved Temporarily</H1>
The document has moved <A
HREF="http://www.macfaq.com/macfaq/index.htm
l">here</A>.<P>
</BODY></HTML>
© 1997 Elliotte Rusty Harold
5/12/2017
• HTML is returned for browsers that don't
understand redirects, but most modern
browsers do not display this and jump
straight to the page specified in the Location
header instead.
• Because redirects can change the site which
a user is connecting without their
knowledge so redirects are not arbitrarily
followed by URLConnections.
© 1997 Elliotte Rusty Harold
5/12/2017
To Learn More
• Java Network Programming
– O’Reilly & Associates, 1997
– ISBN 1-56592-227-1
• Web Client Programming with Java
– http://www.digitalthink.com/catalog/cs/
cs308/index.html
© 1997 Elliotte Rusty Harold
5/12/2017