© 1999 Elliotte Rusty Harold8/16/2015 URLs, InetAddresses, and URLConnections High Level Network...

© 1999 Elliotte Rusty Harold 04/19/23

URLs, InetAddresses, and URLConnections

High Level Network Programming

Elliotte Rusty Harold

[email protected]

http://metalab.unc.edu/javafaq/slides/


We will learn how Java handles

• Internet Addresses

• URLs

• CGI

• URLConnection

• Content and Protocol handlers


I assume you

• Understand basic Java syntax and I/O

• Have a user’s view of the Internet

• No prior network programming experience


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


Some Background

• Hosts

• Internet Addresses

• Ports

• Protocols


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.


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


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


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


Creating InetAddresses

• There are no public InetAddress() constructors. Arbitrary addresses may not be created.

• All addresses that are created must be checked with DNS


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);}


Other ways to create InetAddress objects

public static InetAddress[] getAllByName(String host) throws UnknownHostException

public static InetAddress getLocalHost() throws UnknownHostException


Getter Methods

• public boolean isMulticastAddress()• public String getHostName()• public byte[] getAddress()• public String getHostAddress()


Utility Methods

• public int hashCode()• public boolean equals(Object o)• public String toString()


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


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


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://www.faqs.org/rfc/


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


W3C Standards

• HTTP

• HTML

• XML

• RDF

• MathML

• SMIL

• P3P


URLs

• A URL, short for "Uniform Resource Locator", is a way to unambiguously identify the location of a resource on the Internet.


Example URLs

http://java.sun.com/file:///Macintosh%20HD/Java/Docs/JDK%201.1.1%20docs/api/

java.net.InetAddress.html#_top_http://www.macintouch.com:80/newsrecent.shtmlftp://ftp.info.apple.com/pub/mailto:[email protected]://utopia.poly.eduftp://mp3:[email protected]:21000/c%3a/stuff/mp3/

http://[email protected]/ http://metalab.unc.edu/nywc/comps.phtml?

category=Choral+Works


The Pieces of a URL

• the protocol, aka scheme

• the authority– user info

user namepassword

– host name or address– port

• the path, aka file

• the ref, aka section or anchor

• the query string


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


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.


Finding Protocol Handlers

• When the virtual machine creates a URL object, it 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.


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:– file – ftp – gopher – http – mailto

–appletresource –doc –netdoc –systemresource –verbatim


URL Constructors

• There are four (six in 1.2) constructors in the java.net.URL class.

public URL(String u) throws MalformedURLExceptionpublic 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 url) throws MalformedURLException

public URL(String protocol, String host, int port, String file, URLStreamHandler handler) throws MalformedURLException

public URL(URL context, String url, URLStreamHandler handler) throws MalformedURLException


Constructing URL Objects

• An absolute URL like http://www.poly.edu/fall97/grad.html#cstry { URL u = newURL("http://www.poly.edu/fall97/grad.html#cs");

}catch (MalformedURLException e) {}


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) {}


Including the Port

URL u = null;try { u = new URL("http", "www.poly.edu", 8000, "/fall97/grad.html#cs");

} catch (MalformedURLException e) {}


Relative URLs

• Many HTML files contain relative URLs.

• Consider the page http://metalab.unc.edu/javafaq/index.html

• On this page a link to “books.html" refers to http://metalab.unc.edu/javafaq/books.html.


Constructing Relative URLs

• The fourth constructor creates URLs relative to a given URL. For example,try { URL u1 = new URL("http://metalab.unc.edu/index.html");

URL u2 = new URL(u1, ”books.html");}catch (MalformedURLException e) {}

• This is particularly useful when parsing HTML.


Parsing URLs

• The java.net.URL class has five methods to split a URL into its component parts. These are: public String getProtocol() public String getHost() public int getPort() public String getFile() public String getRef()


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) { }


Parsing URLs

• JDK 1.3 adds three more: public String getAuthority() public String getUserInfo() public String getQuery()


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://java.sun.com, then it's set to "/".


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 BufferedReader or a BufferedInputStream.


Webcatimport 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 (IOException e) { System.err.println(e);} } }}


The Bug in readLine()

• What readLine() does:– Sees a carriage return, waits to see if next

character is a line feed before returning

• What readLine() should do:– Sees a carriage return, return, throw away

next character if it's a linefeed


Webcatimport 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); char c; while ((c = br.read()) != -1) { System.out.print(c); } } catch (IOException e) { System.err.println(e);} } }}


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


Normal web surfing uses these two steps:

– The browser requests a page– The server sends the page

• Data flows primarily from the server to the client.


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:


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.


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


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


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.


What the client sends to the server

GET /javafaq/images/cup.gifConnection: Keep-AliveUser-Agent: Mozilla/3.01 (Macintosh; I; PPC)Host: www.oreilly.com:80Accept: image/gif, image/x-xbitmap, image/jpeg, */*


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.


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-AliveUser-Agent: Mozilla/3.01 (Macintosh; I; PPC)Host: www.digitalthink.com:80Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*


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/2.01Date: Sat, 02 Aug 1997 07:52:46 GMTAccept-ranges: bytesLast-modified: Tue, 29 Jul 1997 15:06:46 GMT

Content-length: 2810Content-type: text/html


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


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.


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


The URLEncoder class

• The java.net.URLEncoder class contains a single static method which encodes strings in x-www-form-url-encoded formatURLEncoder.encode(String s)


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+Composers


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


The URLDecoder class

• In Java 1.2 the java.net.URLDecoder class contains a single static method which decodes strings in x-www-form-url-encoded formatURLEncoder.decode(String s)


GET URLsString 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) { //...


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.


URLConnections vs. URLs

• Can send output as well as read input

• Can post data to CGIs

• Can read headers from a connection


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. (optional)

5. The response header information is read using getHeaderField().


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().


For example,

try { URL u = new URL("http://www.sd99.com/");

URLConnection uc = u.openConnection();

uc.connect(); InputStream in = uc.getInputStream();

// read the data... } catch (IOException e) { //...


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("Last-modified");


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


For example

String key = null;for (int i=1; (key = uc.getHeaderFieldKey(i))!=null); i++) {

System.out.println(key + ": " + uc.getHeaderField(key));

}


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)


• The long returned by getHeaderFieldDate() can be converted into a Date object using a Date() constructor like this:

String s = uc.getHeaderFieldDate("Last-modified", 0);

Date lm = new Date(s);


Six Convenience Methods

• These return the values of six particularly common header fields:

public int getContentLength()public String getContentType()public String getContentEncoding()public long getExpiration()public long getDate()public long getLastModified()


try { URL u = new URL("http://www.sdexpo.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);}


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


Eight Steps:1.Construct the URL.

2.Call the URL’s openConnection() method to create the URLConnection object.

3.Pass true to the URLConnection’s setDoOutput() method

4.Create the data you want to send, preferably as a byte array.


5.Call getOutputStream() to get an output stream object.

6.Write the byte array calculated in step 5 onto the stream.

7.Close the output stream.

8.Call getInputStream() to get an input stream object. Read from it as usual.


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: 60Content-type: text/x-www-form-urlencodedHost: utopia.poly.edu:56435

username=Sadie%2C+Julie&realname=Women+Composers


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 x-www-form-urlencoded format.


• 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.


For example,

URLConnection uc = u.openConnection();

uc.setDoOutput(true);uc.setDoInput(true);


• The request line and MIME header are sent as soon as the URLConnection connects. Then getOutputStream() returns an output stream on which you can write the x-www-form-urlencoded name-value pairs.


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.


Recall

• a typical HTTP response from a web server begins like this:

HTTP/1.0 200 OKServer: Netscape-Enterprise/2.01Date: Sat, 02 Aug 1997 07:52:46 GMTAccept-ranges: bytesLast-modified: Tue, 29 Jul 1997 15:06:46 GMT

Content-length: 2810Content-type: text/html


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.


HTTP Protocols• Java 1.0 only supports GET and POST

requests to HTTP servers

• Java 1.1/1.2 supports GET, POST, HEAD, OPTIONS, PUT, DELETE, and TRACE.

• The protocol is chosen with the setRequestMethod(String method) method.

• A java.net.ProtocolException, a subclass of IOException, is thrown if an unknown protocol is specified.


getRequestMethod()

• The getRequestMethod() method returns the string form of the request method currently set for the URLConnection. GET is the default method.


disconnect()

• The disconnect() method of the HttpURLConnection class closes the connection to the web server.

• Needed for HTTP/1.1 Keep-alive


For example,try { URL u = new URL("http://www.amnesty.org/");

HttpURLConnection huc = (HttpURLConnection) u.openConnection();

huc.setRequestMethod("PUT"); huc.connect(); OutputStream os = huc.getOutputStream(); int code = huc.getResponseCode(); if (code >= 200 && < 300) { // put the data... } huc.disconnect();}catch (IOException e) { //...


usingProxy

• The boolean usingProxy() method returns true if web connections are being funneled through a proxy server, false if they're not.


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.


GET /~elharo/macfaq/index.html HTTP/1.0

HTTP/1.1 302 Moved TemporarilyDate: Mon, 04 Aug 1997 14:21:27 GMTServer: Apache/1.2b7Location: http://www.macfaq.com/macfaq/index.htmlConnection: closeContent-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.html">here</A>.<P>

</BODY></HTML>


• 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.


Following RedirectsHttpURLConnection.setFollowRedirects(true) method says that connections will follow redirect instructions from the web server. Untrusted applets are not allowed to set this.

HttpURLConnection.getFollowRedirects() returns true if redirect requests are honored, false if they're not.


To Learn More

• Java Network Programming– O’Reilly & Associates, 1997– ISBN 1-56592-227-1

• Java I/O– O’Reilly & Associates, 1999– ISBN 1-56592-485-1

• Web Client Programming with Java– http://www.digitalthink.com/catalog/cs/

cs308/index.html


Questions?

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© 1999 Elliotte Rusty Harold8/16/2015 URLs, InetAddresses, and URLConnections High Level Network...

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