Java Network Programming, Part 1: URLs, URLConnections, and InetAddresses

Java Network Programming, Part 1: URLs, URLConnections, and InetAddresses

Elliotte Rusty Harold

elharo@metalab.unc.edu

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

Please turn off all

We will learn how Java handles

I assume you

Applet Network Security Restrictions

Some Background

Hosts

Internet addresses

Domain Name System (DNS)

The InetAddress Class

Creating InetAddresses

The getByName() factory method

Other ways to create InetAddress objects

Getter Methods

Utility Methods

Ports

Protocols

IETF RFCs

W3C Standards

W3C Standards

URLs

Example URLs

The Pieces of a URL

The java.net.URL class

Content and Protocol Handlers

Finding Protocol Handlers

Supported Protocols

URL Constructors

Constructing URL Objects

try {
  URL u = new URL("http://www.poly.edu/fall97/grad.html#cs");
}
catch (MalformedURLException e) {}

Constructing URL Objects in Pieces

Including the Port

try {
  URL u = new URL("http",
   "www.poly.edu", 8000, "/fall97/grad.html#cs");
   // work with the URL
}
catch (MalformedURLException e) {}

Relative URLs

Constructing Relative URLs

Parsing URLs

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

Missing Pieces

Reading Data from a URL

Webcat

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

The Bug in readLine()

Webcat

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 = new BufferedInputStream(u.openStream());
       InputStreamReader isr = new InputStreamReader(in, "8859_1");
       
       char c;
       while ((c = isr.read()) != -1) {
        System.out.write(c);
       }

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

 }

}

CGI

Normal web surfing uses these two steps:

Forms

CGI

GET and POST

HTTP

A Typical HTTP Connection

  1. Client opens a socket to port 80 on the server.

  2. Client sends a GET request including the name and path of the file it wants and the version of the HTTP protocol it supports.

  3. The client sends a MIME header.

  4. The client sends a blank line.

  5. The server sends a MIME header

  6. The server sends the data in the file.

  7. The server closes the connection.

What the client sends to the server

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

MIME

Browser Request MIME Header

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, */*

Server Response MIME Header

Query Strings

URL Encoding

For example,

The URLEncoder class

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

    Example

    String eqs = "Author=" + URLEncoder.encode("Sadie,
Julie");
eqs += "&";
eqs += "Title=";
eqs += URLEncoder.encode("Women Composers");

    The URLDecoder class

    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) { //...

    URLConnections

    URLConnections vs. URLs

    URLConnection six 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().

    6. The data is read from an InputStream

    I/O Across a URLConnection

    For example, 

    try { 
  URL u = new URL("http://www.sdexpo.com/");
  URLConnection uc = u.openConnection();
  uc.connect(); 
  InputStream in = uc.getInputStream();
  // read the data...
}
catch (IOException e) { //...

    Reading Header Data

    getHeaderFieldKey()

    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()

    Example

    Six Convenience Methods

    Example

    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

    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 POST request includes

    Example

    URLConnection uc = u.openConnection();
uc.setDoOutput(true);
uc.setDoInput(true);

    Details

    HttpURLConnection

    Recall

    Response Codes

    HTTP Protocols

    getRequestMethod()

    disconnect()

    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 && port < 300) { 
   // put the data...
  }
  huc.disconnect();
}
catch (IOException e) { //...

    usingProxy

    Redirect Instructions

    Example

    
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.html">here</A>.<P>
</BODY></HTML>

    Example

    Following Redirects

    To Learn More

    Java Network Programming cover

    Questions?

    Index | Cafe con Leche
    Copyright 2000 Elliotte Rusty Harold
    elharo@metalab.unc.edu
    Last Modified November 8, 2000