Clients and Servers
26-Jul-16
URL review
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A URL has the syntax:
 protocol://hostname:port/path#anchor
import java.net.*;
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URL url = new URL(String);
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Constructs a URL object from a text string
MalformedURLException
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This is the package that defines sockets, URLs, etc.
This exception is thrown if the given String cannot be parsed by
newURL(String)
We have used URLs to display a page in an applet:
 appletContext.showUrl(URL)
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HTTP review
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HTTP is a protocol--a formal description of a language
that computers use to communicate
An HTTP message consists of three parts:
– The request or the response line
– A request line typically contains either GET or PUT
– A response line contains the status code, such as 404 Not Found
– A header section
– Contains name-value pairs, such as Content-type: text/html
– Ends with a blank line
– The body of the message
– The body is optional
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Using a URL
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URLConnection c = url.openConnection();
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c.getHeaderField(name)
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Returns the value of the named header field (as a String)
Frequently used fields have shorthand methods, for example,
c.getLastModified() = c.getHeaderField("last-modified")
getHeaderField(int)
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The URLConnection is the basic way to access the resource information
Returns the value of the int-th header field (as a String)
The 0-th header field is the status line
c.getInputStream()
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Returns an InputStream containing the “content” of the resource
url.openStream() is shorthand for
url.openConnection().getInputStream()
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Socket review
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A socket is a low-level software device for connecting
two programs (possibly on different computers)
together
new Socket(String host, int port)
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Creates a client socket and makes the connection
Methods include getInputStream(), getOutputStream(),
and close()
new ServerSocket(int port)
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Creates a server socket that listens on the specified port
accept() returns a Socket that can be used for I/O
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accept() is a blocking method, so multithreading is highly desirable
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How to write a server
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ServerSocket server = new ServerSocket(port)
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Socket client = server.accept();
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accept() blocks while it waits for a connection
InputStream inStream = client.getInputStream();
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The port should be a number above 1024
InputStreamReader reader = new InputStreamReader(inStream);
BufferedReader input = new BufferedReader(reader);
char ch = input.read(), String s = input.readLine()
OutputStream outStream = client.getOutputStream();
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PrintWriter output = new PrintWriter(outStream, true);
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true is so that you auto-flush, that is, don’t fill the buffer
output .print(X), output .println(X), output .println()
input.close(), output.close(), server.close(), client.close()
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How to write a client
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Socket server = new Socket(ip_address, port)
 The ip_address can be the String "localhost"
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InputStream inStream = server.getInputStream();
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As on the previous slide
OutputStream outStream = server.getOutputStream();
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This method makes the actual connection
As on the previous slide
input.close(), output.close(), server.close()
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As on the previous slide
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How to write an HTTP server
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An HTTP server is just a server that follows the HTTP
protocol (request/status line, header, blank line, body)
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There are two versions of HTTP: 1.0 and 1.1
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Since HTTP is a text-based protocol, compliance is easy
HTTP 1.0 is simpler and should be used if the special features
of 1.1 are not required
The most important change in HTTP 1.1 is that it can
accommodate proxy servers
The client and server must agree which version of
HTTP is being used
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Most HTTP servers can use both
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Proxy servers
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Proxies are important because they allow more than one
server to use the same IP address
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There aren’t enough IP addresses to go around
If you have a lot of clients, you need a lot of servers--but the
user should not have to try multiple IP addresses
client
server
has IP address
client
client
client
client
Without a proxy
client
client
proxy
has IP address
server
server
With a proxy
client
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Multithreading
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server.accept() is a blocking call--Java stops and waits for a
response before it continues
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This is only acceptable if the server never has more than one client
A server needs to have a separate thread for each client
There are two ways to create a Thread:
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Write a class that extends Thread
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Override the public void run() method
Create an instance of your class and call its (inherited) start() method
Write a class that implements Runnable
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Implement the public void run() method
Create an instance of your class
Create a Thread object with this instance as a parameter to the constructor
Call the Thread object’s start() method
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Synchronization
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While an object is being modified by one thread, no other thread
should try to access it
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You can synchronize an object:
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synchronized (obj) { code that uses/modifies obj }
synchronized is a statement type, like if or while
No other code that is synchronized on this object can use or modify the
object at the same time
You can synchronize a method:
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This leads to unpredictable (and difficult to debug) results
synchronized void addOne(arg1, arg2, ...) { code }
synchronized is a method modifier, like public or abstract
Only one synchronized method in a class can be used at a time (but this
doesn’t restrict other, non-synchronized methods)
Synchronization can really hurt efficiency (and response time)
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It can be very difficult to make a program both safe and efficient
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A synchronization analogy
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Imagine that you have a building with two entrances
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Synchronization is like this
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One entrance is always kept locked, and has a single key
The other entrance is never locked
Code that always uses the locked (synchronized) entrance has
to wait for other code to exit and hand over the key
Any code that uses the unlocked entrance can go into the
building at any time, regardless of what other code may be
there
Thus, synchronizing code only protects you from other
synchronized code!
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The End
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