Clients and Servers 13-Sep-19

URL review A URL has the syntax: protocol://hostname:port/path#anchor import java.net.*; This is the package that defines sockets, URLs, etc. URL url = new URL(String); Constructs a URL object from a text string MalformedURLException 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)

HTTP review 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

Using a URL URLConnection c = url.openConnection(); The URLConnection is the basic way to access the resource information c.getHeaderField(name) 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) Returns the value of the int-th header field (as a String) The 0-th header field is the status line c.getInputStream() Returns an InputStream containing the “content” of the resource url.openStream() is shorthand for url.openConnection().getInputStream()

Socket review A socket is a low-level software device for connecting two programs (possibly on different computers) together new Socket(String host, int port) Creates a client socket and makes the connection Methods include getInputStream(), getOutputStream(), and close() new ServerSocket(int port) Creates a server socket that listens on the specified port accept() returns a Socket that can be used for I/O accept() is a blocking method, so multithreading is highly desirable

How to write a server ServerSocket server = new ServerSocket(port) The port should be a number above 1024 Socket client = server.accept(); accept() blocks while it waits for a connection InputStream inStream = client.getInputStream(); InputStreamReader reader = new InputStreamReader(inStream); BufferedReader input = new BufferedReader(reader); char ch = input.read(), String s = input.readLine() OutputStream outStream = client.getOutputStream(); PrintWriter output = new PrintWriter(outStream, true); 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()

How to write a client Socket server = new Socket(ip_address, port) The ip_address can be the String "localhost" This method makes the actual connection InputStream inStream = server.getInputStream(); As on the previous slide OutputStream outStream = server.getOutputStream(); input.close(), output.close(), server.close()

How to write an HTTP server An HTTP server is just a server that follows the HTTP protocol (request/status line, header, blank line, body) Since HTTP is a text-based protocol, compliance is easy There are two versions of HTTP: 1.0 and 1.1 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 Most HTTP servers can use both

Proxy servers Proxies are important because they allow more than one server to use the same IP address 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 proxy has IP address server With a proxy client server has IP address Without a proxy

Multithreading server.accept() is a blocking call--Java stops and waits for a response before it continues 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: Write a class that extends Thread Override the public void run() method Create an instance of your class and call its (inherited) start() method Write a class that implements Runnable 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

Synchronization While an object is being modified by one thread, no other thread should try to access it This leads to unpredictable (and difficult to debug) results You can synchronize an object: 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: 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) It can be very difficult to make a program both safe and efficient

A synchronization analogy Imagine that you have a building with two entrances One entrance is always kept locked, and has a single key The other entrance is never locked Synchronization is like this 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!

The End