Clients and Servers Jan 14 2019 URL review A URL has the syntax protocol hostname port path anchor import java net URL url new URL String Constructs a URL object from a text string MalformedURLException 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 2 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 header section A request line typically contains either GET or PUT A response line contains the status code such as 404 Not Found Contains name value pairs such as Content type text html Ends with a blank line The body of the message The body is optional 3 Using a URL URLConnection c url openConnection 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 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 Returns an InputStream containing the content of the resource url openStream is shorthand for url openConnection getInputStream 4 Sockets 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 5 How to write a server ServerSocket server new ServerSocket port Socket client server accept accept blocks while it waits for a connection InputStream inStream client getInputStream 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 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 6 How to write a client Socket server new Socket ip address port The ip address can be the String localhost InputStream inStream server getInputStream As on the previous slide OutputStream outStream server getOutputStream This method makes the actual connection As on the previous slide input close output close server close As on the previous slide 7 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 There are two versions of HTTP 1 0 and 1 1 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 Most HTTP servers can use both 8 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 client client server has IP address client client Without a proxy client client proxy has IP address server server With a proxy client 9 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 10 Synchronization While an object is being modified by one thread no other thread should try to access it 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 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 It can be very difficult to make a program both safe and efficient 11 A synchronization analogy Imagine that you have a building with two entrances Synchronization is like this 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 12 The End 13
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