Java for Interfaces and Networks

Transcription

1 Java for Interfaces and Networks Threads and Networking Federico Pecora School of Science and Technology Örebro University Federico Pecora Java for Interfaces and Networks Lecture 4 1 / 25

2 Outline 1 2 Introduction to Networking in Java Federico Pecora Java for Interfaces and Networks Lecture 4 2 / 25

3 Outline 1 2 Introduction to Networking in Java Federico Pecora Java for Interfaces and Networks Lecture 4 3 / 25

4 Threads and processes Processes: several executing program instances (multi-tasking) Threads: several paths of execution within the same process (multi-threading) Processes maintain considerable state information, have separate address spaces, and interact only through IPC (inter-process communication) mechanisms Multiple threads typically share the state information of a process and share memory and other resources directly Context switching between threads in the same process is typically faster than context switching between processes Federico Pecora Java for Interfaces and Networks Lecture 4 4 / 25

5 : simple example BoringThread.java 1 public class BoringThread extends Thread { 2 public void run() { 3 while (true) {} 4 } //run 5 public static void main(string args) { 6 BoringThread t1 = new BoringThread(); 7 t1.start(); 8 BoringThread t2 = new BoringThread(); 9 t2.start(); 10 } //main 11 } //class BoringThread Federico Pecora Java for Interfaces and Networks Lecture 4 5 / 25

6 : simple example Let the class that should be a thread inherit from Thread Define what should happen when the thread runs in the run() method Invoke the start() method in order to start the thread (this results in calling run()) Whe run() returns, the thread dies Federico Pecora Java for Interfaces and Networks Lecture 4 5 / 25

7 : a more interesting example BoringThread.java 1 public class BoringThread2 extends Thread { 2 static int numberofthreads = 0; 3 int threadnumber = ++numberofthreads; 4 public void run() { 5 int nrreps = 0; 6 while (true) { 7 System.out.println("Thread " + threadnumber + 8 " has run " + ++nrreps + " iterations."); 9 } 10 } //run 11 public static void main(string args) { 12 BoringThread2 t1 = new BoringThread2(); 13 BoringThread2 t2 = new BoringThread2(); 14 t1.start(); 15 t2.start(); 16 } //main 17 } //class BoringThread2 Federico Pecora Java for Interfaces and Networks Lecture 4 6 / 25

8 : a more interesting example linux> java BoringThread2 Thread 1 has run 1 iterations. Thread 1 has run 2 iterations. Thread 1 has run 3 iterations.... Thread 2 has run 1155 iterations. Thread 2 has run 1156 iterations. Thread 1 has run 1173 iterations. Thread 2 has run 1157 iterations. Thread 2 has run 1158 iterations. Thread 1 has run 1174 iterations. Thread 1 has run 1175 iterations.... Federico Pecora Java for Interfaces and Networks Lecture 4 6 / 25

9 Thread scheduling Do not design your threads under the assumption that they will run in any particular order if that is desired, you must synchronize them Uses of threads performance gain if multiple processors are available facilitating communication between logical elements of your programs (e.g., servers, users, multiple clients) doing several things at once (e.g., running the GUI while waiting for data) You need to think about how threads are scheduled only when at least one of your threads heavily uses the processor for long time periods intermediate results of that thread s execution are important Federico Pecora Java for Interfaces and Networks Lecture 4 7 / 25

10 Thread scheduling Federico Pecora Java for Interfaces and Networks Lecture 4 7 / 25

11 Thread scheduling The thread scheduler decides when a thread gets to run Green thread scheduling, for OSs which do not support threads (e.g., Windows 3.1) the JVM supplies the threading logic, based on thread priorities threads are called user threads Native thread scheduling, e.g., Linux, Windows XP the OS supplies the threading logic, based on thread priorities and (often) time-slicing threads are called native threads Federico Pecora Java for Interfaces and Networks Lecture 4 7 / 25

12 Thread synchronization: motivation Let s see an example in which thread synchronization is important An application that repeatedly writes a string to the console every write will be presented further ahead on the line astring astring astring astring astring three threads, one writing every 100ms, one every 200ms and one every 300ms Federico Pecora Java for Interfaces and Networks Lecture 4 8 / 25

13 Thread synchronization: motivation Presenter.java 1 public class Presenter extends Thread { 2 private String message; private double sleeptime; 3 private int steps; 4 public Presenter(String message, double sleeptime) { 5 this.message = message; 6 this.sleeptime = sleeptime; 7 this.steps = 0; 8 this.start(); 9 } 10 public void run() { 11 while (true) { 12 for (int i = 0; i < steps; ++i) System.out.print(" "); 13 ++this.steps; 14 System.out.println(this.message); 15 try { Thread.sleep((int)(this.sleepTime * 1000)); } 16 catch (InterruptedException e) { 17 System.out.println("Oh! Someone woke me up!"); 18 } } } //catch, while, run 19 //continues... Federico Pecora Java for Interfaces and Networks Lecture 4 8 / 25

14 Thread synchronization: motivation Presenter.java (cont.) 20 public static void main(string[] args) { 21 Presenter t1 = new Presenter("FirstThread", 0.1); 22 Presenter t2 = new Presenter("SecondThread", 0.2); 23 Presenter t3 = new Presenter("ThirdThread", 0.3); 24 } //main 25 } //class Presenter linux> java Presenter FirstThread SecondThread ThirdThread FirstThread SecondThread FirstThread ThirdThread FirstThread SecondThread FirstThread FirstThread ThirdThread SecondThread FirstThread FirstThread SecondThread FirstThread ThirdThread FirstThread SecondThread FirstThread FirstThread ThirdThread SecondThread FirstThread FirstThread SecondThread Federico Pecora Java for Interfaces and Networks Lecture 4 8 / 25

15 Thread synchronization: motivation It is possible that the thread printouts do not occur in the order they are expected to linux> java Presenter... FirstThread... ThirdThread SecondThread SecondThread FirstThread FirstThread FirstThread SecondThread This is because the threads are not synchronized Federico Pecora Java for Interfaces and Networks Lecture 4 8 / 25

16 Thread synchronization: motivation Presenter.java (frag.) 1 String syncobject = "foo"; 2 public void run() { 3 while (true) { 4 synchronized(syncobject) { 5 for (int i = 0; i < steps; ++i) System.out.print(" "); 6 ++this.steps; 7 System.out.println(this.message); 8 } //synchronized 9 try { Thread.sleep((int)(this.sleepTime * 1000)); } 10 catch (InterruptedException e) { 11 System.out.println("Oh! Someone woke me up!"); 12 } //catch 13 } //while 14 } //run Every object has an intrinsic lock associated with it Critical sections can be defined over shared objects Federico Pecora Java for Interfaces and Networks Lecture 4 8 / 25

17 Extending Thread vs. implementing Runnable Java provides two ways of implementing threaded applications by extending the Thread class by implementing the Runnable interface We have seen the former in the previous examples Extending Thread can be a problem when we our class should already inherit from another class A way around the impossiblity to extend multiple classes is to implement interface Runnable Federico Pecora Java for Interfaces and Networks Lecture 4 9 / 25

18 Threads as Runnable classes: example BoringRunnable.java 1 public class BoringRunnable implements Runnable { 2 public void run() { 3 while (true) {} 4 } //run 5 public static void main(string[] args) { 6 BoringRunnable r1 = new BoringRunnable(); 7 Thread t1 = new Thread(r1, "Thread 1"); 8 t1.start(); 9 BoringRunnable r2 = new BoringRunnable(); 10 Thread t2 = new Thread(r2, "Thread 2"); 11 t2.start(); 12 } //main 13 } //class BoringRunnable Federico Pecora Java for Interfaces and Networks Lecture 4 10 / 25

19 Threads as Runnable classes: example Let the class that needs to be a thread implement the Runnable interface Define what should happen when the thread runs in the run() method Instantiate your implementation of Runnable Create Thread objects around the Runnables Invoke the start() method of the created Threads Federico Pecora Java for Interfaces and Networks Lecture 4 10 / 25

20 are part of the language, not external libraries as in C++ The JVM and/or the OS provide thread scheduling Can be defined by extending the class Thread Can be defined by implementing the interface Runnable Thread synchronization obtained by synchronizing against any object with the synchronize keyword Federico Pecora Java for Interfaces and Networks Lecture 4 11 / 25

21 Thread states Initial state: A program has created a thread s Thread object, but the thread object s start() method has not yet been called Runnable state: This is a thread s default state after the call to start() completes, a thread becomes runnable whether or not that thread is using the processor Blocked state: Occurs when a thread executes the sleep(), wait(), or join() methods, when a thread attempts to read data not yet available from a network, and when a thread waits to acquire a lock NB: Thread schedulers determine which runnable thread to assign to the processor Terminating state: Occurs when a thread returns from the run() method Federico Pecora Java for Interfaces and Networks Lecture 4 12 / 25

22 Synchronized One can use synchronized to isolate critical sections, i.e., parts of code that should not be interrupted this can be a block of code that synchronizes over an Object s lock synchronize can be used to indicate that an entire method is a critical section i.e., the Object whose lock is used is this This occurs typically when different threads need to access shared data structures An example: a Buffer contains data a WriterThread puts data into the Buffer a ReaderThread picks data out of the Buffer Federico Pecora Java for Interfaces and Networks Lecture 4 13 / 25

23 Buffer management example Buffer.java 1 class Buffer { 2 private String[] queue; 3 private int number; 4 public Buffer (int size) { 5 queue = new String[size]; 6 number = 0; 7 } //Buffer 8 public synchronized void put(string str) { 9 queue[number++] = str; 10 } //put 11 public synchronized String get() { 12 String s = queue[0]; 13 for (int i = 1; i < number; ++i) 14 queue[i - 1] = queue[i]; 15 --number; 16 return s; 17 } //get 18 } //class Buffer Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

24 Buffer management example WriterThread.java 1 class WriterThread extends Thread { 2 Buffer b; 3 public WriterThread(Buffer b) { 4 this.b = b; 5 } 6 public void run() { 7 int nrreps = 0; 8 while (true) { 9 String s = "String number " + ++nrreps; 10 System.out.println("WriterThread: wrote " + s); 11 b.put(s); 12 } 13 } //run 14 } //class WriterThread Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

25 Buffer management example ReaderThread.java 1 class ReaderThread extends Thread { 2 Buffer b; 3 public ReaderThread(Buffer b) { 4 this.b = b; 5 } 6 public void run() { 7 while (true) { 8 String s = b.get(); 9 System.out.println("ReaderThread: read " + s); 10 } 11 } //run 12 } //class ReaderThread Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

26 Buffer management example BufferExample.java 1 public class BufferExample { 2 public static void main(string[] args) { 3 Buffer b = new Buffer(100); 4 WriterThread w = new WriterThread(b); 5 ReaderThread r = new ReaderThread(b); 6 w.start(); 7 r.start(); 8 } //main 9 } //class BufferExample Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

27 Buffer management example BufferExample.java 1 public class BufferExample { 2 public static void main(string[] args) { 3 Buffer b = new Buffer(100); 4 WriterThread w = new WriterThread(b); 5 ReaderThread r = new ReaderThread(b); 6 w.start(); 7 r.start(); 8 } //main 9 } //class BufferExample Although synchronize ensures that reading and writing never occur at the same time, there remains a problem... Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

28 Buffer management example BufferExample.java 1 public class BufferExample { 2 public static void main(string[] args) { 3 Buffer b = new Buffer(100); 4 WriterThread w = new WriterThread(b); 5 ReaderThread r = new ReaderThread(b); 6 w.start(); 7 r.start(); 8 } //main 9 } //class BufferExample The reader can attempt to read when the buffer is empty, and the writer can attempt to write when the buffer is full Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

29 Buffer management example BufferExample.java 1 public class BufferExample { 2 public static void main(string[] args) { 3 Buffer b = new Buffer(100); 4 WriterThread w = new WriterThread(b); 5 ReaderThread r = new ReaderThread(b); 6 w.start(); 7 r.start(); 8 } //main 9 } //class BufferExample Solution: the reader should wait for the writer to write when the buffer is empty, and the writer should wait for the reader to read when the buffer is full Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

30 Buffer management example Buffer.java (put() method) 1 public synchronized void put(string str) { 2 while(number == queue.length) { 3 try { 4 System.out.println("Buffer.put: Waiting.."); 5 wait(); 6 } 7 catch (InterruptedException e) { 8 System.out.println("Buffer.put: Oh!"); 9 } 10 } 11 queue[number++] = str; 12 notify(); 13 } //put Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

31 Buffer management example Buffer.java (get() method) 1 public synchronized String get() { 2 while(number == 0) { 3 try { 4 System.out.println("Buffer.get: Waiting.."); 5 wait(); 6 } 7 catch (InterruptedException e) { 8 System.out.println("Buffer.get: Oh!"); 9 } 10 } 11 String s = queue[0]; 12 for (int i = 1; i < number; ++i) 13 queue[i - 1] = queue[i]; 14 --number; 15 notify(); 16 return s; 17 } // get Federico Pecora Java for Interfaces and Networks Lecture 4 14 / 25

32 wait and notify A thread forces itself to wait for a prerequisite for continued execution to exist before it continues The waiting thread assumes that some other thread will create that condition and then notify it wait() induces the thread to wait for a notification puts a thread in blocked state notify() notify a waiting thread that it can stop waiting puts the notified thread in runnable state notifyall() notify all waiting threads to go to runnable state Federico Pecora Java for Interfaces and Networks Lecture 4 15 / 25

33 wait and notify Observe the following race condition 1 thread A tests a condition and discovers it must wait 2 thread B sets the condition and calls notify() to inform A to resume execution, but since A is not yet waiting, nothing happens 3 thread A waits, by calling wait() 4 because of the prior notify() call, A waits indefinitely Why can the race condition not happen in our buffer management example? Because our threads invoke wait() and notify() from within a synchronized context if you do not do this, you will get an IllegalMonitorStateException! Federico Pecora Java for Interfaces and Networks Lecture 4 16 / 25

34 wait and notify Observe the following race condition 1 thread A tests a condition and discovers it must wait 2 thread B sets the condition and calls notify() to inform A to resume execution, but since A is not yet waiting, nothing happens 3 thread A waits, by calling wait() 4 because of the prior notify() call, A waits indefinitely Why can the race condition not happen in our buffer management example? Because our threads invoke wait() and notify() from within a synchronized context if you do not do this, you will get an IllegalMonitorStateException! Federico Pecora Java for Interfaces and Networks Lecture 4 16 / 25

35 Introduction to Networking in Java Outline 1 2 Introduction to Networking in Java Federico Pecora Java for Interfaces and Networks Lecture 4 17 / 25

36 Introduction to Networking in Java Networking in Java Java provides good support for networking Unlike C/C++, networked applications in Java are not platform dependent Support for applets Support for TCP and UDP through classes in java.net URL, URLConnection, Socket, and ServerSocket classes all use TCP to communicate over the network DatagramPacket, DatagramSocket, and MulticastSocket classes are for use with UDP Provides programmatic access to network parameters (via class java.net.networkinterface) Federico Pecora Java for Interfaces and Networks Lecture 4 18 / 25

37 Introduction to Networking in Java Networking basics Computers communicate to each other using either TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) When you write Java programs that communicate over the network, you are programming at the application layer TCP provides a reliable flow of data at the transport level UDP provides an unreliable flow of data at the transport level (e.g., where the order of data is not important) Server PORT TCP Client Federico Pecora Java for Interfaces and Networks Lecture 4 19 / 25

38 Introduction to Networking in Java Networking basics: TCP and UDP Applications for which order and integrity of data is important (e.g., a browser) are built on TCP e.g., the application level protocols HTTP, FTP, SSH Data packets sent over UDP should not depend on each other e.g., the ping command, a network time server The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer Server PORT TCP Client Federico Pecora Java for Interfaces and Networks Lecture 4 20 / 25

39 Introduction to Networking in Java Networking basics: Ports A server is a process that listens and responds to requests on a specific port Ports are identified by port number (16 bits, that is up to 65535) Ports can also have a name (e.g., smtp = 25, ssh = 22, http = 80) Ports are reserved ( Well Known Ports ) Ports can be reserved for various services ( Registered Ports, e.g., 1433 for Microsoft SQL Server, 6000 for the X11, for Quake) Ports are dynamic or private Server PORT TCP Client Federico Pecora Java for Interfaces and Networks Lecture 4 21 / 25

40 Introduction to Networking in Java A simple example: get-a-web-page applet An applet that provides a button Pressing the button induces the page to load the URL ShowURLApplet.java 1 import java.applet.*; 2 import java.awt.*; 3 import java.awt.event.*; 4 import java.net.*; 5 import javax.swing.*; 6 //continues... Federico Pecora Java for Interfaces and Networks Lecture 4 22 / 25

41 Introduction to Networking in Java A simple example: get-a-web-page applet ShowURLApplet.java (cont.) 7 public class ShowURLApplet extends JApplet 8 implements ActionListener { 9 public void actionperformed(actionevent event) { 10 this.handle_button(); 11 } //actionperformed 12 private void handle_button() { 13 try { 14 URL onionurl = new URL(" 15 getappletcontext().showdocument(onionurl); 16 } //try 17 catch (java.net.malformedurlexception e) { 18 //Error in web address 19 } 20 } //handle_button 21 //continues... Federico Pecora Java for Interfaces and Networks Lecture 4 22 / 25

42 Introduction to Networking in Java A simple example: get-a-web-page applet ShowURLApplet.java (cont.) 22 public void init() { 23 Container cp = getcontentpane(); 24 cp.setlayout(new FlowLayout()); 25 JButton button = new JButton("Load The Onion"); 26 button.addactionlistener(this); 27 cp.add(button); 28 } //init 29 } //ShowURLApplet Federico Pecora Java for Interfaces and Networks Lecture 4 22 / 25

43 Introduction to Networking in Java Network communication with sockets A simple echo server server listens on port 2000 (using ServerSocket) client connects to the port (using Socket) user enters strings on the client... client sends each string to the server... server prints the string and sends it back to the client server terminates connection if client sends quit or empty string Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

44 Introduction to Networking in Java Network communication with sockets Client.java 1 import java.net.*; 2 import java.io.*; 3 public class Client { 4 public static final int PORT = 2000; 5 public static void main(string[] args) 6 throws IOException { 7 InetAddress addr; 8 if (args.length >= 1) 9 addr = InetAddress.getByName(args[0]); 10 else 11 addr = InetAddress.getByName(null); 12 Socket socket = new Socket(addr, PORT); 13 System.out.println("The new socket: " + socket); 14 //continues... Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

45 Introduction to Networking in Java Network communication with sockets Client.java (cont.) 15 BufferedReader in = new BufferedReader( 16 new InputStreamReader(socket.getInputStream())); 17 PrintWriter out = new PrintWriter( 18 new BufferedWriter( 19 new OutputStreamWriter( 20 socket.getoutputstream())), true); 21 //true: PrintWriter is line buffered 22 BufferedReader kbd_reader = new BufferedReader( 23 new InputStreamReader(System.in)); 24 String buf; 25 //continutes... Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

46 Introduction to Networking in Java Network communication with sockets Client.java (cont.) 26 while (true) { 27 buf = kbd_reader.readline(); 28 System.out.println("From keyboard: " + buf); 29 System.out.println("To server: " + buf); 30 out.println(buf); 31 String inline = in.readline(); 32 if (inline == null) break; 33 System.out.println("From server: " + inline); 34 } 35 } //main 36 } //Client Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

47 Introduction to Networking in Java Network communication with sockets Server.java 1 import java.io.*; 2 import java.net.*; 3 public class Server { 4 public static final int PORT = 2000; 5 public static void main(string[] args) 6 throws IOException { 7 ServerSocket s = new ServerSocket(PORT); 8 System.out.println("Server-socket: " + s); 9 System.out.println("Servern listening..."); 10 Socket socket = s.accept(); 11 System.out.println("Connection accepted"); 12 System.out.println("The new socket: " + socket); 13 //continues... Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

48 Introduction to Networking in Java Network communication with sockets Server.java (cont.) 14 BufferedReader in = new BufferedReader( 15 new InputStreamReader(socket.getInputStream())); 16 PrintWriter out = new PrintWriter( 17 new BufferedWriter( 18 new OutputStreamWriter( 19 socket.getoutputstream())), true); 20 //true: PrintWriter is line buffered 21 while (true) { 22 String inline = in.readline(); 23 System.out.println("Server received: " + inline); 24 if (inline == null inline.equals("quit")) 25 break; 26 out.println("you said " + inline + " "); 27 } 28 } //main 29 } //Server Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

49 Introduction to Networking in Java Network communication with sockets linux> java Server Server-socket: ServerSocket[addr=\ / ,port=0,\ localport=2000] Servern listening... Connection accepted The new socket: Socket[addr=/ ,\ port=38893,localport=2000] Server received: Hello Server received: My name is Kalle Server received: See ya... Server received: quit linux> linux> java Client The new socket: Socket[addr=\ localhost/ ,port=2000,\ localport=38893] Hello From keyboard: Hello To server: Hello From server: You said Hello My name is Kalle From keyboard: My name is Kalle To server: My name is Kalle From server: You said My name is Kalle See ya... From keyboard: See ya... To server: See ya... From server: You said See ya... quit From keyboard: quit To server: quit linux> Federico Pecora Java for Interfaces and Networks Lecture 4 23 / 25

50 Introduction to Networking in Java Multi-thread client/server applications In the previous example, both client and server are single-thread server listens to on a port, and then gives all its attention to the first client that connects client is either reading form keyboard or sending/waiting for response from server Multi-threaded version server has N+1 threads, one for listening and one for dealing with each of the N clients client has two threads, one for reading from keyboard, one for sending/receiving to/from server Federico Pecora Java for Interfaces and Networks Lecture 4 24 / 25

51 Introduction to Networking in Java Threads and Networking Thank you! Federico Pecora Java for Interfaces and Networks Lecture 4 25 / 25