Threads in Java

1
Threads in Java

• Shon Vick
• CMSC 331

2
What Is a Thread?

• Familiar with writing sequential programs
• At any given time during the runtime of the
  program, there is a single point of execution.
• The program has a beginning, an execution
  sequence, and an end
• So do threads except that they run within a
  program

3
Threads run within Programs

• A thread itself is not a program it cannot run
  on its own.
• A Thread runs within a program and can execute a
  set of instructions completely independent of
  other threads in the same process

4
Why are threads useful?

• Can be doing multiple things (multiple execution
  paths) at once.
• Example Browser
• Loading an applet
• Scrolling down a page
• Printing

5
Threads

• Lightweight process
• A thread is similar to a real process in that a
  thread and a running program are threads of
  execution
• A thread takes advantage of the resources
  allocated for that program instead of having to
  allocate those resources again

6
Threads

• A thread has its own stack and program counter
  for example.)
• The code running within the thread works only
  within the context implied by the stack and PC so
  also called a execution context

7
A Thread Example
// This class extends the Thread class class
MyThread extends Thread // Override the
Thread's class run() method public void run()
System.out.println("Hello there, from "
getName())  
From Berg Chapter 11 see references
8
An example
public class ExtendThr   static public void
main(String s)   MyThread Thread_a,
Thread_b // Define the threads   Thread_a
new MyThread() // Create MyThread object
Thread_b new MyThread() // Create another
MyThread object   Thread_a.start() // Start
the run() method in thread a
Thread_b.start() // Start the run() method in
thread b
From Berg Chapter 11 see references
9
Creating and Starting a Thread
public class Echo extends Thread private
String word private int waitTime // in
millisecs public Echo(String word, int
waitTime) this.word word this.waitTime
waitTime   Modified From Arnold and
Gosling Chapter 9 see references
10
Creating and Starting a Thread
public void run( ) try while (true)
System.out.print(word )
sleep(waitTime)
catch (InterruptedException e) return()
// end the thread  
11
Creating and Starting a Thread
public static void main(String args) Echo
e1 new Echo(Foo, 200) Echo e2
new Echo(Bar, 200)
12
An Alternate Mechanism
// Class the implements the Runnable
interface class MyThread2 implements Runnable
public void run() System.out.println("Hi
there, from "
Thread.currentThread().getName())
From Berg Chapter 11 see references
13
An Alternate Mechanism
public class RunnableThr   static public void
main(String s) MyThread2 work2do //
Object that has work to be done Thread
a_thread, b_thread // A thread object on which
to run the work // Create the MyThread2
object work2do new MyThread2()
14
An Alternate Mechanism
// Create a thread with the MyThread2 object as
its target a_thread new Thread(work2do)
b_thread new Thread(work2do) // Start
the thread which will call run() in the target
a_thread.start() b_thread.start()

15
A Sample Use

• Suppose that we have a server that has many
  clients connected to it
• We would like to start a new thread for each
  client connection
• What happens if we do not?
• Our server waits for a connection with accept and
  when a connection is made a new thread to handle
  that connection is created
• Example from Berg Chapter 4 see references

16
A Server Example
public class MultiThreadedServer   public
static void main( String args ) throws
Exception // create a server side
socket - same as before ServerSocket server
new ServerSocket( 9999 )
Need a server socket to listen for connections
17
A Server Example
block
while( true ) Socket socket
server.accept() new ThreadedSocket( socket
).start()
Start the thread
Pass the socket along
Create a Thread object
18
A Server Example
class ThreadedSocket extends Thread
ThreadedSocket( Socket socket ) this.socket
socket //
Remember the socket
19
A Server Example
public void run() try // create the
Objects for the Client
ObjectOutputStream output
new ObjectOutputStream( socket.getOutputStream()
) output.writeObject(
stuffForClient) socket.close()
catch( Exception e ) e.printStackTrace()

20
Sharing Data

• In our previous example the socket class was
  never shared by more than one thread because
  every accept inside the while results in a new
  thread
• What if that were not the case?
• Suppose we had the notion of an account that
  would be shared across threads

21
Synchronized Methods
public class Account private double
balance public Account(double initial)
balance initial public synchronized
double getBalance() return balance
public synchronized double deposit(double amt)
balance amt
One at a time
22
Synchronized Methods

• With synchronized two or more running threads are
  guaranteed not to interfere with one another
• If there are read and write methods they should
  be synchronized
• Another reason for accessors
• Static methods may also be locked class wide
  lock
• Why doesnt the constructor need to be
  synchronized?

23
Synchronized Statements

• The Synchronized statement enables you to execute
  synchronized code that locks an object without
  invoking a Synchronized method
• The Synchronized statement has two parts
• An object to be locked
• A statement to execute when the lock is obtained

24
Synchronized Statements
// in some class public static void abs(int
values) synchronized (values) for (int
i 0 i lt values.length I) if
(valuesi lt 0 ) valuesi - valuesi

25
Selected References

• Advanced Techniques for Java Developers,Berg
  Fritzinger, Chapter 11 Wiley, 1999
• The online Java Tutorial http//java.sun.com/docs/
  books/tutorial/essential/threads/
• The Java Programming Language , K.Arnold and J.
  Gosling , Addison-Wesley , 1996 Chapter 9