OPERATING SYSTEM

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OPERATING SYSTEM

• TUTORIAL 3

http//www.cs.concordia.ca/k_karthi/comp346.html
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Topics

• Exception Handling contd
• What is Thread
• Thread Class, Scheduling
• Thread Life Cycle
• Useful Methods of Thread Class
• Useful Methods of Object Class

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Exception Handling contd.. Multiple catch
clauses class MultiCatch public static void
main(String args) try int a
args.length System.out.println(a
a) int b 42/a int c
1 c42 99 catch
(ArithmeticException e)
System.out.println(Divide by 0 e)
catch(ArrayIndexOutOfBoundsException e)
System.out.println(Array index oob e)
System.out.println(After try catch
blocks)
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Thread

• A thread is a single sequential flow of control
  within a program. A single thread has a
  beginning, a sequence, and an end . However, a
  thread itself is not a program it cannot run on
  its own. Rather, it runs within a program.
• Multithread more than one threads can exist and
  run concurrently. They can perform different
  tasks in parallel. Multithreading is unique in
  Java.
• Example when we download a video clip, we can
  playback the clip before the ending of download.
  One thread download it, another thread start
  playing.

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Thread Scheduling

• There are two types of threads
• Green thread Java 1.2.1 and older versions use
  priority-based scheduling. If a higher-priority
  thread becomes ready, preemption occurs and
  processor is given to the higher-priority thread.
  So, the previously running thread must wait. For
  the same priority threads there is no preemption.
• Native thread Scheduling depends on the native
  OS. This is not platform independent! But it
  supports multiple CPU.

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Thread Class (1)

• We can declare a class to be a subclass of
  Thread. This subclass should override the run
  method of class Thread.
• For example, a thread that computes primes with
  in a range could be written as follows
• class PrimeThread extends Thread
• long minNum, maxNum
• PrimeThread(long _minNum, long _maxNum)
• // constructor
• minNum _minNum maxNum _maxNum
• public void run() //execution point
• //compute primes in between minNum and maxNum

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Thread Class (2)

• An instance of this subclass can then be
  allocated and started.
• The following code would then create a thread and
  start it running
• PrimeThread p new PrimeThread(100, 200)
• p.start()

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Thread Life Cycle
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Useful Methods of Thread Class (1)

• start( ) Causes this thread to begin execution
  the Java Virtual Machine calls the run method of
  this thread. When we create an object of Thread
  Class it will not start execution.
• run( ) This is the actual body of the Thread.
  User shall write necessary codes to do their task
  here.
• Don't call a Thread's run method directly. If
  you do, you will not start a new thread you must
  use start which creates a new thread and then
  calls run for you.
• Thread t new Thread( )
• // Note we call t.start(), not t.run()
• t.start()

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Useful Methods of Thread Class (2)

• yield( ) Causes the currently executing thread
  object to temporarily pause and allow other
  threads to execute. If we use priority-based
  scheduling, yield will allow the highest priority
  ready thread to run. In equal priority
  scheduling, a running thread will leave the CPU
  instantly by yield and according to Round Robin
  Scheduling algorithm next ready thread will run.
• sleep( ) Causes the currently executing thread
  to sleep (temporarily cease execution) for the
  specified number of milliseconds.
• join( ) Waits for this thread to die. It
  Throws InterruptedException - if another thread
  has interrupted the current thread

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Some Useful Methods of Any Java Class

• Class Object is the root of the class
  hierarchy. Every class has Object as a
  superclass. All objects, including arrays,
  implement the methods of this class.
• wait( ) when a running thread calls wait the
  thread enters a waiting state for the particular
  object on which wait was called.
• notify( ) Wakes up a single thread that is
  waiting on this object's monitor. If more than
  one threads are waiting on this object, one of
  them is chosen to be awakened. The choice is
  arbitrary.
• notifyall( ) Wakes up all threads that are
  waiting on this object's monitor.

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Synchronized block (1)

• A synchronized block in java acts like a monitor.
• class XYZ
• public synchronized void method()
• ...
• if (...)
• ...
• wait()
• ...
• ...
• No two threads can be executing inside of a
  synchronized block at the same time
• A synchronized block maintains a queue of FCFS to
  have access inside
• If a thread waits inside a synchronized block
  other thread can enter

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Java Threads "synchronized" Keyword
contd.. The primary tool for managing
coordination between threads in Java programs is
the "synchronized" keyword. To make your
programs thread-safe, you must identify what
data will be shared across threads. If you are
writing data that may be read later by another
thread and vice versa, then that data is shared,
and you must synchronize when accessing
it. http//java.sun.com/docs/books/tutorial/essen
tial/threads/multithreaded.html
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References

• Deitel Deitel, JAVA How to Program Prentice
  Hall
• The JAVA Tutorial
• http//java.sun.com/docs/books/tutorial/
• http//java.sun.com/products/jdk/1.2/docs/api/java
  /lang/Thread.html