Cup 9: Threads

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Cup 9 Threads

• Special Topics Java

Dr. Tim Margush Department of Mathematics and
Computer Science The University of Akron
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Whats a Thread?

• Thread of Control
• A sequential flow of control within a program
• Threads are distinct execution sequences
  occurring simultaneously within one programs
  memory space
• Allows efficient usage of single or multiple
  processors

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Obtaining a New Thread

• Define a class that
• extends java.lang.Thread
• overrides the run() method
• Instantiate an object of that class
• this creates a new thread

• Define a class that
• implements Runnable
• implements the run() method
• Instantiate an object of that class
• this creates a new thread

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Starting a Thread

• start() //duh!
• before starting, a thread does not use any system
  resources
• start schedules the thread for execution (it is
  in the runnable state)
• the threads run() method is what gets executed
• the thread may yield to other threads, but
  continues to execute when possible

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Not Runnable?

• A Thread becomes Not Runnable if
• its sleep method is called
• becomes runnable after the specified time period
  has elapsed
• it calls a wait method
• becomes runnable when notified that the wait
  condition has been met
• it is blocked on I/O
• becomes runnable when the IO finishes

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Stop Thread! Stop!

• A Thread stops (becomes dead) when its run method
  terminates
• A Thread has a stop() method that can also be
  used to terminate it
• throws a ThreadDeath error which ultimately
  causes the termination
• do not catch the error or the Thread will not die
  (unless you re-throw it)

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User and Daemon Threads

• Daemon Thread
• a subordinate thread
• an application will halt when only Daemon Threads
  are left running
• setDaemon(true)
• call before starting the Thread

• User Thread
• independent from the creating Thread
• should be designed with some stopping criterion
• Threads created from Daemon threads are Daemons
  by default

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Let Sleeping Threads Lie

• Thread method sleep(long millisec)
• Suspends Thread execution for specified time -
  allowing other Threads to run
• May throw an InterruptedException
• caused by calling the Threads interrupt() method
• this wakes up a sleeping thread
• the exception should be caught
• sleep(long millisecs, long nanosecs)

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Yielding to Another Thread

• Thread method yield()
• allows waiting Threads a chance to execute
• allows current Thread to continue if other
  Threads are not able to run

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Scheduling Conflicts

• Thread method setPriority(int)
• higher priority Threads will execute first (if
  several Threads are ready to run)
• low priority Threads will be preempted if a
  higher priority Thread becomes runnable
• equal priority Threads may share processor
  resources (time-slice)
• system dependent

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Joining Threads

• aThread.join()
• suspends the current Thread until the named
  Thread dies
• you may specify a maximum time to wait
• InterruptedException may occur

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Thread Relatives

• Unsynchronized Threads
• should not use the same resources
• no need to communicate between Threads
• Synchronized Threads
• must access shared resources, probably not at the
  same time
• may need to communicate with each other

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Synchronization

• synchronized method_a()...
• synchronized method_b()...
• Two Threads cannot be executing in these methods
  on the same object at the same time
• different objects - OK
• If methods are not synchronized, different
  Threads can run in them at the same time (even
  within a single method and even if a synchronized
  method is executing)

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InterThread Communication

• anObject.wait()
• suspends the current Thread that is synchronized
  on this object
• You may specify a maximum wait time (in
  milliseconds)
• releases the Thread's lock
• the current Thread must have the lock on the
  object, or this will fail
• InterruptedException is possible

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Notifying Waiting Threads

• anObject.notify()
• anObject.notifyAll()
• Places one (or all) Threads waiting for a lock on
  this object in the runnable state
• This may occur automatically if the maximum wait
  time has elapsed
• The current Thread must have a lock on this
  object