Threads In Java CSS 2267 Under Construction

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Threads In Java - CSS 2267 Under Construction

• Elliott Evans

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Threads and Multi-threading

• Concept of simultaneous independent
  programs/processes running in same computer
• Each in its own execution state
• Running in parallel
• Can have the same form of multiple independent
  parallel activity inside on program/process
• Inside one process, multiple independent threads
  of execution
• Arises naturally when one process needs to
  support simultaneous independent activities
• As in GUIs or doing heavy I/O
• You can write Java programs with such multiple
  independent threads of activity
• Unconsciously, or
• With intended and controlled effects in a run()
  method

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Running a Thread

• Extend the Thread class and then use start( )
• As in first example at http//clem.mscd.edu/evan
  sell/LECTURES/CodeExamplesThreads.htmlCreateThrea
  ds
• Class Show1 class Show1 extends Thread
• Has a run( ) method public void run()
• How outsider runs it
• Show1 s1 new Show1("First Real") s1.start()
• Implement the Interface Runnable
• As in second example at http//clem.mscd.edu/eva
  nsell/LECTURES/CodeExamplesThreads.htmlCreateThre
  ads
• Class Show2 class Show2 implements Runnable
• Has a run( ) method public void run()
• How outsider runs it
• Show2 i2 new Show2()
• Thread s2 new Thread(i2, "First Interface")
• s2.start()
• Regardless the run() method does the work of the
  thread

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States of a Thread in Java VERY Simplified
To be born
start( )
Ready
Various forms of waiting
yield( ) or pre-empted
scheduled
Run (Running)
run( ) falls out
DEAD
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The Class Thread

• See http//java.sun.com/j2se/1.5.0/docs/api/java/
  lang/Thread.html
• Constructors Many
• Usually pass a name for Thread
• Plus, possibly a Runnable object
• Important static methods
• Get current running thread Thread
  currentThread()
• Make a thread sleep for a while void sleep(long
  millis)
• Give up CPU to let others run void yield( )
• Important non-static methods
• Get and set priority (getPriority( ),
  setPriority( ))
• Get and set name (getName( ), setName( ))
• Get started start( )
• Run and wait on join(long millis)

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States of a Thread in Java SimplifiedLetting
Go!
To be born
start( )
Ready
Time expired, or interrupt
Got I/O
Sleep (Sleeping)
Other forms of waiting
Blocked (I/O, etc.)
yield( ) or pre-empted
scheduled
Wait for I/O
sleep( )
Run (Running)
run( ) falls out
DEAD
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Object Locks and Synchronization

• Standard paradigm for a thread to call methods
  in other objects
• Famous producer/consumer model
• What if two different threads want to run in the
  same method of the same object?
• How to prevent then from messing each other up?
• Each object has a lock on it
• Objects class can designate code as synchronized
• Only one thread can get into a set of
  synchronized code at a time
• See example at http//clem.mscd.edu/evansell/LEC
  TURES/CodeExamplesThreads.htmlSynchThreads
• Mechanisms to control and discipline access are
  not in the thread code, but rather in the code of
  the object being accessed!

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Methods and Mechanisms in Object Class Useful to
Threads

• Mechanisms synchronizing a block of code
• Two forms of synchronized
• Rules of synchronized
• Method notify() Helping to unblock others
• Applies to those waiting for the lock on object
  you are in
• How do you let go?
• Waiting to access something
• Method wait()
• see if you get notified
• see if what you are waiting for happened
• Repetitive wait-ing
• The complementary and dual nature of notify and
  wait

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Simplified Producer - Consumer

• See http//clem.mscd.edu/evansell/LECTURES/CodeEx
  amplesThreads.htmlSynchThreads

SThing
put() wait() for empty
thingVal
get() wait() for full
All outside threads that want to access SThing
have to wait for LOCK!
LOCK
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States of a Thread in Java - Ultimate
To be born
start( )
Ready
Time expired, or interrupt
Obtained lock
Got I/O
notified
Sleep (Sleeping)
Wait (Waiting)
Seeking Lock (on object)
Blocked (I/O, etc.)
yield( ) or pre-empted
scheduled
Entering synchronized code in object
Wait for I/O
wait( )
sleep( )
Run (Running)
run( ) falls out
DEAD
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To be born
start( )
Ready
Time expired, or interrupt
Obtained lock
Got I/O
notified
Sleep (Sleeping)
Wait (Waiting)
Seeking Lock (on object)
Blocked (I/O, etc.)
yield( ) or pre-empted
scheduled
Entering synchronized code in object
Wait for I/O
wait( )
sleep( )
Run (Running)
run( ) falls out
DEAD