Object Locks

1
Object Locks

• When a thread calls a synchronized method, the
  object becomes locked
• Analogy
• Locking inside a room/booth to perform a task ,
  such as a telephone call.
• Other Threads/People have to Wait till you
  open the door and signal them that the
  object/phone is ready to be accessed.

2
Scheduling

• Thread Scheduler
• Decides which thread, that is waiting and ready
  gets the first access to the object
• Thread leaves a synchronized method, it
  relinquishes the object lock.
• So that other threads can now obtain the lock and
  perform the necessary operations

3
Number of Locks

• Two different Objects of the same class, can be
  locked by different threads
• However, an objects lock can be owned by only one
  thread at any given point of time.
• But a thread can hold locks of multiple objects
  at the same time.

4
A simple case

• Check Balance
• Transfer Amount
• Synchronized void transfer(bank)
• Check Balance
• TransferAmount
• What if there is not enuf money to transfer?
• Since it is synchronized, no other thread can
  enter the bank object

5
Wait()

• Lets wait till there is enuf money
• Synchronized void transfer(bank)
• Check Balance
• Wait()
• TransferAmount
• Use the wait() method of the Object class if you
  need to wait inside a synchronized method.

6
Wait()

• When wait() is called inside a synchronized
  method, current thread is blocked and gives up
  object lock.
• Wait() is a method of class Object and not of
  class Thread.
• If a thread holds the locks to multiple objects,
  then the call to wait unlocks only the object on
  which wait() is called

7
Waitlist

• Once a thread calls wait() method, it enters the
  wait list of the object
• The thread is now blocked
• Until the thread is removed from the waitlist the
  scheduler ignores it and does not have a chance
  to continue running.

8
Notify()

• To remove the thread from waitlist, some other
  thread must call notify() or notifyAll().
• notifyAll() removes all threads from the objects
  list
• Notify() removes only one arbitrarily chosen
  thread.
• When threads are removed from waiting list, they
  are runnable and scheduler can activate them

9
Scheduling Rules

• If two or more threads modify an object, declare
  the methods that carry out modification as
  synchronized. Read only that are affected by
  modifications must be synchronized.
• If a thread must wait for the state of an object
  to change, it should wait inside the object, by
  entering a synchronized method and calling wait

10
Scheduling Rules

• Dont spend significant amount in synchronized
  method.
• Whenever a method changes the state of an object
  it should call notifyAll(). That gives waiting
  threads a chance to see if circumstances have
  changed

11
Scheduling Rules

• Remember wait() and notifyAll()/notify() are
  methods of Object class, not the Thread Class.
  Double-check that your calls to wait() are
  matched by a notification on the same object.

12
Deadlock

• Quite possible to create code that deadlocks
• Thread 1 holds lock on A
• Thread 2 holds lock on B
• Thread 1 is trying to acquire a lock on B
• Thread 2 is trying to acquire a lock on A
• Deadlock!
• Not easy to detect when deadlock hasoccurred
• other than by the fact that nothing is
  happening

13
Deadlock Wait graphs
14
Waitgraph - example
15
Deadlock -Causes

• Four preconditions that must be present for a
  deadlock to occur
• Mutual exclusion Only one process can use a
  resource at a time.
• Hold and wait A process must hold the resources
  while awaiting assignment of other resources.
• No preemption No resource can be forcibly
  removed from a process holding it.
• Circular wait A closed chain of processes
  exists, such that each process holds at least one
  resource needed by the next process in the chain.

16
Livelock

• Deadlock arises when blocked threads cannot
  execute
• Livelock occurs when threads actually are
  executing, but no work gets done.
• E.g. one thread keeps performing work that gets
  undone by another thread, so it must keep
  starting over

17
Livelock

• A condition that occurs when two or more
  processes continually change their state in
  response to changes in the other processes. The
  result is that none of the processes will
  complete.
• i.e. A situation where the computing power is
  excessively consumed by interrupt processes.

18
Deadlock versus Livelock

• Livelock is similar to deadlock in that no
  progress is made.
• Livelock differs from deadlock in that processing
  continues to take place, rather than just waiting
  in an idle loop.
• In livelock no process is blocked or waiting for
  anything.

19
Deadlock versus Livelock
The hallway is wide enough for two people to
pass. If on the same side of the hallway, a
polite strategy is to step to the other side. A
more belligerent strategy is to wait for the
other person to move. Two belligerent people will
suffer in deadlock, glaring face to face in front
of each other. Two polite people could suffer
from livelock if they repeatedly side-step
simultaneously. (No conclusions on morality are
to be inferred from the fact that one polite and
one belligerent person don't have any problems.)