Mutual Exclusion

1
Mutual Exclusion
2
Race Condition

• In many practical multithreaded application, two
  or more threads need to share the same object.
• Imagine what might happen if two or more threads
  are accessing the same object at the same time.
• Depending on the order in which the object is
  accessed, the corrupted objects may result.
• Race condition refers to such situation.

3
Setting Mutual Exclusion Zone

• You can set a method as a restricted region by
  defining it as synchronized.
• public synchronized void access()
• You can also set a block of code as a restricted
  region by enclosing it with
• synchronized
• // you code here
• Every Java object has the key(called monitor) to
  these synchronized region. (Only one key per an
  object)
• For a thread to enter these synchronized region
  should get the key(of the object).
• Since the key can be assigned to only one thread
  at a time, we can avoid race condition.

4
How To Get a Key?

• A thread is assigned the key when it enters a
  synchronized region if the key is not owned by
  any other thread.
• In other words, no thread is inside any of the
  synchronized regions.
• When multiple threads try to enter the region at
  the same time, only one thread is picked up and
  given the key.
• When a thread is inside a synchronized region,
  any other threads which access the region are put
  into waiting queue.
• We refer this situation as waiting for the key.
• When a thread returns the key(or finished with
  the region), the first thread in the waiting
  queue is picked up and given the key.

5
Giving Up the Key

• Quite often, a thread may need to suspend itself
  inside a synchronized region. (See the handout)
• wait() method is used in such situation.
• When a thread calls wait(), it puts itself into
  another waiting queue.
• A thread in this queue can return to synchronized
  region only when other thread calls
  notify()/notifyAll() method.
• In other words, it is another thread that can
  wake up suspended threads.
• Note that a suspended thread can never be
  rescheduled until it is notified.