Semaphore

1
Semaphore

• A special type of lock.
• Similar to lock as it can be used to prevent
  access to shared data when the data is locked.
• A lock that has a value.
• A semaphore value represents the number of
  permits.
• Package java.util.concurrentpublic class
  Semaphore public Semaphore(long permits)

2
Semaphore Value

• public class Semaphore public Semaphore(long
  permits) public void acquire() public void
  acquire(long permits) public void
  release() public void release(long permits)
• acquire()
• grabs a permit and decrements the semaphore value
  ( of permits)
• release()
• returns a permit and increments the semaphore
  value ( of permits)
• acquire() blocks
• when the semaphore value ( of permits) becomes 0
• until a permit is available.
• acquire() does not block
• as far as the semaphore value is greater than 0
  (as far as permits are available).

3

• The semaphore value can represent
• the number of locks that can be granted.
• e.g., a semaphore can be used to simulate a
  reader-writer lock (with the max number of
  readers) by having
• a reader acquire one permit, and
• a writer acquire all permits.
• the maximum number of objects in an object pool.
• e.g., connection pooling
• A pool of instantiated connection objects
• Connections are recycled to send data (e.g.,
  commands and queries) to servers (e.g., a web
  server and database)
• No runtime overhead to dynamically instantiate
  connections
• The max number of simultaneous connections is
  bounded.

4
Comparison with Lock

• A semaphore is mostly same as a lock
• if the max number of permits is 1.
• Differences
• No condition objects are available in a
  semaphore.
• No newCondition() supported.
• ReentrantLock has this method.
• No nested locking in a semaphore
• A semaphore can acquire multiple permits.

5
Barrier

• A rendezvous point for multiple threads.
• All threads meet (or are synchronized) at a
  rendezvous point
• before any of them are permitted to pass the
  point.

6
Barrier and join()

• Whats the differences between a barrier and
  join()?
• When join() returns
• a target thread exits
• no way to access data stored in a corresponsing
  Runnable object.
• A barrier provides access to data stored in a
  Runnable object
• even after a corresponding thread exits.

7
CyclicBarrier

• java.util.concurrent.CyclicBarrier
• public class CyclicBarrier public
  CyclicBarrier( int parties ) public
  CyclicBarrier( int parties, Runnable
  barrierAction) public int await() public
  int getParties() public int
  getNumberWaiting() ..

• Other threads
• class BarrieredRuunable impl Runnable
  private CyclicBarrier barrier private
  int result public void run()
  result barrier.await()
  public int getResult return result
• Class BarrierAction impl Runnable private
  int finalResult 0 public void run()
  for(Runnable r list) finalResult
  r.getResult()

• Rendezvous thread
• BarrierdRunnable runnablefor(int i iltN i)
  runnable new BarrierdRunnable()
  list.add( runnable ) new Thread( runnable
  ).start()CyclicBarrier barrier new
  CyclicBarrier(N, new BarrierAction())