Lecture Note 12

buninnateSoftware and s/w Development

Nov 18, 2013 (3 years and 8 months ago)




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.concurrent;

public class Semaphore{

public Semaphore(long permits)

… }


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)


grabs a permit and decrements the semaphore value (# of permits)


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).


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.


Comparison with Lock

A semaphore is mostly same as a lock

if the max number of permits is 1.


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.



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.


Barrier and join()

What’s the differences between a barrier and

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.




public class CyclicBarrier{

public CyclicBarrier( int parties )

public CyclicBarrier( int parties, Runnable barrierAction)

public int await()

public int getParties()

public int getNumberWaiting()

…….. }

Rendezvous thread

BarrierdRunnable runnable;

for(int i; i<N; i++){

runnable = new BarrierdRunnable();

list.add( runnable );

new Thread( runnable ).start();


CyclicBarrier barrier =

new CyclicBarrier(N,

new BarrierAction());

Other threads

class BarrieredRuunable impl Runnable{

private CyclicBarrier barrier = …;

private int result;

public void run(){

result = …;



public int getResult{

return result; }


Class BarrierAction impl Runnable{

private int finalResult = 0;

public void run(){

for(Runnable r: list)

finalResult += r.getResult(); } }