Package nl.nn.adapterframework.util

Class Semaphore

java.lang.Object

nl.nn.adapterframework.util.Semaphore

public class Semaphore
extends Object

A semaphore is a flag used to check whether a resource is currently being used by another thread or process and wait for the other process to release it.

For example, if a process wants to use the printer, it first needs to make sure the printer is available by checking to see if the semaphore has been set. If it is set, it needs to wait to the process that currently has it is finished. however, If the printer were free, the process would set the semaphore and start using the printer, blocking access to all other processes until it finished.

Semphores are a classical technique for protecting critical sections of code from being simultaneously executed by more than one thread. A semaphore is a generalisation of a monitor. A monitor allows only one thread to lock an object at once. A semaphore allows N processes.

The process of grabbing a semaphore for semi-exclusive use is called downing the semaphore because they are implemented with a countdown integer that decrements for each lock and increments for each unlock. If a semaphore is fully occupied, new threads wanting to use it will wait until some thread releases its lock by upping the semaphore. For a semaphore to work, the check for full, and the decrement must be done all in one atomic uninterruptible instruction. This is done by the release() method.

Author:

Gerrit van Brakel

Constructor Summary

Constructors

Constructor	Description
Semaphore()
Semaphore(int i)

Method Summary

Modifier and Type	Method	Description
void	acquire()	Decrements internal counter, blocking if the counter is already zero or less.
boolean	isReleased()
void	release()	Increments internal counter, possibly awakening the thread wait()ing in acquire()
void	tighten()	non blocking decrements internal counter.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Semaphore

public Semaphore()

Semaphore

public Semaphore(int i)

Method Detail

acquire

public void acquire()
             throws InterruptedException

Decrements internal counter, blocking if the counter is already zero or less.

Throws:

InterruptedException - passed from this.wait().

tighten

public void tighten()

non blocking decrements internal counter.

release

public void release()

Increments internal counter, possibly awakening the thread wait()ing in acquire()

isReleased

public boolean isReleased()