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Incorrect semapore behavior in the simplest task




Just understand with the concepts of Mutex and Semaphore, therefore I decided to solve the simplest problem about 5 philosophers.

About the task itself: there are 5 people (philosophers) and a table that enlists 2 people. It is necessary to feed all people and at the same time not to create a conflict situation when 2 people will try to sit at the table.

The idea of ​​the solution seems very simple: Create semiphore with 2 permit , and in run each stream deals with 1 of these Permit . After the delay occurs (the philosopher eats), the place is released – 1 permit is released.

full solution code, as it is relatively small:

import java.util.concurrent.semaphore;
Public Class MainsemapHore {
  Private Static Class Philosopher EXTENDS Thread {
    Private Semaphore Semaphore;
    Static int table_capacity = 0;
    Philosopher (String Name, Semaphore Semaphore) {
      This.setname (Name);
      this.semaphore = semapore;
    Public void Run () {
      super.run ();
      Try {
        System.out.Printf ("% s came to eat. \ N", getname ());
        semapore.acquire (1);
        Table_capacity ++;
        System.Out.printf ("% s sat at the table. At the table now:% D philosophers. \ N", getName (), Table_Capacity);
        Try {
          SLEEP (10);
        } Catch (InterruptedException E) {
          E.PrintStackTrace ();
        System.out.Printf ("% s was rooted and got out of the table. At the table now:% D philosophers. \ N", getName (),
        semaphore.release (1);
      } Catch (InterruptedException E) {
        E.PrintStackTrace ();
      Finally {
        semaphore.release ();
  Public Static Void Main (String [] Args) {
    Semaphore Sem = New Semaphore (2, True);
    Philosopher ph1 = new philosopher ("Aristotle", SEM);
    Philosopher ph2 = new philosopher ("Plato", SEM);
    Philosopher PH3 = New Philosopher (Socrates, SEM);
    Philosopher PH4 = New Philosopher ("Diogen", SEM);
    Philosopher PH5 = NEW PHILOSOPHER ("PROTAGOR", SEM);
    ph1.start ();
    ph2.start ();
    ph3.start ();
    pp4.start ();
    ph5.start ();

Alas, in practice, the idea is not perfect. To be more accurate, it does not work completely. In the screenshot, it can be seen below that 3 philosophers are sitting at some point at some point:

Even if you reduce the number of places (permit ) to one, there will be a moment when the table will be 2 (and then 3) of the philosopher!

I absolutely do not understand what may be the problem.

Answer 1, Authority 100%

First Table_capacity Must be Atomicinteger , because there are two streams can be at the same time, and secondly you have two semaphore.release (1); one in try , other in finally and it turns out that one past philosophers fully frees semaphore

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