Javaマルチスレッドの例では、リソースにアクセスするスレッドの数を制限するために

Semaphore`と

Mutex`を使用する方法を示します。


  1. Semaphores

    – アクセス可能なスレッドの数を制限する

リソース。例として、ファイルに同時にアクセスするための最大10個の接続を制限する。


  1. Mutex

    – 一度にリソースにアクセスするスレッドは1つだけです. 例、いつ

クライアントがファイルにアクセスしているときに、他の誰も同じファイルに同時にアクセスする必要はありません。

1.セマフォ

4つのATMを備えたATMキュービクルを考えると、「セマフォ」は4人だけが同時にアクセスできることを確認できます。

SemaphoreTest.java 

package com.techfou;

import java.util.concurrent.Semaphore;

public class SemaphoreTest {

   //max 4 people
    static Semaphore semaphore = new Semaphore(4);

    static class MyATMThread extends Thread {

        String name = "";

        MyATMThread(String name) {
            this.name = name;
        }

        public void run() {

            try {


                System.out.println(name + " : acquiring lock...");
                System.out.println(name + " : available Semaphore permits now: "
                                + semaphore.availablePermits());

                semaphore.acquire();
                System.out.println(name + " : got the permit!");

                try {

                    for (int i = 1; i <= 5; i++) {

                        System.out.println(name + " : is performing operation " + i
                                + ", available Semaphore permits : "
                                + semaphore.availablePermits());

                       //sleep 1 second
                        Thread.sleep(1000);

                    }

                } finally {

                   //calling release() after a successful acquire()
                    System.out.println(name + " : releasing lock...");
                    semaphore.release();
                    System.out.println(name + " : available Semaphore permits now: "
                                + semaphore.availablePermits());

                }

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

        }

    }

    public static void main(String[]args) {

        System.out.println("Total available Semaphore permits : "
                + semaphore.availablePermits());

        MyATMThread t1 = new MyATMThread("A");
        t1.start();

        MyATMThread t2 = new MyATMThread("B");
        t2.start();

        MyATMThread t3 = new MyATMThread("C");
        t3.start();

        MyATMThread t4 = new MyATMThread("D");
        t4.start();

        MyATMThread t5 = new MyATMThread("E");
        t5.start();

        MyATMThread t6 = new MyATMThread("F");
        t6.start();

    }
}

出力は変わることがありますが、ロックと解放のフローは多かれ少なかれ同じでなければなりません。 

Total available Semaphore permits : 4
A : acquiring lock...
D : acquiring lock...
C : acquiring lock...
B : acquiring lock...
B : available Semaphore permits now: 4
C : available Semaphore permits now: 4
E : acquiring lock...
F : acquiring lock...
F : available Semaphore permits now: 2
F : got the permit!
F : is performing operation 1, available Semaphore permits : 1
D : available Semaphore permits now: 4
A : available Semaphore permits now: 4
D : got the permit!
D : is performing operation 1, available Semaphore permits : 0
E : available Semaphore permits now: 2
C : got the permit!
B : got the permit!
C : is performing operation 1, available Semaphore permits : 0
B : is performing operation 1, available Semaphore permits : 0
F : is performing operation 2, available Semaphore permits : 0
D : is performing operation 2, available Semaphore permits : 0
C : is performing operation 2, available Semaphore permits : 0
B : is performing operation 2, available Semaphore permits : 0
F : is performing operation 3, available Semaphore permits : 0
D : is performing operation 3, available Semaphore permits : 0
C : is performing operation 3, available Semaphore permits : 0
B : is performing operation 3, available Semaphore permits : 0
F : is performing operation 4, available Semaphore permits : 0
D : is performing operation 4, available Semaphore permits : 0
C : is performing operation 4, available Semaphore permits : 0
B : is performing operation 4, available Semaphore permits : 0
D : is performing operation 5, available Semaphore permits : 0
F : is performing operation 5, available Semaphore permits : 0
B : is performing operation 5, available Semaphore permits : 0
C : is performing operation 5, available Semaphore permits : 0


D : releasing lock...
F : releasing lock...
D : available Semaphore permits now: 1
A : got the permit!
A : is performing operation 1, available Semaphore permits : 0
F : available Semaphore permits now: 1
E : got the permit!


E : is performing operation 1, available Semaphore permits : 0
B : releasing lock...
B : available Semaphore permits now: 1
C : releasing lock...
C : available Semaphore permits now: 2
A : is performing operation 2, available Semaphore permits : 2
E : is performing operation 2, available Semaphore permits : 2
A : is performing operation 3, available Semaphore permits : 2
E : is performing operation 3, available Semaphore permits : 2
A : is performing operation 4, available Semaphore permits : 2
E : is performing operation 4, available Semaphore permits : 2
A : is performing operation 5, available Semaphore permits : 2
E : is performing operation 5, available Semaphore permits : 2
A : releasing lock...
A : available Semaphore permits now: 3
E : releasing lock...
E : available Semaphore permits now: 4

上記の出力を慎重に観察すると、一度に操作を実行する最大4人（C、B、F、D）が存在することがわかり、人々AとEが待機しています。そのうちの1人がロックを解除すると（DとF）、AとEはそれを取得してすぐに再開します。

2.ミューテックス

`ミューテックス`はアクセスカウント1のセマフォーです。銀行でロッカーを使う状況を考えてみましょう。通常、ロッカールームに入ることができるのは1人だけです。

MutexTest.java 

package com.techfou;

import java.util.concurrent.Semaphore;

public class SemaphoreTest {

   //max 1 people
    static Semaphore semaphore = new Semaphore(1);

    static class MyLockerThread extends Thread {

        String name = "";

        MyLockerThread(String name) {
            this.name = name;
        }

        public void run() {

            try {

                System.out.println(name + " : acquiring lock...");
                System.out.println(name + " : available Semaphore permits now: "
                                + semaphore.availablePermits());

                semaphore.acquire();
                System.out.println(name + " : got the permit!");

                try {

                    for (int i = 1; i <= 5; i++) {

                        System.out.println(name + " : is performing operation " + i
                                + ", available Semaphore permits : "
                                + semaphore.availablePermits());

                       //sleep 1 second
                        Thread.sleep(1000);

                    }

                } finally {

                   //calling release() after a successful acquire()
                    System.out.println(name + " : releasing lock...");
                    semaphore.release();
                    System.out.println(name + " : available Semaphore permits now: "
                                + semaphore.availablePermits());

                }

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

        }

    }

    public static void main(String[]args) {

        System.out.println("Total available Semaphore permits : "
                + semaphore.availablePermits());

        MyLockerThread t1 = new MyLockerThread("A");
        t1.start();

        MyLockerThread t2 = new MyLockerThread("B");
        t2.start();

        MyLockerThread t3 = new MyLockerThread("C");
        t3.start();

        MyLockerThread t4 = new MyLockerThread("D");
        t4.start();

        MyLockerThread t5 = new MyLockerThread("E");
        t5.start();

        MyLockerThread t6 = new MyLockerThread("F");
        t6.start();

    }
}

出力は変わることがありますが、ロックと解放の流れは同じでなければなりません。 

Total available Semaphore permits : 1
A : acquiring lock...
B : acquiring lock...
A : available Semaphore permits now: 1
C : acquiring lock...
B : available Semaphore permits now: 1
C : available Semaphore permits now: 0
A : got the permit!
D : acquiring lock...
E : acquiring lock...
A : is performing operation 1, available Semaphore permits : 0
E : available Semaphore permits now: 0
D : available Semaphore permits now: 0
F : acquiring lock...
F : available Semaphore permits now: 0
A : is performing operation 2, available Semaphore permits : 0
A : is performing operation 3, available Semaphore permits : 0
A : is performing operation 4, available Semaphore permits : 0
A : is performing operation 5, available Semaphore permits : 0
A : releasing lock...
A : available Semaphore permits now: 1
B : got the permit!
B : is performing operation 1, available Semaphore permits : 0
B : is performing operation 2, available Semaphore permits : 0
B : is performing operation 3, available Semaphore permits : 0
B : is performing operation 4, available Semaphore permits : 0
B : is performing operation 5, available Semaphore permits : 0
B : releasing lock...
B : available Semaphore permits now: 1
C : got the permit!
C : is performing operation 1, available Semaphore permits : 0
C : is performing operation 2, available Semaphore permits : 0
C : is performing operation 3, available Semaphore permits : 0
C : is performing operation 4, available Semaphore permits : 0
C : is performing operation 5, available Semaphore permits : 0
C : releasing lock...
C : available Semaphore permits now: 1
E : got the permit!
E : is performing operation 1, available Semaphore permits : 0
E : is performing operation 2, available Semaphore permits : 0
E : is performing operation 3, available Semaphore permits : 0
E : is performing operation 4, available Semaphore permits : 0
E : is performing operation 5, available Semaphore permits : 0
E : releasing lock...
E : available Semaphore permits now: 1
D : got the permit!
D : is performing operation 1, available Semaphore permits : 0
D : is performing operation 2, available Semaphore permits : 0
D : is performing operation 3, available Semaphore permits : 0
D : is performing operation 4, available Semaphore permits : 0
D : is performing operation 5, available Semaphore permits : 0
D : releasing lock...
D : available Semaphore permits now: 1
F : got the permit!
F : is performing operation 1, available Semaphore permits : 0
F : is performing operation 2, available Semaphore permits : 0
F : is performing operation 3, available Semaphore permits : 0
F : is performing operation 4, available Semaphore permits : 0
F : is performing operation 5, available Semaphore permits : 0
F : releasing lock...
F : available Semaphore permits now: 1

見ることができるように、ここでは一度に1つのスレッドしか実行されません。これはミューテックスの役割です。

参考文献


  1. https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore.html

    [Semaphore

Javadocs]。

http://stackoverflow.com/questions/771347/what-is-mutex-and-semaphore-in-java-what-is-the-main-difference

[What

Javaではmutexとsemaphoreはありますか？主な違いは何ですか？]。ウィキペディア –

セマフォ]。

http://stackoverflow.com/questions/5291041/is-there-a-mutex-in-java

[Is

そこにJavaのミューテックス？]。

http://www.oracle.com/technetwork/articles/javase/index-140767.html

[同時

J2SE 5.0によるプログラミング]。

http://www.javaworld.com/article/2076797/java-concurrency/programming-java-threads-in-the-real-world—​part-2.html—​プログラミング

現実世界のJavaスレッド、第2回]


java thread


mutex


semaphore


thread