Java Thread – Mutex and Semaphore example

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Java multi threads example to show you how to use Semaphore and Mutex to limit the number of threads to access resources.

  1. Semaphores – Restrict the number of threads that can access a resource. Example, limit max 10 connections to access a file simultaneously.
  2. Mutex – Only one thread to access a resource at once. Example, when a client is accessing a file, no one else should have access the same file at the same time.

1. Semaphore

Consider an ATM cubicle with 4 ATMs, Semaphore can make sure only 4 people can access simultaneously.

SemaphoreTest.java

package com.mkyong;

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();

	}
}

Output may vary, but the flow of locking and releasing should be more or less same.


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

Observe the above output carefully, you will see that there are maximum 4 people (C, B, F, D) to perform an operation at a time, the people A and E are waiting. As soon as one of them release the lock (D and F), A and E will acquire it and resumes immediately.

2. Mutex

Mutex is the Semaphore with an access count of 1. Consider a situation of using lockers in the bank. Usually the rule is that only one person is allowed to enter the locker room.

MutexTest.java

package com.mkyong;

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();

	}
}

Output may vary, but the flow of locking and releasing should be same.


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

As it can be seen, only one thread executes at a time here. This is the role of a Mutex.

References

  1. Semaphore Javadocs
  2. What is mutex and semaphore in Java ? What is the main difference?
  3. Wikipedia – Semaphore
  4. Is there a Mutex in Java?
  5. Concurrent Programming with J2SE 5.0
  6. Programming Java threads in the real world, Part 2

Datsabk

A technically sound person, oriented towards learning things logically rather than technically. It is my logical approach that has helped me learn and take up any programming language and start with coding. With a responsibility of Java based Web development professionally, I highly believe in imparting knowledge online for any technical subject that I can handle.

8 Comments
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Ramesh
7 years ago

I don’t see any mutex lock implementation

2
Reply
Iftekhar
6 years ago
Reply to  Ramesh

thats so sad

1
Reply
rICH rICHIE
7 years ago

Dude, paste the code for Mutex – you pasted the same code twice.

1
Reply
Koko
7 years ago
Reply to  rICH rICHIE

in the article first lines is stated that a Mutex is a semaphore only allowing 1 and only 1 concurrent acces to the ressource.
Hence why the code “look” the same, but isn’t
First code:
// max 4 people
static Semaphore semaphore = new Semaphore(4);
and
Second code:
// max 1 people
static Semaphore semaphore = new Semaphore(1);

But yes this doesn’t make it a proper mutex since the Semaphore can still be unlocked from another thread
EDIT:
After reading a bit, is seem there is no Mutex class, but something that work very much like is ReentrantLock
See https://stackoverflow.com/a/46547469

2
Reply
Sangdeuk
3 years ago

Thanks.
This is very useful to make sense about it.
I will refer to your material 🙂

0
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Lorenzo Di Dio
5 years ago

Why for(int i =1; i <= 5; i++) ?

0
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Feroz
5 years ago

static objects belongs to class and not specifically to an instance of a class.

static will ensure all thread instances of the class (A to F) access a shared resource – class variable. You would not want each object to have its own semaphore of count 4, right.

0
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vikas_luv
9 years ago

why Semapore object is defined static here?

0
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