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