Java Thread Mutex and Semaphore example
By：Roy.LiuLast updated：2019-08-11
Java multi threads example to show you how to use Semaphore and Mutex to limit the number of threads to access resources.
- Semaphores – Restrict the number of threads that can access a resource. Example, limit max 10 connections to access a file simultaneously.
- 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
- Semaphore Javadocs
- What is mutex and semaphore in Java ? What is the main difference?
- Wikipedia - Semaphore
- Is there a Mutex in Java?
- Concurrent Programming with J2SE 5.0
- Programming Java threads in the real world, Part 2
From：一号门
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