2.Binary Semaphore(Mutex)
A semaphore initialized to one, and which is used such that it only has at most one permit available, can serve as a mutual exclusion lock. This is more commonly known as a binary semaphore, because it only has two states: one permit available, or zero permits available.
When used in this way, the binary semaphore has the property (unlike many Lock implementations), that the “lock” can be released by a thread other than the owner (as semaphores have no notion of ownership). This can be useful in some specialized contexts, such as deadlock recovery.
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.
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.
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();
}
}