Пример многопоточности Java, чтобы показать вам, как использовать Семафор и Мьютекс для ограничения количества потоков для доступа к ресурсам.
Семафоры– Ограничьте количество потоков, которые могут получить доступ к ресурсу. Например, ограничьте максимальное количество подключений 10 для одновременного доступа к файлу.Мьютекс– Только один поток для одновременного доступа к ресурсу. Например, когда клиент обращается к файлу, никто другой не должен иметь доступ к тому же файлу одновременно.
1. Семафор
Рассмотрим ячейку банкомата с 4 рычагами, Семафор может гарантировать, что только 4 человека могут получить доступ одновременно.
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();
}
}
Выход может отличаться, но поток блокировки и разблокировки должен быть более или менее одинаковым.
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
Внимательно наблюдайте за приведенным выше выводом, вы увидите, что для выполнения операции одновременно требуется максимум 4 человека (C, B, F, D), люди A и E ждут. Как только один из них снимет блокировку (D и F), A и E получат ее и немедленно возобновят работу.
2. Мьютекс
Мьютекс является Семафор с числом обращений 1. Рассмотрим ситуацию с использованием шкафчиков в банке. Обычно правило таково, что в раздевалку разрешается входить только одному человеку.
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();
}
}
Выход может отличаться, но поток блокировки и разблокировки должен быть одинаковым.
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
Как видно, здесь одновременно выполняется только один поток. Это роль мьютекса.
Рекомендации
- Семафорные Javadocs
- Что такое мьютекс и семафор в Java? В чем главное отличие?
- Википедия – Семафор
- Есть ли мьютекс в Java?
- Параллельное программирование с J2SE 5.0
- Программирование потоков Java в реальном мире, Часть 2
Оригинал: “https://mkyong.com/java/java-thread-mutex-and-semaphore-example/”