解释semaphore前先看下以下例子：
例1
有10个线程同时执行SemaphoreTest任务，从执行结果可以看出，10个线程是同时执行的，等待一段时间后结束。
package com.hs.example.base.multithread.day01;
public class SemaphoreTest implements Runnable {
@Override
public void run() {
try {
System.out.println(Thread.currentThread().getName() + " start ...");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " end ...");
}
public static void main(String[] args) {
SemaphoreTest semaphoreTest = new SemaphoreTest();
for (int i = 0; i < 10; i++) {
new Thread(semaphoreTest, "t" + i).start();
}
}
}
t2 start ... t3 start ... t6 start ... t7 start ... t0 start ... t1 start ... t4 start ... t5 start ... t8 start ... t9 start ... t2 end ... t3 end ... t0 end ... t6 end ... t4 end ... t7 end ... t1 end ... t9 end ... t5 end ... t8 end ...
例2
在例2中，同样有10个线程执行同一个任务，但是使用了Semaphore工具类，初始化Semaphore类的入参为2，在run方法开始时调用 semaphore.acquire(); 方法，结束时调用 semaphore.release(); 方法，字面理解为请求和释放，以下程序的意思是当有多个线程执行时，只允许2个线程同时执行，从输出结果交替输出start和end也可看出这一点，而 semaphore.acquire(); 方法的意思是当线程执行到这一步时需要获得semaphore的允许才可继续执行
import java.util.concurrent.Semaphore;
public class SemaphoreTest2 implements Runnable {
private Semaphore semaphore;
public SemaphoreTest2(Semaphore semaphore) {
this.semaphore = semaphore;
}
@Override
public void run() {
try {
semaphore.acquire();
System.out.println(Thread.currentThread().getName() + " start ...");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
semaphore.release();
System.out.println(Thread.currentThread().getName() + " end ...");
}
public static void main(String[] args) {
Semaphore semaphore = new Semaphore(2);
SemaphoreTest2 semaphoreTest = new SemaphoreTest2(semaphore);
for (int i = 0; i < 10; i++) {
new Thread(semaphoreTest, "t" + i).start();
}
}
}
t0 start ... t1 start ... t0 end ... t3 start ... t2 start ... t1 end ... t3 end ... t2 end ... t8 start ... t5 start ... t8 end ... t9 start ... t6 start ... t5 end ... t9 end ... t4 start ... t6 end ... t7 start ... t4 end ... t7 end ...
