Java_多線程之Semaphore

轉載自大佬https://www.cnblogs.com/klbc/p/9500947.html

總結了一下：

Semaphore實現爲一種基於計數的信號量，Semaphore管理着一組虛擬的許可集合，這種許可可以作爲某種憑證，來管理資源，以及線程池的最大併發線程。Semaphore 是 synchronized 的加強版，作用是控制線程的併發數量。

具體方法及解釋大佬博客都有。

這裏貼上魔改代碼（線程池實現）例子：

public class SemaphoreDemo {

		public static void main(String args[]) {
		        final int clientCount = 3;//最多併發數
		        final int totalRequestCount = 10;//線程總數
		        Semaphore semaphore = new Semaphore(clientCount);
		        ExecutorService executorService = Executors.newCachedThreadPool();
		        
		        SemaphoreService ss=new SemaphoreService();
		        MyTesk mt=new MyTesk(ss);
		        		
		        
		        for (int i = 0; i < totalRequestCount; i++) {
		        	executorService.execute(mt);
		        	
		        }
		        executorService.shutdown();
		    }
}
class SemaphoreService {

    private static SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

    private Semaphore semaphore = new Semaphore(3);// 同步關鍵類，構造方法傳入的數字是多少，則同一個時刻，只運行多少個進程同時運行制定代碼

    public void doSomething() {
        try {
            /**
             * 在 semaphore.acquire() 和 semaphore.release()之間的代碼，同一時刻只允許制定個數的線程進入，
             * 因爲semaphore的構造方法是3，則同一時刻只允許3個線程進入，其他線程只能等待。
             * */
            semaphore.acquire();
            System.out.println(Thread.currentThread().getName() + ":doSomething start-" + getFormatTimeStr());
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + ":doSomething end-" + getFormatTimeStr());
            
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
        	System.out.println("可用通路數：" + semaphore.availablePermits());
        	semaphore.release();
		}
    }

    public static String getFormatTimeStr() {
        return sf.format(new Date());
    }
}
class MyTesk implements Runnable{
    private SemaphoreService service;

    public MyTesk(SemaphoreService service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        this.service.doSomething();
    }

pool-1-thread-3:doSomething start-2020-02-22 15:12:03.131
pool-1-thread-2:doSomething start-2020-02-22 15:12:03.131
pool-1-thread-1:doSomething start-2020-02-22 15:12:03.131
pool-1-thread-1:doSomething end-2020-02-22 15:12:05.132
pool-1-thread-2:doSomething end-2020-02-22 15:12:05.132
pool-1-thread-3:doSomething end-2020-02-22 15:12:05.132