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Semaphore是一個二進制信號量，只有0和1兩個值。如果線程想要訪問一個共享資源，它必須先獲得信號量。如果信號量的內部計數器大于0，那么信號量減1，并允許訪問這個資源。否則，如果信號量計數器等于0，線程會等待直至計數器大于0。

所以說計數器大于0，說明有資源可用。計數器等于0，說明沒有資源可用。

同時Semaphore提供了一個帶有boolean參數的構造方法，true代表公平鎖，false代表非公平鎖，默認實現是非公平鎖。

我們使用Semaphore信號量來重寫PrintQueue的例子。

首先實現一個PrintQueue打印隊列，有一個Semaphore信號量用來并發訪問控制。打印之前使用acquire()方法獲取信號量，執行完畢后使用release()方法釋放信號量。每次打印等待一個隨機時間，模擬打印耗時。

public class PrintQueue {private Semaphore semaphore;public PrintQueue() {//semaphore = new Semaphore(1); //非公平的semaphore = new Semaphore(1, true); //公平的}public void printJob(Object document) {try {semaphore.acquire();long duration = (long)(Math.random() * 10000);System.out.printf("%s: Print a job duration %d seconds.\n",Thread.currentThread().getName(), duration / 1000);Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();} finally {semaphore.release();}} }

創建Job線程類，模擬打印請求。

public class Job implements Runnable{private PrintQueue printQueue;public Job(PrintQueue printQueue) {this.printQueue = printQueue;}@Overridepublic void run() {System.out.printf("%s: Going to print a Job.\n", Thread.currentThread().getName());printQueue.printJob(new Object());System.out.printf("%s: The Job has been printed.\n", Thread.currentThread().getName());} }

主方法類中啟動10個打印Job線程。

public class Main {public static void main(String[] args) {PrintQueue printQueue = new PrintQueue();Thread[] threads = new Thread[10];for (int i = 1; i < 10; i++) {threads[i] = new Thread(new Job(printQueue));}for (int i = 1; i < 10; i++) {threads[i].start();}} }

查看日志，每次只有一個打印Job可以執行打印程序，其他線程處于WAITING狀態。

Thread-0: Going to print a Job. Thread-8: Going to print a Job. Thread-7: Going to print a Job. Thread-6: Going to print a Job. Thread-5: Going to print a Job. Thread-4: Going to print a Job. Thread-0: Print a job duration 2 seconds. Thread-3: Going to print a Job. Thread-2: Going to print a Job. Thread-1: Going to print a Job. Thread-0: The Job has been printed. Thread-8: Print a job duration 8 seconds. Thread-8: The Job has been printed. Thread-7: Print a job duration 8 seconds. Thread-7: The Job has been printed. Thread-6: Print a job duration 9 seconds. Thread-6: The Job has been printed. Thread-5: Print a job duration 6 seconds. Thread-5: The Job has been printed. Thread-4: Print a job duration 7 seconds. Thread-4: The Job has been printed. Thread-3: Print a job duration 4 seconds. Thread-3: The Job has been printed. Thread-2: Print a job duration 1 seconds. Thread-2: The Job has been printed. Thread-1: Print a job duration 1 seconds. Thread-1: The Job has been printed.

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轉載于:https://my.oschina.net/nenusoul/blog/794378

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