寫一個併發計數器

首先給出一個併發場景下，錯誤的計數器方法：

public class CountExample {
    // 請求總數
    public static int clientTotal = 5000;
    // 同時併發執行的線程數
    public static int threadTotal = 200;
    public static int count = 0;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }

    private static void add() {
        count++;
    }
}

運行多次輸出的結果不一樣：

第一次輸出：

21:45:05.233 [main] INFO com.tim.concurrency.example.CountExample - count:4975.

第二次輸出：

21:47:08.113 [main] INFO com.tim.concurrency.example.CountExample - count:4972.

分析：

上述代碼想實現的功能是200個併發線程進行5000次請求，並對cout值進行加1的操作。

改進策略1 （通過AtomicLong）：

@Slf4j
public class CountExample {
    // 請求總數
    public static int clientTotal = 5000;
    // 同時併發執行的線程數
    public static int threadTotal = 200;
    public static AtomicLong count = new AtomicLong(0);

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }

    private static void add() {
        count.getAndAdd(1);
    }
}

改進策略2（通過synchronized同步字段）：

public class CountExample {
    // 請求總數
    public static int clientTotal = 5000;
    // 同時併發執行的線程數
    public static int threadTotal = 200;
    public static int count = 0;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }

    private synchronized static void add() {
        count++;
    }
}

改進策略3（通過LongAdder）：

@Slf4j
public class CountExample {
    // 請求總數
    public static int clientTotal = 5000;
    // 同時併發執行的線程數
    public static int threadTotal = 200;
    public static LongAdder count = new LongAdder();

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }
    private static void add() {
        count.increment();
    }
}