使用ThreadLocal实现的计数器

最新推荐文章于 2025-06-20 13:58:57 发布

Jack_Weng

最新推荐文章于 2025-06-20 13:58:57 发布

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CC 4.0 BY-SA版权

文章标签： Java ThreadLocal

本文链接：https://blog.youkuaiyun.com/wj310298/article/details/79047110

本文介绍了一种利用ThreadLocal实现计数器的方法，并解决了进程回收导致计数丢失的问题。通过不同版本迭代优化，最终实现了稳定可靠的全局计数器。

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今天脑子里闪过使用ThreadLocal实现计数器的念头，百度了一下，没有讲到怎么聚合所有进程各自的计数器值。所以自己实现一个，代码如下。

import java.util.WeakHashMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThradeLocalTest {
    static class Counter {
        private static class Entry {
            long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;

            long count = 0;
        }

        private static Lock lock = new ReentrantLock();
        private static WeakHashMap<Thread, Entry> map = new WeakHashMap<>();

        private static ThreadLocal<Entry> local = ThreadLocal.withInitial(Entry::new);

        public void increase() {
            Entry entry = local.get();
            long count = entry.count;
            if (count == 0) {
                lock.lock();
                try {
                    map.put(Thread.currentThread(), entry);
                } finally {
                    lock.unlock();
                }
            }
            local.get().count = count + 1;
        }

        public long getAll() {
            return map.entrySet().stream().map(entry->entry.getValue().count).reduce(0L, Long::sum);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        int number = 100;
        Thread[] threads = new Thread[number];
        for (int i = 0; i < number; i++) {
            threads[i] = new Thread(()->{
                for (int j = 0; j < 100_000_000; j++) {
                    counter.increase();
                }
            });
        }
        for (Thread thread1 : threads) {
            thread1.start();
        }
        System.out.println(counter.getAll());
        for (Thread thread2 : threads) {
            thread2.join();
        }
        System.out.println(counter.getAll());
    }
}

该代码有个问题，就是如果Thread被回收了，对应的计数就丢失了，所以需要自己实现一下存储计数使用的map，改完之后发代码。

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadLocalTest {
    static class Counter {
        private static class Entry {
            long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;

            long count = 0;
        }

        private volatile long removedTotal = 0;
        long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;

        private Lock lock = new ReentrantLock();
        private Map<WeakReference<Thread>, Entry> map = new HashMap<>();
        private ReferenceQueue<Object> queue = new ReferenceQueue<>();

        private final ThreadLocal<Entry> local = ThreadLocal.withInitial(Entry::new);

        public void increase() {
            Entry entry = local.get();
            long count = entry.count;
            if (count == 0) {
                lock.lock();
                try {
                    map.put(new WeakReference<>(Thread.currentThread(), queue), entry);
                } finally {
                    lock.unlock();
                }
            }
            expunge();
            local.get().count = count + 1;
        }

        private void expunge() {
            for (Object x; (x = queue.poll()) != null; ) {
                synchronized (queue) {
                    lock.lock();
                    try {
                        Entry e = map.get(x);
                        map.remove(x);
                        removedTotal += e.count;
                    } finally {
                        lock.unlock();
                    }
                }
            }
        }

        public long getAll() {
            return map.entrySet().stream().map(entry->entry.getValue().count).reduce(0L, Long::sum) + removedTotal;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        int number = 4;
        Thread[] threads = new Thread[number];
        for (int i = 0; i < number; i++) {
            threads[i] = new Thread(()->{
                for (int j = 0; j < 100_000_000; j++) {
                    counter.increase();
                }
            });
        }
        for (Thread thread1 : threads) {
            thread1.start();
        }
        for (Thread thread2 : threads) {
            thread2.join();
        }
        for (int i = 0; i < number; i++) {
            threads[i] = new Thread(()->{
                for (int j = 0; j < 100_000_000; j++) {
                    counter.increase();
                }
            });
        }
        long start = System.currentTimeMillis();
        for (Thread thread1 : threads) {
            thread1.start();
        }
        for (Thread thread2 : threads) {
            thread2.join();
        }
        System.out.println(System.currentTimeMillis() - start);
        System.out.println(counter.getAll());

        start = System.currentTimeMillis();
        for (long i = 0; i < 800_000_000L; i++) {}
        System.out.println(System.currentTimeMillis() - start);
    }
}

极端情况啥的还没测试，没有触及gc。

又修改了一个版本，不使用ThreadLocal了，创建一个subCounter用于每个线程计数。

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadLocalTest {
    static class Counter {
        private static class Entry {
            long count = 0;
        }

        public interface SubCounter {
            void increase();
        }

        private class SubCounterImpl implements SubCounter {
            private final Entry entry;

            SubCounterImpl(Entry entry) {
                this.entry = entry;
            }

            public void increase() {
                entry.count++;
            }
        }

        private volatile long removedTotal = 0;
        long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;

        private final Lock lock = new ReentrantLock();
        private final Map<WeakReference<SubCounter>, Entry> map = new HashMap<>();
        private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

        public SubCounter createSubCounter() {
            expunge();
            Entry entry = new Entry();
            SubCounter subCounter = new SubCounterImpl(entry);
            lock.lock();
            try {
                map.put(new WeakReference<>(subCounter, queue), entry);
            } finally {
                lock.unlock();
            }
            return subCounter;
        }

        private void expunge() {
            for (Object x; (x = queue.poll()) != null; ) {
                synchronized (queue) {
                    lock.lock();
                    try {
                        Entry e = map.get(x);
                        map.remove(x);
                        removedTotal += e.count;
                    } finally {
                        lock.unlock();
                    }
                }
            }
        }

        public long getAll() {
            expunge();
            return map.entrySet().stream().map(entry->entry.getValue().count).reduce(0L, Long::sum) + removedTotal;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        int number = 4;
        Thread[] threads = new Thread[number];
        for (int i = 0; i < number; i++) {
            threads[i] = new Thread(new Runnable() {
                Counter.SubCounter subCounter = counter.createSubCounter();
                @Override
                public void run() {
                    for (int j = 0; j < 100_000_000; j++) {
                        subCounter.increase();
                    }
                }
            }
            );
        }
        for (Thread thread1 : threads) {
            thread1.start();
        }
        for (Thread thread2 : threads) {
            thread2.join();
        }
        for (int i = 0; i < number; i++) {
            threads[i] = new Thread(new Runnable() {
                Counter.SubCounter subCounter = counter.createSubCounter();
                @Override
                public void run() {
                    for (int j = 0; j < 100_000_000; j++) {
                        subCounter.increase();
                    }
                }
            }
            );
        }
        long start = System.currentTimeMillis();
        for (Thread thread1 : threads) {
            thread1.start();
        }
        for (Thread thread2 : threads) {
            thread2.join();
        }
        System.out.printf("Time:%d\n", System.currentTimeMillis() - start);
        System.out.printf("Count:%d\n", counter.getAll());

        start = System.currentTimeMillis();
        int intCount = 0;
        for (long i = 0; i < 800_000_000L; i++) {
            intCount++;
        }
        System.out.printf("Time:%d\n", System.currentTimeMillis() - start);
        System.out.printf("Count:%d\n", intCount);

        // 测试垃圾回收
        Counter counter2 = new Counter();
        start = System.currentTimeMillis();
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 200_000; j++) {
                counter2.createSubCounter().increase();
            }
        }
        System.out.printf("Time:%d\n", System.currentTimeMillis() - start);
        System.out.printf("Count:%d\n", counter2.getAll());
        System.out.printf("counter size:%d\n", counter.map.size());
        System.out.printf("counter2 size:%d\n", counter2.map.size());
    }
}

配置最大运行内存为2M，运行结果如下：

Time:29
Count:800000000
Time:1193
Count:800000000
Time:5945
Count:400000
counter size:5
counter2 size:504