Java根据某个key加锁怎么实现

一、背景

日常开发中，有时候需要根据某个 key 加锁，确保多线程情况下，对该 key 的加锁和解锁之间的代码串行执行。
大家可以借助每个 key 对应一个 ReentrantLock ，让同一个 key 的线程使用该 lock 加锁；每个 key 对应一个 Semaphore ，让同一个 key 的线程使用 Semaphore 控制同时执行的线程数。

二、参考代码

接口定义

public interface LockByKey {

    /**
     * 加锁
     */
    void lock(T key);

    /**
     * 解锁
     */
    void unlock(T key);
}

2.1 同一个 key 只能一个线程执行

2.1.1 代码实现

每个 key 对应一个 ReentrantLock ，让同一个 key 的线程使用该 lock 加锁。

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.ReentrantLock;

public class DefaultLockByKeyImpl implements LockByKey {

    private final Map lockMap = new ConcurrentHashMap<>();

    /**
     * 加锁
     */
    @Override
    public void lock(T key) {
        // 如果key为空，直接返回
        if (key == null) {
            throw new IllegalArgumentException("key 不能为空");
        }
        
        // 获取或创建一个ReentrantLock对象
        ReentrantLock lock = lockMap.computeIfAbsent(key, k -> new ReentrantLock());
        // 获取锁
        lock.lock();
    }


    /**
     * 解锁
     */
    @Override
    public void unlock(T key) {
        // 如果key为空，直接返回
        if (key == null) {
            throw new IllegalArgumentException("key 不能为空");
        }

        // 从Map中获取锁对象
        ReentrantLock lock = lockMap.get(key);
        // 获取不到报错
        if (lock == null) {
            throw new IllegalArgumentException("key " + key + "尚未加锁");
        }
        // 其他线程非法持有不允许释放
        if (!lock.isHeldByCurrentThread()) {
            throw new IllegalStateException("当前线程尚未持有，key:" + key + "的锁，不允许释放");
        }
        lock.unlock();
    }
}

注意事项：
（1）参数合法性校验
（2）解锁时需要判断该锁是否为当前线程持有

2.1.2 编写单测

import com.google.common.collect.Lists;
import org.junit.Test;

import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class DefaultLockByKeyImplTest {

    private final LockByKey lockByKey = new DefaultLockByKeyImpl<>();

    private final CountDownLatch countDownLatch = new CountDownLatch(7);
    private final ExecutorService executorService = Executors.newFixedThreadPool(10);

    @Test
    public void test() throws InterruptedException {
        List keys = Lists.newArrayList("a", "a", "a", "b", "c", "b", "d");
        Set executingKeySet = new HashSet<>();

        for (int i = 0; i < keys.size(); i++) {
            String key = keys.get(i);
            int finalI = i;
            executorService.submit(() -> {
                lockByKey.lock(key);
                if (executingKeySet.contains(key)) {
                    throw new RuntimeException("存在正在执行的 key:" + key);
                }
                executingKeySet.add(key);

                try {
                    System.out.println("index:" + finalI + "对 [" + key + "] 加锁 ->" + Thread.currentThread().getName());
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    System.out.println("index:" + finalI + "释放 [" + key + "] ->" + Thread.currentThread().getName());
                    lockByKey.unlock(key);
                    executingKeySet.remove(key);
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await();
    }
}

如果同一个 key 没释放能够再次进入，会抛出异常。
也可以通过日志来观察执行情况：

index:0对 [a] 加锁 ->pool-1-thread-1
index:6对 [d] 加锁 ->pool-1-thread-7
index:4对 [c] 加锁 ->pool-1-thread-5
index:3对 [b] 加锁 ->pool-1-thread-4
index:6释放 [d] ->pool-1-thread-7
index:4释放 [c] ->pool-1-thread-5
index:0释放 [a] ->pool-1-thread-1
index:3释放 [b] ->pool-1-thread-4

index:1对 [a] 加锁 ->pool-1-thread-2
index:5对 [b] 加锁 ->pool-1-thread-6
index:1释放 [a] ->pool-1-thread-2
index:5释放 [b] ->pool-1-thread-6

index:2对 [a] 加锁 ->pool-1-thread-3
index:2释放 [a] ->pool-1-thread-3

2.2、同一个 key 可以有 n个线程执行

2.2.1 代码实现

每个 key 对应一个 Semaphore ，让同一个 key 的线程使用 Semaphore 控制同时执行的线程数。

import lombok.SneakyThrows;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Semaphore;

public class SimultaneousEntriesLockByKey implements LockByKey {

    private final Map semaphores = new ConcurrentHashMap<>();

    /**
     * 最大线程
     */
    private int allowed_threads;

    public SimultaneousEntriesLockByKey(int allowed_threads) {
        this.allowed_threads = allowed_threads;
    }

    /**
     * 加锁
     */
    @Override
    public void lock(T key) {
        Semaphore semaphore = semaphores.compute(key, (k, v) -> v == null ? new Semaphore(allowed_threads) : v);
        semaphore.acquireUninterruptibly();
    }


    /**
     * 解锁
     */
    @Override
    public void unlock(T key) {
        // 如果key为空，直接返回
        if (key == null) {
            throw new IllegalArgumentException("key 不能为空");
        }

        // 从Map中获取锁对象
        Semaphore semaphore = semaphores.get(key);
        if (semaphore == null) {
            throw new IllegalArgumentException("key " + key + "尚未加锁");
        }
        semaphore.release();
        if (semaphore.availablePermits() >= allowed_threads) {
            semaphores.remove(key, semaphore);
        }
    }

2.2.2 测试代码

import com.google.common.collect.Lists;
import org.junit.Test;

import java.time.LocalDateTime;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class SimultaneousEntriesLockByKeyTest {

    private final int maxThreadEachKey = 2;
    private final LockByKey lockByKey = new SimultaneousEntriesLockByKey<>(maxThreadEachKey);

    private final CountDownLatch countDownLatch = new CountDownLatch(7);
    private final ExecutorService executorService = Executors.newFixedThreadPool(10);

    @Test
    public void test() throws InterruptedException {
        List keys = Lists.newArrayList("a", "a", "a", "b", "c", "b", "d");
        Map executingKeyCount = Collections.synchronizedMap(new HashMap<>());

        for (int i = 0; i < keys.size(); i++) {
            String key = keys.get(i);
            int finalI = i;
            executorService.submit(() -> {
                lockByKey.lock(key);
                executingKeyCount.compute(key, (k, v) -> {
                    if (v != null && v + 1 > maxThreadEachKey) {
                        throw new RuntimeException("超过限制了");
                    }
                    return v == null ? 1 : v + 1;
                });
                try {
                    System.out.println("time:" + LocalDateTime.now().toString() + " ,index:" + finalI + "对 [" + key + "] 加锁 ->" + Thread.currentThread().getName() + "count:" + executingKeyCount.get(key));
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    System.out.println("time:" + LocalDateTime.now().toString() + " ,index:" + finalI + "释放 [" + key + "] ->" + Thread.currentThread().getName() + "count:" + (executingKeyCount.get(key) - 1));
                    lockByKey.unlock(key);
                    executingKeyCount.compute(key, (k, v) -> v - 1);
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await();
    }
}

输出：

time:2023-03-15T20:49:57.044195 ,index:6对 [d] 加锁 ->pool-1-thread-7count:1
time:2023-03-15T20:49:57.058942 ,index:5对 [b] 加锁 ->pool-1-thread-6count:2
time:2023-03-15T20:49:57.069789 ,index:1对 [a] 加锁 ->pool-1-thread-2count:2
time:2023-03-15T20:49:57.042402 ,index:4对 [c] 加锁 ->pool-1-thread-5count:1
time:2023-03-15T20:49:57.046866 ,index:0对 [a] 加锁 ->pool-1-thread-1count:2
time:2023-03-15T20:49:57.042991 ,index:3对 [b] 加锁 ->pool-1-thread-4count:2
time:2023-03-15T20:49:58.089557 ,index:0释放 [a] ->pool-1-thread-1count:1
time:2023-03-15T20:49:58.082679 ,index:6释放 [d] ->pool-1-thread-7count:0
time:2023-03-15T20:49:58.084579 ,index:4释放 [c] ->pool-1-thread-5count:0
time:2023-03-15T20:49:58.083462 ,index:5释放 [b] ->pool-1-thread-6count:1
time:2023-03-15T20:49:58.089576 ,index:3释放 [b] ->pool-1-thread-4count:1
time:2023-03-15T20:49:58.085359 ,index:1释放 [a] ->pool-1-thread-2count:1
time:2023-03-15T20:49:58.096912 ,index:2对 [a] 加锁 ->pool-1-thread-3count:1
time:2023-03-15T20:49:59.099935 ,index:2释放 [a] ->pool-1-thread-3count:0