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
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public interface LockByKey<T> { /** * 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。
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import java.util.Map;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.locks.ReentrantLock;public class DefaultLockByKeyImpl<T> implements LockByKey<T> { private final Map<T, ReentrantLock> 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
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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<String> lockByKey = new DefaultLockByKeyImpl<>(); private final CountDownLatch countDownLatch = new CountDownLatch(7); private final ExecutorService executorService = Executors.newFixedThreadPool(10); @Test public void test() throws InterruptedException { List<String> keys = Lists.newArrayList("a", "a", "a", "b", "c", "b", "d"); Set<String> 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：
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index:0对 [a] 加锁 ->pool-1-thread-1index:6对 [d] 加锁 ->pool-1-thread-7index:4对 [c] 加锁 ->pool-1-thread-5index:3对 [b] 加锁 ->pool-1-thread-4index:6释放 [d] ->pool-1-thread-7index:4释放 [c] ->pool-1-thread-5index:0释放 [a] ->pool-1-thread-1index:3释放 [b] ->pool-1-thread-4index:1对 [a] 加锁 ->pool-1-thread-2index:5对 [b] 加锁 ->pool-1-thread-6index:1释放 [a] ->pool-1-thread-2index:5释放 [b] ->pool-1-thread-6index:2对 [a] 加锁 ->pool-1-thread-3index: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。
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import lombok.SneakyThrows;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.Semaphore;public class SimultaneousEntriesLockByKey<T> implements LockByKey<T> { private final Map<T, Semaphore> 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
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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<String> 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<String> keys = Lists.newArrayList("a", "a", "a", "b", "c", "b", "d"); Map<String, Integer> 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
三、总结
本文结合自己的理解和一些参考代码，给出自己的示例，希望对大家有帮助。
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原文链接：https://blog.csdn.net/w605283073/article/details/129568863
