我要评论一、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<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。
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
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：
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<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
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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