java 使用ConcurrentHashMap和计数器实现锁
在某些场景下,我们想让线程根据某些业务数据进行排队,简单代码如下：
import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.atomic.AtomicInteger; public class TestServiceImpl { private static ConcurrentHashMap<Long, LockObj> lockMap = new ConcurrentHashMap<Long, LockObj>(40); public void test(Long userId){ LockObj lock = tryLock(userId); synchronized (lock) { try{ //处理业务 } finally{ unLock(lock); } } } private LockObj tryLock(Long key) { LockObj curVal = new LockObj(key); LockObj preVal = lockMap.putIfAbsent(key, curVal); if (null == preVal) { curVal.inc(); return curVal; } else{ preVal.inc(); } return preVal; } private void unLock(LockObj lock){ if (lock.dec() <= 0){ lockMap.remove(lock.getKey()); } } public class LockObj { private long key = 0; private AtomicInteger count = new AtomicInteger(0); public LockObj(long key){ this.key = key; } public int inc(){ return count.incrementAndGet(); } public int dec(){ return count.decrementAndGet(); } public long getKey(){ return key; } @Override public String toString() { return "LockObj [key=" + key + ", count=" + count + "]"; } } }
按照userId来排队,如果每个线程处理数据后不释放锁的话，那么可以不利用计数器。但是加了释放锁的操作，则必须加上计算器。因为当线程把锁释放掉后，还没来得及退出synchronized 代码块时，另外一个线程调用了tryLock方法，那该线程将拿到另外一个对象的锁，导致利用synchronized 关键字进行userId排队失败。
也可以利用guava的API来实现。
import com.google.common.collect.Interner; import com.google.common.collect.Interners; public class TestServiceImpl { Interner<String> pool = Interners.newWeakInterner(); public void test(Long userId) throws OspException { synchronized ( pool.intern(String.valueOf(userId))){ //处理业务操作 } } }