利用ConcurrentHashMap和计数器实现锁

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我是靠谱客的博主昏睡小刺猬，这篇文章主要介绍利用ConcurrentHashMap和计数器实现锁，现在分享给大家，希望可以做个参考。

概述

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：

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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 + "]";
        }
    }

}

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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。

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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
        }
    }
}