ConcurrentHashMap的优雅使用

ConcurrentHashMap的优雅使用

相对于hashMap,ConcurrentHashMap则具有原子性的读写特性，但是ConcurrentHashMap的聚合方法（例如：size、isEmpty 和 containsValue…）在并发场景下，可能只是一个参考值，不可用于流程的控制。在一个线程pullAll的时候，另一个线程获取size，此时的size就是错误的。

要想避免上述问题，则可以通过添加synchronized来控制

import lombok.extern.slf4j.Slf4j;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.TimeUnit;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.LongStream;


@Slf4j
public class ConcurrentHashMapMain {

    private static int THREAD_COUNT = 10;
    private static int ITEM_COUNT = 1000;

    //生成指定大小的ConcurrentHashMap
    private static ConcurrentHashMap<String, Long> getConcurrentHashMapData(int count) {
        return LongStream.range(1, count+1)
                //针对基础类型参数需要添加boxed才可用collect。
                .boxed()
                //新建一个Stream 内容为1到count，然后以这个流创建一个ConcurrentHashMap
                .collect(Collectors.toConcurrentMap(i -> UUID.randomUUID().toString(), Function.identity(),
                        (o1, o2) -> o1, ConcurrentHashMap::new));
    }


    public static void main(String[] args) throws InterruptedException {
        errorTest();
        successTest();
    }

    public static void errorTest() throws InterruptedException {

        //创建900个item的ConcurrentHashMap
        ConcurrentHashMap<String, Long> concurrentHashMap = getConcurrentHashMapData(ITEM_COUNT - 100);
        log.info("concurrentHashMap.size{}", concurrentHashMap.size());


        ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
        forkJoinPool.execute(() -> IntStream.rangeClosed(1, 12).parallel().forEach(i -> {
            int needAddNumber = ITEM_COUNT - concurrentHashMap.size();
            log.info("concurrentHashMap.size{}", concurrentHashMap.size());
            concurrentHashMap.putAll(getConcurrentHashMapData(needAddNumber));
        }));
        forkJoinPool.shutdown();
        forkJoinPool.awaitTermination(10, TimeUnit.MINUTES);

        log.info("after add, concurrentHashMap.size{}", concurrentHashMap.size());
    }


    public static void successTest() throws InterruptedException {
        ConcurrentHashMap<String, Long> concurrentHashMap = getConcurrentHashMapData(ITEM_COUNT - 100);
        log.info("concurrentHashMap.size{}", concurrentHashMap.size());

        ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
        forkJoinPool.execute(() -> IntStream.rangeClosed(1, 12).parallel().forEach(i -> {
            //给concurrentHashMap 加个锁，保证只有一个线程可以处理
            synchronized (concurrentHashMap) {
                int gap = ITEM_COUNT - concurrentHashMap.size();
                log.info("concurrentHashMap.size{}", concurrentHashMap.size());
                concurrentHashMap.putAll(getConcurrentHashMapData(gap));
            }
        }));
        forkJoinPool.shutdown();
        forkJoinPool.awaitTermination(10, TimeUnit.MINUTES);
        log.info("after add, concurrentHashMap.size{}", concurrentHashMap.size());

    }
}

运行结果为：

09:33:44.235 [main] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-9] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-11] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-6] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-2] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-15] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-4] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-1] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-13] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-8] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.244 [ForkJoinPool-1-worker-10] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.249 [ForkJoinPool-1-worker-11] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1264
09:33:44.249 [ForkJoinPool-1-worker-1] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1272
09:33:44.250 [main] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - after add, concurrentHashMap.size1900
09:33:44.254 [main] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.255 [ForkJoinPool-2-worker-9] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size900
09:33:44.256 [ForkJoinPool-2-worker-10] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.256 [ForkJoinPool-2-worker-10] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.256 [ForkJoinPool-2-worker-8] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.256 [ForkJoinPool-2-worker-1] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-13] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-2] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-11] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-15] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-4] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-6] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.257 [ForkJoinPool-2-worker-9] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - concurrentHashMap.size1000
09:33:44.258 [main] INFO com.arno.train.ConcurrentHash.ConcurrentHashMapMain - after add, concurrentHashMap.size1000