使用线程安全的集合替代ConcurrentHashMap例如CopyOnWriteArrayList,写入时拷贝数组副本,加锁只锁定单次写入的线程。或者直接用 Collections.synchronizedMap 包装 HashMap 来实现线程安全。
分段锁技术,仅锁定链表或树链节点,而不是整个Map。
分批加载数据,每次只加载一部分进Map。
避免高并发写入,提前扩容存储空间。
并发写入ConcurrentHashMap需要非常小心,通过加锁、合并函数、线程安全集合、分段锁等技术来保证数据一致性。
@RestController
@RequestMapping("concurrenthashmapmisuse")
@Slf4j
public class ConcurrentHashMapMisuseController {
private static int THREAD_COUNT = 10;
private static int ITEM_COUNT = 1000;
private ConcurrentHashMap<String, Long> getData(int count) {
return LongStream.rangeClosed(1, count)
.boxed()
.collect(Collectors.toConcurrentMap(i -> UUID.randomUUID().toString(), Function.identity(),
(o1, o2) -> o1, ConcurrentHashMap::new));
}
@GetMapping("wrong")
public String wrong() throws InterruptedException {
ConcurrentHashMap<String, Long> concurrentHashMap = getData(ITEM_COUNT - 100);
log.info("init size:{}", concurrentHashMap.size());
ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
forkJoinPool.execute(() -> IntStream.rangeClosed(1, 10).parallel().forEach(i -> {
int gap = ITEM_COUNT - concurrentHashMap.size();
log.info("gap size:{}", gap);
concurrentHashMap.putAll(getData(gap));
}));
forkJoinPool.shutdown();
forkJoinPool.awaitTermination(1, TimeUnit.HOURS);
log.info("finish size:{}", concurrentHashMap.size());
return "OK";
}
@GetMapping("right")
public String right() throws InterruptedException {
ConcurrentHashMap<String, Long> concurrentHashMap = getData(ITEM_COUNT - 100);
log.info("init size:{}", concurrentHashMap.size());
ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
forkJoinPool.execute(() -> IntStream.rangeClosed(1, 10).parallel().forEach(i -> {
synchronized (concurrentHashMap) {
int gap = ITEM_COUNT - concurrentHashMap.size();
log.info("gap size:{}", gap);
concurrentHashMap.putAll(getData(gap));
}
}));
forkJoinPool.shutdown();
forkJoinPool.awaitTermination(1, TimeUnit.HOURS);
log.info("finish size:{}", concurrentHashMap.size());
return "OK";
}
}
学习：Java 业务开发常见错误 100 例学习笔记
