GetAndPut操作线程安全性

问题描述

　　　在使用map时经常有这样的诉求：首先通过get判断元素是否存在，如果存在则返回；否则，通过put添加新的元素，整个逻辑要求是线程安全的。

putIfAbsent

Hystrix 熔断器Factory的实现逻辑如下所示。
　　优点：性能比使用synchronized高；
　　缺点：并发高时，会有额外的HystrixCircuitBreakerImpl实例被创建；

public static class Factory {
        private static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();

        public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
            // this should find it for all but the first time
            HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());
            if (previouslyCached != null) {
                return previouslyCached;
            }

            // if we get here this is the first time so we need to initialize

            // Create and add to the map ... use putIfAbsent to atomically handle the possible race-condition of
            // 2 threads hitting this point at the same time and let ConcurrentHashMap provide us our thread-safety
            // If 2 threads hit here only one will get added and the other will get a non-null response instead.
            HystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));
            if (cbForCommand == null) {
                // this means the putIfAbsent step just created a new one so let's retrieve and return it
                return circuitBreakersByCommand.get(key.name());
            } else {
                // this means a race occurred and while attempting to 'put' another one got there before
                // and we instead retrieved it and will now return it
                return cbForCommand;
            }
        }

   }

synchronized

注意：加锁后需要再次get判断value是否存在。

public static HystrixCommandMetrics getInstance(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, HystrixThreadPoolKey threadPoolKey, HystrixCommandProperties properties) {
        // attempt to retrieve from cache first
        HystrixCommandMetrics commandMetrics = metrics.get(key.name());
        if (commandMetrics != null) {
            return commandMetrics;
        } else {
            synchronized (HystrixCommandMetrics.class) {
                HystrixCommandMetrics existingMetrics = metrics.get(key.name());
                if (existingMetrics != null) {
                    return existingMetrics;
                } else {
                    HystrixThreadPoolKey nonNullThreadPoolKey;
                    if (threadPoolKey == null) {
                        nonNullThreadPoolKey = HystrixThreadPoolKey.Factory.asKey(commandGroup.name());
                    } else {
                        nonNullThreadPoolKey = threadPoolKey;
                    }
                    HystrixCommandMetrics newCommandMetrics = new HystrixCommandMetrics(key, commandGroup, nonNullThreadPoolKey, properties, HystrixPlugins.getInstance().getEventNotifier());
                    metrics.putIfAbsent(key.name(), newCommandMetrics);
                    return newCommandMetrics;
                }
            }
        }
    }