java - Oracle Java ConcurrentHashMap 的错误实现？

Question

我正在测试 ConcurrentHashMap关于 Oracle 的 Java 8 实现:

ConcurrentMap<String, String> concurrentMap = new ConcurrentHashMap<>();
String result = concurrentMap.computeIfAbsent("A", k -> "B");
System.out.println(result);  // "B"
result = concurrentMap.putIfAbsent("AA", "BB");
System.out.println(result);  // null

Javadoc of computeIfAbsent是这么说的

Implementation Requirements:

The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent:

if (map.get(key) == null) {
    V newValue = mappingFunction.apply(key);
    if (newValue != null)
        return map.putIfAbsent(key, newValue);
}

它说然后返回当前值，如果现在不存在则返回 null。那么它不应该返回 null 吗？假设 putIfAbsent 也返回 null。

我在这里错过了什么？

Answer 1

ConcurrentMap.computeIfAbsent 的代码示例没有反射(reflect)实际意图，很可能是 putIfAbsent 的非直觉行为导致的错误，而实现遵循记录的意图。这已在 JDK-8174087 中报告 和 fixed in Java 9

请注意 Map.computeIfAbsent 的契约(Contract)是

Implementation Requirements:

The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent:

if (map.get(key) == null) {
    V newValue = mappingFunction.apply(key);
    if (newValue != null)
        map.put(key, newValue);
}

省略 return 语句。但是明明说

Returns:

the current (existing or computed) value associated with the specified key, or null if the computed value is null

是ConcurrentMap.computeIfAbsent的文档试图合并并发方面，落入 putIfAbsent 的非直观行为:

Implementation Requirements:

The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent:

if (map.get(key) == null) {
    V newValue = mappingFunction.apply(key);
    if (newValue != null)
        return map.putIfAbsent(key, newValue);
}

但它还是说

Returns:

the current (existing or computed) value associated with the specified key, or null if the computed value is null

并且记录的意图应该优先于代码示例。请注意，实际的 default implementation of ConcurrentMap.computeIfAbsent符合记录的意图:

@Override
default V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
    Objects.requireNonNull(mappingFunction);
    V v, newValue;
    return ((v = get(key)) == null &&
            (newValue = mappingFunction.apply(key)) != null &&
            (v = putIfAbsent(key, newValue)) == null) ? newValue : v;
}

所以 the implementation of ConcurrentHashMap.computeIfAbsent确实符合关于返回值的 ConcurrentMap.computeIfAbsent 和 Map.computeIfAbsent 的文档意图，并且也等效于提供的 default 实现通过接口(interface)。

为了完整起见，Map.computeIfAbsent 的默认 实现是

default V computeIfAbsent(K key,
        Function<? super K, ? extends V> mappingFunction) {
    Objects.requireNonNull(mappingFunction);
    V v;
    if ((v = get(key)) == null) {
        V newValue;
        if ((newValue = mappingFunction.apply(key)) != null) {
            put(key, newValue);
            return newValue;
        }
    }

    return v;
}