Java 8+ ConcurrentHashMap 锁 strip 化

Question

我一直在阅读 Concurency in Practice作者:布赖恩·戈茨。

在关于 Lock Striping 的章节中写到 ConcurrentHashMap使用 16 个桶来改进多线程的多线程访问:

Lock splitting can sometimes be extended to partition locking on a variablesized set of independent objects, in which case it is called lock striping. For example, the implementation of ConcurrentHashMap uses an array of 16 locks, each of which guards 1/16 of the hash buckets; bucket N is guarded by lock N mod 16.

我已经阅读了那些问题:

ConcurrentHashMap locking

Need simple explanation how “lock striping” works with ConcurrentHashMap

但是这些答案对 Java 版本 <= 7 有效。

对于 Java 8+，行为似乎发生了显着变化。对于 Java 8+，似乎不是为段而是为表中的特定节点(transient volatile ConcurrentHashMap.Node<K, V>[] table;)获取锁。例如 putVal操作:

ConcurrentHashMap.Node var7;

.... ///retrive node for var7

synchronized(var7) {
....
}

还有来自 Java8 + 领域的 DEFAULT_CONCURRENCY_LEVEL和类 Segment似乎未在实现中使用(它仅在私有(private)方法 writeObject::ObjectOutputStream 中使用，并且此方法未在 ConcurrentHashMap 实现中的任何地方调用)。

1. ConcurrentHashMap 发生如此重大变化的原因是什么？实现？

2. 如果类 Segment是未使用的，也是像DEFAULT_CONCURRENCY_LEVEL这样的字段也未使用 - 为什么不从实现中删除它 - 是否出于某些历史原因？

3. 如果我们不锁定段，就像以前的 Java 版本 <7 一样，仅锁定特定节点是否足够？如果是 - 为什么？这是否意味着我们在这里不需要锁 strip 化？

Answer 1

What is the cause of such significant change in ConcurrentHashMap implementation?

减少内存占用(原因之一)。

We do not want to waste the space required to associate a distinct lock object with each bin, so instead use the first node of a bin list itself as a lock. Locking support for these locks relies on builtin "synchronized" monitors.

^{openjdk > jdk8 > java.util.concurrent.ConcurrentHashMap.java > Line 314}

---

If class Segment is unused and also field like DEFAULT_CONCURRENCY_LEVEL is also unused - why not get rid of it from implementation - is it for some historical reasons?

确保序列化兼容性。

We also declare an unused Segment class that is instantiated in minimal form only when serializing.

^{openjdk > jdk8 > java.util.concurrent.ConcurrentHashMap.java > Line 486}

/**
 * The default concurrency level for this table. Unused but
 * defined for compatibility with previous versions of this class.
 */
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

^{openjdk > jdk8 > java.util.concurrent.ConcurrentHashMap.java > Line 526}

/**
 * Stripped-down version of helper class used in previous version,
 * declared for the sake of serialization compatibility
 */
static class Segment<K,V> extends ReentrantLock implements Serializable {
    private static final long serialVersionUID = 2249069246763182397L;
    final float loadFactor;
    Segment(float lf) { this.loadFactor = lf; }
}

^{openjdk > jdk8 > java.util.concurrent.ConcurrentHashMap.java > Line 1366}

---

If we are not locking on segments, like it used to be for Java version < 7, is locking only on specific node sufficient?

没有。

Using the first node of a list as a lock does not by itself suffice though: When a node is locked, any update must first validate that it is still the first node after locking it, and retry if not. Because new nodes are always appended to lists, once a node is first in a bin, it remains first until deleted or the bin becomes invalidated (upon resizing).

^{openjdk > jdk8 > java.util.concurrent.ConcurrentHashMap.java > Line 320}