为什么 TreeBin 在 ConcurrentHashMap 中维护一个读写锁？
why TreeBin maintains a read-write lock in ConcurrentHashMap?
ConcurrentHashMap的TreeBin维护着一个寄生读写锁。谁能告诉我为什么要维护读写锁？
代码中有注释是这样解释TreeBin寄生锁策略的：
/*
* TreeBins also require an additional locking mechanism. While
* list traversal is always possible by readers even during
* updates, tree traversal is not, mainly because of tree-rotations
* that may change the root node and/or its linkages. TreeBins
* include a simple read-write lock mechanism parasitic on the
* main bin-synchronization strategy: Structural adjustments
* associated with an insertion or removal are already bin-locked
* (and so cannot conflict with other writers) but must wait for
* ongoing readers to finish. Since there can be only one such
* waiter, we use a simple scheme using a single "waiter" field to
* block writers. However, readers need never block. If the root
* lock is held, they proceed along the slow traversal path (via
* next-pointers) until the lock becomes available or the list is
* exhausted, whichever comes first. These cases are not fast, but
* maximize aggregate expected throughput.
*/
"why" 由后面的评论解释：
/**
* TreeNodes used at the heads of bins. TreeBins do not hold user
* keys or values, but instead point to list of TreeNodes and
* their root. They also maintain a parasitic read-write lock
* forcing writers (who hold bin lock) to wait for readers (who do
* not) to complete before tree restructuring operations.
*/
简而言之，总体锁定策略是避免在读取路径上锁定。所以map分为"bins"，每个bin有很多reader / single writer lock.
除了 reader 如果存在锁争用实际上不会被阻塞。相反，他们遍历整个箱子……回头检查锁是否已被释放。 "parasitic" 锁定就是它的实现。
ConcurrentHashMap的TreeBin维护着一个寄生读写锁。谁能告诉我为什么要维护读写锁？
代码中有注释是这样解释TreeBin寄生锁策略的：
/*
* TreeBins also require an additional locking mechanism. While
* list traversal is always possible by readers even during
* updates, tree traversal is not, mainly because of tree-rotations
* that may change the root node and/or its linkages. TreeBins
* include a simple read-write lock mechanism parasitic on the
* main bin-synchronization strategy: Structural adjustments
* associated with an insertion or removal are already bin-locked
* (and so cannot conflict with other writers) but must wait for
* ongoing readers to finish. Since there can be only one such
* waiter, we use a simple scheme using a single "waiter" field to
* block writers. However, readers need never block. If the root
* lock is held, they proceed along the slow traversal path (via
* next-pointers) until the lock becomes available or the list is
* exhausted, whichever comes first. These cases are not fast, but
* maximize aggregate expected throughput.
*/
"why" 由后面的评论解释：
/**
* TreeNodes used at the heads of bins. TreeBins do not hold user
* keys or values, but instead point to list of TreeNodes and
* their root. They also maintain a parasitic read-write lock
* forcing writers (who hold bin lock) to wait for readers (who do
* not) to complete before tree restructuring operations.
*/
简而言之，总体锁定策略是避免在读取路径上锁定。所以map分为"bins"，每个bin有很多reader / single writer lock.
除了 reader 如果存在锁争用实际上不会被阻塞。相反，他们遍历整个箱子……回头检查锁是否已被释放。 "parasitic" 锁定就是它的实现。