为什么ConcurrentHashMap中的get方法是阻塞的？

Question

I have a question on ConcurrentHashMap in java. It internally calls readValueUnderLock. Why locking is required in case of get operation. And in which case this condition will be true (Entry.value==null) which cause readValueUnderLock to be called)

Answer 1

From the source code java doc comment of readValueUnderLock

  /**
     * Reads value field of an entry under lock. Called if value
     * field ever appears to be null. This is possible only if a
     * compiler happens to reorder a HashEntry initialization with
     * its table assignment, which is legal under memory model
     * but is not known to ever occur.
     */

From this link

Not quite. You are right that it should never be called. However, the JLS/JMM can be read as not absolutely forbidding it from being called because of weaknesses in required ordering relationships among finals vs volatiles set in constructors (key is final, value is volatile), wrt the reads by threads using the entry objects. (In JMM-ese, ordering constraints for finals fall outside of the synchronizes-with relation.) That's the issue the doc comment (pasted below) refers to. No one has ever thought of any practical loophole that a processor/compiler might find to produce a null value read, and it may be provable that none exist (and perhaps someday a JLS/JMM revision will fill in gaps to clarify this), but Bill Pugh once suggested we put this in anyway just for the sake of being conservatively pedantically correct. In retrospect, I'm not so sure this was a good idea, since it leads people to come up with exotic theories.

Answer 2

In order to read a value from a hash map the code must find the value first. If another thread adds a value while the first thread is finding the value it could derail the search. Essentially the hash map could do something like:

calculate hash
go to location hash in the array
look to see if there's a list
iterate through the list until value is found

If this list is say an array list and the other thread needs to resize it, this would be a big problem for the thread iterating through it.