java - Volatile and ArrayBlockingQueue and perhaps other concurrent objects -


i understand (or @ least think do;) ) principle behind volatile keyword. when looking concurrenthashmap source, can see nodes , values declared volatile, makes sense because value can written/read more 1 thread:

static class node<k,v> implements map.entry<k,v> {     final int hash;     final k key;     volatile v val;     volatile node<k,v> next;     ... }

however, looking arrayblockingqueue source, it's plain array being updated/read multiple threads:

private void enqueue(e x) {     // assert lock.getholdcount() == 1;     // assert items[putindex] == null;     final object[] items = this.items;     items[putindex] = x;     if (++putindex == items.length)         putindex = 0;     count++;     notempty.signal(); }

how guaranteed value inserted items[putindex] visible thread, providing element inside array not volatile (i know declaring array doesnt have effect anyhow on elements themselves) ? couldn't thread hold cached copy of array?

thanks

notice enqueue private. calls (offer(e), offer(e, long, timeunit), put(e)). notice every 1 of looks like:

public void put(e e) throws interruptedexception {     checknotnull(e);     final reentrantlock lock = this.lock;     lock.lockinterruptibly();     try {         // stuff.         enqueue(e);     } {         lock.unlock();     } }

so can conclude every call enqueue protected lock.lock() ... lock.unlock() don't need volatile because lock.lock/unlock memory barrier.


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