同步块中的Java内存模型

Question

I have confuse about memory model in java, the example as following code :

 /**
  * one thread write request and wait for response
  */
public AbstractPacket writeMessageAndWaitForResp(AbstractPacket packet, int waitTimeoutInSecond) {
        if (!(packet instanceof SendToRouter)) {
            throw new IllegalArgumentException("this msg can not be sent to router!");
        }

        int command = packet.getResponseCommand();
        AbstractPacket[] slot = new AbstractPacket[]{NullPacket.NULL};
        synchronized (("" + this.getFactoryId() + this.getRouterNo() + command).intern()) {// prevent same command request re-entry
            command2RespMap.put(command, slot);
            synchronized (slot) { // prevent notify before wait
                ChannelFuture future = writeMessage(packet);
                if (future == null) {
                    command2RespMap.remove(command);
                    return null;
                }

                try {
                    slot.wait(waitTimeoutInSecond * 1000);
                } catch (InterruptedException e) {
                    logger.error(e.getMessage(), e);
                }
            }
            command2RespMap.remove(command);
        }
        AbstractPacket result = slot[0]; // get out the result outof slot array
        if (result == NullPacket.NULL) {
            logger.error("receive sync message timeout!");
            return null;
        }
        return result;
}

 /**
  * another thread write response and notify the waiter
  */
 public void routerResponse(AbstractPacket packet) {
        int command = packet.getHeadCommand();
        AtomicReference<AbstractPacket> slot = command2RespMap.get(command);
        if (slot == null || slot.get() != NullPacket.NULL) {
            logger.error("command : {} request not exist !", command);
            return;
        }
        synchronized (slot) {
            slot[0] = packet;
            slot.notify();
        }
}

My question is , in the first function, I got the result from slot in index 0 out of the synchronized block on slot variable.

does this mean the slot[0] may not contains the value set by the second function in another thread ?

very thanks !

Answer 1

synchronized key words, as a memory barrier, guarantees 2 things

1. The compiler might swap the ordering of some statements in your program. However, upon seeing a synchronized block, any statement in the block won't be re-ordered or mixed with the statements outside of the synchronized block.

2. Inside the block, any variables would have it's value fetched from main memory and the thread local copy and/or the L2 cache/register cached value would be discarded. Upon the end of the block, the variables modified in the block would be written back to the main memory so that all other threads would read up the same value.

So, since you refer to slot[0] outside of the synchronized block, there is no guarantee that this reference would be consistent with the value you updated inside the synchronized block, also no guarantee if it would be a thread local cached value or from main memory.

BTW as @chrylis mentioned, your block is super expensive and you need to consider at least moving the wait() out.