I got it, thanks!
(2012/12/26 19:17), Florent Guillaume wrote:
> On Tue, Dec 25, 2012 at 2:45 PM, Chen Huajun<chenhj@cn.fujitsu.com> wrote:
>> Now ,Could you explain the the detail of following issues?
>>> At first glance, there are a couple of issues:
>>>
>>> 1. Double-Checked Locking in reportHostStatus, which is bad form
>>
>> I knows a problem of Double-Checked Locking,while used in singleton pattern
>> as following.
>>
>> public static Singleton getInstance() {
>> if (instance == null) {
>> synchronized (Singleton.class) {
>> if (instance == null) {
>> instance = new Singleton();
>> }
>> }
>> }
>> return instance;
>> }
>>
>> because JVM would run "instance = new Singleton(); " as that :
>>
>> mem = allocate();
>> instance = mem;
>> ctorSingleton(instance);
>>
>> Do you think my code has the same problem or just it looks ugly?
>
> Double-checked locking generally is incorrect, and does not work. It
> ONLY works if you're double-checking a volatile variable and using
> Java>= 5.
> Please read http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
>
>
>>> 3. No need for 'volatile' if you're also using 'synchronized'
>>
>> 'synchronized' is only used for write,'volatile' is for read.
>
> This sentence shows you're confused about synchronization and
> multi-threading in Java.
>
>> It's for performance and is a bit complex.(It may be a excessive design)
>> I worry about JVM will optimize the following code
>>
>> HashMap<String,HostStatus> newHostStatusMap = (HashMap<String,
>> HostStatus>) hostStatusCache.clone();
>> newHostStatusMap.put(hostSpecKey, hostStatus);
>> hostStatusCache=newHostStatusMap;
>> as:
>> hostStatusCache=(HashMap<String, HostStatus>) hostStatusCache.clone();
>> hostStatusCache.put(hostSpecKey, hostStatus);
>>
>> do you know if it will happen?
>
> Of course it can. The JVM is free to reorder a lot of things while
> respecting the Java Memory Model.
>
> All this means you shouldn't try to play games with the JVM, just use
> a basic lock or synchronization primitive where you need it.
>
> Florent
>
> --
> Florent Guillaume, Director of R&D, Nuxeo
> Open Source, Java EE based, Enterprise Content Management (ECM)
> http://www.nuxeo.com http://www.nuxeo.org +33 1 40 33 79 87
>
>
--
Best Regards,
Chen Huajun
