Strange Issue With Static Global Variable in Native

[Dan Xu]

Hi Chris,

Thanks for your detailed explanation. I understand the issue now. 
Thanks!

-Dan


On Mon 09 Dec 2013 02:09:52 AM PST, Chris Hegarty wrote:
> On 08/12/13 17:39, Dan Xu wrote:
>>
>> On 12/08/2013 01:34 AM, Chris Hegarty wrote:
>>>
>>> On 07/12/2013 18:12, Dan Xu wrote:
>>>> ...
>>>>> Just so I understand, did you use a JNI global when caching the
>>>>> reference?
>>>>>
>>>>> -Alan
>>>> Hi Alan,
>>>>
>>>> What is a JNI global?
>>>
>>> http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html#wp1242
>>>
>>>
>>>
>>> > What I use here is a static global variable. I
>>>> have uploaded this file to
>>>> http://cr.openjdk.java.net/~dxu/8025437/DefaultProxySelector1.c.
>>>> You can
>>>> search for "no_proxy". I have declared this variable as "static
>>>> jobject'
>>>> at the beginning, and initialized it in function initJavaClass().
>>>
>>> You can only store global, not local, refs across multiple
>>> calls/contexts.
>>>
>>> In your case it would look something like:
>>>
>>> static jobject setNoProxy(JNIEnv *env) {
>>>               jobject empty_proxy = NULL;
>>>               jfieldID pr_no_proxyID = NULL;
>>>                 pr_no_proxyID = (*env)->GetStaticFieldID(env,
>>> proxy_class,
>>>                         "NO_PROXY", "Ljava/net/Proxy;");
>>>                 if (proxy_class && pr_no_proxyID) {
>>>                     printf("create NO_PROXY\n");
>>>                     empty_proxy = (*env)->GetStaticObjectField(env,
>>> proxy_class, pr_no_proxyID);
>>>
>>>                     empty_proxy = (*env)->NewGlobalRef(env,
>>> empty_proxy);
>>>                 }
>>>                 return empty_proxy;
>>> }
>>>
>>> -Chris.
>>
>> Do you mean (*env)->GetStaticObjectField() return me a local reference?
>
> Yes. Quote from the spec, "All Java objects returned by JNI functions
> are local references. "
>
>> When shall I call NewGlobalRef()?
>
> I included the NewGlobalRef call in the code above (a modified version
> of setNoProxy). Directly after GetStaticObjectField.
>
> In Proxy.java, this static field has
>> already been created by "public final static Proxy NO_PROXY = new
>> Proxy();" And I still need call NewGlobalRef() to make it global, right?
>
> Yes. You are adding a new reference to this object, from JNI code.
> This reference needs to be registered with the GC so it can be updated
> if the referent is relocated.
>
>> But why is it not necessary for me to call NewGlobalRef() for
>> ptype_httpID and ptype_socksID? Thanks!
>
> These are not reference to Java Objects, they are of type jmethodID.
> You can think of them as pointers/offsets into particular type, and
> are not subject to change.
>
> -Chris.
>
>>
>> -Dan