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From "Tingpeng Wu" <wutingp...@gmail.com>
Subject Re: problem about System.loadLibrary and object's address
Date Sun, 29 Apr 2007 10:16:50 GMT

----- Original Message ----- 
From: "LvJimmy,Jing" <firepure@gmail.com>
To: <dev@harmony.apache.org>
Sent: Sunday, April 29, 2007 5:24 PM
Subject: Re: problem about System.loadLibrary and object's address


> 2007/4/29, Tingpeng Wu <wutingpeng@gmail.com>:
>> Hi, xiaofeng,
>>
>> 1. I also feel very unnatural to call it through MyRuntime class, but I don't know
how to call it directly. Since drlvm needs HotSpot to help it to compile the java source code,
if I don't call it though an instance, the HotSpot couldn't distinguish it. If I let the Jit
to add the free() call in a future time, I will not need it any more.
>>
> 
> :) Java static method can be called as SomeClass.StaticMethod, so your
> program can be written as (it is nothing related with compiler)
> public static void main(String[] args)
> {
>    set(10);
>    System.out.println(get());
> }
> 

>> 2. I added an method named "unsigned int get_object_addr(jobject jobj)" in object_handle.cpp
to return the address of jobj . I can call it directly to get the object address in free()
method. Since I could not assure whether it can get the right address, so I add the get_obj_addr()
for test purpose. I will delete it if it can work corredtly.
>>
> 
> Ah, I don't catch well here. Why do you need a free() function here?
> If you alloc a java object (no matter in java or native), you can just
> leave it alone and let GC do everything for you. Free() is too c-style
> for Java developer :)
> In the other way, if you want to alloc some memory for other use, for
> an example, char array in c/c++, you can just keep its address(you can
> keep a jlong as void* ) and free it at last(you'd better free in this
> case or you'll get memory leak). It is such easier to keep/pass a
> jlong value than jobject handle.
> 
My thesis is to redesign the heap layout, and provide the free() method to support reclaiming
the object explicitly. Then in jit module, I will add some analysis to insert some free call
to reclaim the dead object in advance. The purpose is to try to improve the efficiency of
program's execution. 

> 
>> 3. Another quesetion: where is runtime helper you speak of? I am afraid I need do
some investigation on it.
>>
>> Thanks,
>> tingpeng
>> ----- Original Message -----
>> From: "Xiao-Feng Li" <xiaofeng.li@gmail.com>
>> To: <dev@harmony.apache.org>
>> Sent: Sunday, April 29, 2007 4:05 PM
>> Subject: Re: problem about System.loadLibrary and object's address
>>
>>
>> > Tingpeng,
>> >
>> > 1. Your native is static native, why do you call it through an instance?
>> > 2. You probably need to use runtime helper for the object direct free
>> > in Java app, because the default JNI call path will save the object
>> > handle (and passes the reference address as a root entry during GC
>> > enumeration). For your purpose, there is no GC happening in free(),
>> > so it probably works with a stale object reference (freed) in the
>> > object handle.
>> > 3. You can use free() directly to retrieve its object address from the
>> > handle. Why do you need the get_obj_addr() method?
>> >
>> > Thanks,
>> > xiaofeng
>> >
>> > On 4/29/07, Tingpeng Wu <wutingpeng@gmail.com> wrote:
>> >> Thanks, xiaofeng,
>> >> My test program is simple as follows:
>> >>
>> >> public class JNItest
>> >>
>> >> {
>> >>
>> >> static
>> >>
>> >> {
>> >>
>> >> System.loadLibrary("goodluck");
>> >>
>> >> }
>> >>
>> >>
>> >>
>> >> public native static int get();
>> >>
>> >> public native static void set(int i);
>> >>
>> >>
>> >>
>> >> public static void main(String[] args)
>> >>
>> >> {
>> >>
>> >> JNItest test = new JNItest();
>> >>
>> >> test.set(10);
>> >>
>> >> System.out.println(test.get());
>> >>
>> >> }
>> >>
>> >> }
>> >>
>> >> Then I use javah to produce JNItest.h and implement two method in JNItest.cpp.
Then use them to build goodluck.dll. I use the latest verson of drlvm to test it, there is
still an error which reports "Vm launcher meets error and needs shut up (this is translated
from chinese)".
>> >>
>> >> The reason I asked the second problem is I want to get the address of obj
in java to support the reclaimation. I plan to
>> >> MyRuntime class, which has a native method,
>> >> My thought now is as follows:
>> >> 1. provide a new class named MyRuntime which has two native methods.
>> >>     class MyRuntime
>> >>     {
>> >>          static
>> >>          {
>> >>               System.loadLibrary("runtime");
>> >>          }
>> >>
>> >>         //public native static void alloc(……);
>> >>
>> >>          public native static void get_obj_addr(Object obj);
>> >>
>> >>           //public native static void free(Object obj);
>> >>     }
>> >>
>> >> 2.    export the get_object_addr(jobject jobj) in vmcore to support MyRuntime.get_obj_addr(Object
obj)
>> >> 3.    export the free(unsigned size, void* address) in gc to support MyRuntime.free(Object
obj)
>> >> 4.    if above is right, I can call MyRuntime.free(obj) in java method.
>> >>
>> >>
>> >>
>> >> Is it feasible to get address that way to support the reclaimation method?
>> >>
>> >> Thanks,
>> >> tingpeng
>> >>
>> >> ----- Original Message -----
>> >> From: "Xiao-Feng Li" <xiaofeng.li@gmail.com>
>> >> To: <dev@harmony.apache.org>
>> >> Sent: Sunday, April 29, 2007 11:06 AM
>> >> Subject: Re: problem about System.loadLibrary and object's address
>> >>
>> >>
>> >> > Tingpeng, can you post your program if there is no legal issue? (e.g.,
>> >> > open a JIRA issue and attach your code there).
>> >> >
>> >> > For your second question, itis pretty the core part of JVM native
>> >> > interface design. Yes, the handle is used to access Java object
>> >> > indirectly. The idea is to support object movement during GC, then
the
>> >> > real new address of the same object can be stored to the handle. It's
>> >> > not supposed to be used everywhere in the JVM, because that may break
>> >> > the protocol of JNI, causing GC to fail to update the object new
>> >> > address, e.g., if it is put into a register by your C compiler. You
>> >> > can access it in two ways: either always use JNI interface, or
>> >> > guarantee there is GC happening when you access it.
>> >> >
>> >> > Thanks,
>> >> > xiaofeng
>> >> >
>> >> > On 4/29/07, 吴廷鹏 <wutingpeng@gmail.com> wrote:
>> >> >> Hi, all,
>> >> >> when I use drlvm to execute my program, I found that when program
has System.loadLibrary call, there is always an error which reports "java.lang.outofmemoryerror
<no stack trace available>". The same program can run on Hotspot. Why this happens and
how to solve it?
>> >> >>
>> >> >> Another question, I read the implementation code of Object.clone
method in Object_generic.cpp.
>> >> >>
>> >> >> jobject object_clone(JNIEnv *jenv, jobject jobj)
>> >> >> {
>> >> >>     ObjectHandle h = (ObjectHandle) jobj;
>> >> >>
>> >> >>     //aquire the target address and assign it to variable named
result
>> >> >>
>> >> >>     memcpy(result, h->object, size);
>> >> >>
>> >> >> }
>> >> >> According to my comprehension, h->object is the address of java
object. Is it ture? Does this means I can use the same way to get the address of object in
vmcore's other place provided the necessary head file is included?
>> >> >>
>> >> >> Thanks,
>> >> >> tingpeng
>> >> >>
>> >> >
>> >> >
>> >> > --
>> >> > http://xiao-feng.blogspot.com
>> >> >
>> >
>> >
>> > --
>> > http://xiao-feng.blogspot.com
>> >
> 
> 
> -- 
> 
> Best Regards!
> 
> Jimmy, Jing Lv
> China Software Development Lab, IBM
>
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